How To Build A Basement, Underground Room or House – Requirements, Foundations, Walls, Roof & Waterproofing

How To Build an Underground Room, House, Basement or Bunker

• Preliminaries & Building Codes

• Basement Design

• Basement Earthworks

• Basement Substructure & Superstructure

• Basement Roof / Upper Floor

• Underground  Basement Roof Structure & Drainage Systems

• Drain Pipes, Back-Filling and Waterproofing To Retaining Walls

Basement Below Grade / Ground Level

In residential and commercial construction, underground rooms or buildings are called basements. In military warfare, bunkers are usually built underground as a camouflaged fortified chamber that hides and protects people from bombs, missiles and bullets fired from enemy aircraft. In luxury and exclusive opulent homes of the rich and wealthy, it’s very common to find an underground cellar, a small underground room to store wine. However, cellars have humble beginnings dating back to 1700 BCE in the ancient Middle Eastern city of Tel Kabri, but they probably existed much earlier than that in various native cultures. Before the advent of refrigeration, cellars were dug and built by the common villager living in cold climates to store root crops, vegetables, fruits, meat and other farm produce.

Basement Building Codes

Before you embark on building an underground room on your residential or commercial lot, you have to be familiar with building codes in your city or town. Like any new dwelling, structural extension or addition, a building permit is required to build a basement whether it’s part of the house or not. However, area-specific zoning laws may prohibit underground buildings. Find out if basements are allowed in your location. The horizontal and vertical expansion of your residential property is governed by zoning laws which may prohibit or restrict underground buildings in a specific location, just as multi-storey buildings may be restricted in certain areas. If you do get a building permit, the basement cannot be built as you wish. There are rules and regulations that you have to follow to ensure safety and compliance with not just the structural design but also drainage, ventilation, plumbing, heating, waterproofing, insulation, moisture control, flooding risk, ceiling height, lighting, fire safety, wall openings (egress), access (stairs), party walls (underpinning), site preparation and finishes. Unlike normal above-ground buildings, you almost always need a permit to undertake basement finishing and remodelling.

Building a basement or underground room should involve a lot of planning necessitated by the numerous regulation codes attached to it. The biggest risk affecting a poorly built basement is the risk of flooding and moisture penetration. Thus, moisture/water control and drainage is a critical factor when designing a basement. Another critical factor is the walls and foundation. Basement walls are not like ordinary walls above ground level, they have lateral earth loads acting on them. Basement walls are subject to horizontal forces caused by soil and water pressure, making earth settlement more likely. The deeper the basement below ground level, the higher the lateral forces pushing on the walls. Thus, basement walls should be built like retaining walls to withstand lateral earth pressure.

Preliminary Site Investigations

Preliminary site investigations involving the engineer’s geotechnical site report and flood risk assessment should be done prior to producing basement designs, including checking the location of existing underground utility services, watercourses, adjacent buildings (party walls etc.) and so on. The subsurface soil profile of the area, ground terrain, topography, site drainage systems, water table, aquifers, local rainfall patterns, snowmelt  and flood history are important in deciding whether you should build a basement or not.

The engineer should determine the depth of firm soil strata with a much higher load bearing capacity for basement foundations (e.g. crystalline/sedimentary rock bed, firm clay, silty clay and silty sandy clay).

Flooding Risks

Basements are vulnerable to flooding and damage from various water sources such as underground water, surface water, sewers and sewage backflow due to the fact that their floors are way below the base flood elevation (BFE). Structural defects such as cracks in the basement walls, floor, drainage and sewer pipes can allow groundwater to seep in, as well as leaking clean-out end caps and collapsed/clogged weep holes.

Window wells for basements are prone to flooding especially if the drains are clogged. If the drains are clogged, water will rise to window level, seeping in through the window cill and frames. Make sure that the vertical intake drain pipe and grating cover are not clogged or blocked by rubbish, debris, leaves and soil. To prevent rubbish and debris from entering the window well and clogging the drain, you should install a window well cover preferably a transparent cover that lets in sunlight into the basement. Window wells are not just built to drain away flood water, but they also provide an escape route via the egress window which should be big enough to allow an adult person to climb through in case of a fire emergency or other disaster.

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How To Build An Underground Room or Basement

1 – Basement Design

Once you have familiarized yourself with the standard building codes and done site investigations (including flood risk assessments), you should go ahead and design your basement or underground room. The basic and functional features of a basement are:

• Structural retaining walls (built from cast-in-place reinforced concrete or grouted CMU blocks) including external perimeter HDPE perforated drainage pipes and filling.
• Reinforced concrete foundations
• Reinforced concrete floor slab
• Waterproofing to floor and walls
• Egress windows (window well / light well)
• Staircase and roof opening
• Outdoor stairway for walkout basement
• Roof:  If the basement is under a single storey or multi-storey house, the upper floors will simultaneously function as roofs for the basement. Upper floors are either a reinforced concrete slab, timber joist floor or steel beam floor.
• Bunker Roof:  If the basement is not attached to any upper floor (i.e. exposed basement), then you will need to build a bunker roof.
• Roof insulation and waterproofing
• Wall Finishes (Drywall, framing, thermal insulation and internal waterproofing)
• Floor Finishes
• Ceiling
• Walkout door / entrance cellar door
• Walkout basement outdoor stairwell drain
• Sump pump system
• Internal floor drain
• Combustion room / furnace room

When designing a basement, your structural engineering drawings as well as architectural plan should meet the following conditions prescribed by various municipal building codes prior to being approved. These are not the only code specifications that you have to comply with, there are codes for other elements, fittings, installations and situations that you have to comply with depending on your basement design.

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Basement Floor Plan Requirements for Approval:

Your basement floor plan must be dimensioned and scaled, showing and identifying all rooms by name e.g. bedroom, bathroom, storage, living room and so on.

Using relevant electrical symbols, the plan must show the location of electrical switch board (distribution board), sockets, switches, light fittings, smoke detectors, fans and other installations.

Using relevant architectural symbols and schedules, the plan must show the position of doors and windows as well as their size. Stairs, fire escape routes, egress and window wells must be indicated on the drawings with their dimension detail.

Using relevant architectural/engineering symbols, the position of sanitary plumbing fittings and fixtures must be shown, as well as any HVAC installations like water heaters, furnaces, radiators and air conditioners. The position of stoves, fireplaces, exhaust fans, ducts, fire sprinklers, sinks, showers, bathtubs and W/C must be indicated.

Where alterations, additions and demolitions are being done, the position of existing and new doors, windows and walls must be indicated on the plan. New electrical, HVAC, mechanical and plumbing installations must be shown. Structural modifications like making openings in walls, floor and roof must be shown, as well as the location of beams, columns, lintels, headers, posts and floor joists.

All in all, architectural schedules and tables with item specifications should be provided on the basement plan. The floor to ceiling height should be indicated, as well as the size and product code of insulation in walls and ceilings.

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Minimum Internal Room Size and Space:

Living space occupied by people in a basement should be a minimum of 70 square feet, measured within the room internal perimeter, and the minimum length of each side in the room should be 7 feet. Hallways, passages and stairways inside the basement should be at least 36 inches wide.

The ceiling in habitable rooms, corridors and hallways should be a minimum of 7 feet high. The ceiling in bathrooms, toilets, laundry and other non-living spaces should be at least 6.203 feet high.

A water closet shall be installed in the toilet room, providing a minimum clear space allowance of 15 inches from the centreline of the W/C and room to the side walls, as well as a minimum of 21 inches from the front of the W/C.

Shower cubicles shall have a minimum internal floor gross area of 900 square inches and base dimensions at least 30 x 30 inches, measured from wall to wall.

The stairway shall have a minimum headroom clearance of 6.203 feet high (2000mm) measured vertically from the ceiling to the bottom landing or from the tread nose to the imaginary pitch line parallel to and above the stair slope.

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Egress Window Requirements:

Emergency egress is a means of escape or exit from the basement in the case of a fire or disaster. All basements are required to have an escape route provided by means of an egress window. If the basement has bedrooms separated by and enclosed by internal walls, each bedroom should have its own egress window. An egress window opens out into the window well, which should provide enough space for climbing out into the open air.

Egress Window Clear Opening Size

According to basement building codes in the USA, the minimum size for an egress window in a basement shall be 5.7 square foot, with a clear opening that is at least 20 inches wide and 24 inches high. In metric units, this converts to a clear window opening with a minimum size of 0.53 square metres, which is equal to a minimum width of 508mm and minimum height of 610mm. In Canada, some provincial and municipal codes may have different requirements, for example, in Alberta, the minimum size of an egress window opening is 3.77 sqft (0.35m2), with a minimum width of 15 inches (380mm) and minimum height of 592mm. In the United Kingdom, the minimum size of openable area for an egress window is 0.33m2 and window dimensions should be a minimum of 450x450mm high (18×18 inches high). Just make sure the minimum area of the clear opening as well as the width and height are met. When designing the appropriate window size, adjust the width and height of opening accordingly until the minimum area is obtained.

Egress Window Sill

The window sill for an egress opening shall be no less than 44 inches from the finished floor level (USA basement codes). This is just about the same as UK codes which prescribe a minimum height of 1100mm (44 inches) from floor surface.

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Window Well Requirements

If an egress window sill is situated the ground level or ground formation level, a window well must be dug and built in front of the window. The depth or height of the window well shall commence at least 6 inches below the window sill. In some county municipal codes, the minimum sill to well bottom depth is 3.5 to 4 inches. The enclosing side walls of the window well must project at least 4 inches above the ground level.

The bottom or floor of the window well must be at least 9 square foot. The floor dimensions (width as well as the longitudinal side) of the window well must be at least 36×36 inches in size. The width of the window well shall be measured from the outer side of the basement wall to the inner side of the window well wall.

A ladder shall be installed inside the window well on the side facing the window where the depth of the well exceeds 44 inches (1100mm). The minimum width of the ladder should be 12 inches (305mm) and it should have horizontal rungs spaced at no more than 18 inches (457mm) between them and from the bottom rung to the floor. However, according to OSHA, ladder rungs should be spaced at a minimum distance of 10 inches (250mm) center to center spacing and should not exceed the maximum spacing 14 inches (360mm) apart. The ladder should allow the occupants or escapees to make a footing on the rungs and climb out unhindered. Therefore, a spacing of 3 to 6 inches must be allowed between the face of the ladder and the window well wall. Awnings  or any window well covering which cannot be opened from the inside without struggling, effort or making use of any key or tool are not recommended. The same applies to window screens, grills, burglar bars as well as basement doors. Doors and windows should be easily and effortlessly openable from inside without making use of any key, instrument, tool or complicated skill.

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Smoke Alarms

Building codes state that dwellings should be protected against fire outbreaks using smoke detectors. This includes basements. Smoke alarms must be installed in each and every bedroom or sleeping room. The alarms should not be a plug-in device, but they should be permanently wired to the building electric circuit and interconnected to each other, so that when smoke is detected the sirens will go off at once. Where there is an appliance or furnace making use of solid burnt fuel (e.g. coal, wood, coke, charcoal or anthracite ), a carbon monoxide alarm should be installed.

Mechanical Ventilation and Heating

If windows are not installed in bathrooms and toilets, these rooms should be mechanically ventilated with an exhaust fan and duct system prescribed by building codes, usually a 50 to 80 cfm rated fan and 4 inch minimum diameter ducts. But the airflow measured in cfm (cubic feet per minute) will be determined by the size of your bathroom. Roughly, 1 cfm is needed per square foot of your floor. Multiply this rate with the square footage of your floor to find the required cfm rating of your exhaust fan.

Home heating systems such as furnaces, boilers, heat pumps, radiators, water heaters and hot water tanks should not be installed nor located in living spaces including the bathroom and toilet. They should neither be accessed from any living or habitable space such as bedroom, kitchen or living room. Heating systems should be located in their own separate room. The combustion room must be insulated, ventilated and secured with a fire-rated door.

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The furnace or heater inside the combustion room must be operated by a switch installed by the doorway (on the wall near the door jamb). The switch should easily shut off the system in case of an emergency. For safety and emergency purposes, it’s not a good idea to install the switch near the furnace or boiler. Another safe location for installing a furnace switch is under the basement stairs.

How To Build an Underground Room, House, Basement or Bunker

Once your basement plans are approved by the municipality or town council, you should go ahead and build your basement. The following is a step by step construction procedure or programme of work for a basement:

Basement Below Grade / Ground Level

1 – BASEMENT EARTHWORKS

Site Clearance

Clear site of all vegetation, rubbish, debris, shrubs, bushes and grass,  including cutting down trees and grubbing up roots. Before digging the ground, call the local municipality inspectors or utility operators (Toll free  phone number 811 in the USA)  to come locate and identify underground utility services like water, gas and sewer pipes.

Setting Out Levels  and Profiles

Once your building site is clear, proceed to set out levels on the ground using profile boards and strings. Find a reference point or datum line from which you can set out your measurements and mark the corners with stakes.

Setting out is basically transferring the building drawings (floor plan) to the actual ground where the house will be built. In this case, we are transferring the basement floor plan to the site. We will need to mark the position of the basement corners with stakes as well as outline the external walls with stringlines running between the stakes.

Setting out the building outline allows you to see where the site will be excavated. On flat and level ground, you can start excavating from any point along the stringlines. On undulating or hilly ground with medium and steep slopes, you have to set up stakes at the bottom of the slope as well as on the top edge and middle of the slope. You will need to start excavating from the lowest elevation (lowest ground) going towards the higher points. The aim of cutting the slope is to get flat and level ground from which you can dig foundations for a walk-out basement or excavate a huge pit if you are building an underground basement that is below strip level.

As you can see, mass excavation which may involve open-face excavation on a hillside is always required when building a basement. You will need to hire a bulldozer as well as an excavator to do the job.

Excavate Site To Reduced Level

When you are building a walk-up or walk-out basement situated on hilly or sloping ground, you will need to cut the slope and any elevated ground to reduced levels. Following the setting out lines and stakes on the site, start excavating from the lowest elevation with a bulldozer. Cut the slope until you reach the stakes on the higher edge. Cart-away and dump the excavated material on spoil heaps on the site. After cutting the slope, you should have a fairly level base (reduced level) from which you can start basement pit excavations.

Excavate Basement Pit

The depth of your basement pit will be determined by the design drawings as well as site conditions from preliminary investigations. Let’s say your basement floor-to-ceiling height is 2450mm and the basement is projecting 305mm above ground level. The basement retaining walls are 204mm thick x 2500mm high, resting on 380mm thick x 1220mm wide footings. The floor is a reinforced concrete raft foundation with 230mm edge thickening and 100mm thick slab.

Bulk Excavation – First Stage:

Your first stage of bulk excavation will be excavating from ground level to the bottom of RC retaining walls. Therefore, the depth of excavation will be 2500 – 305mm = 2195mm

Let’s say the gross floor area of your basement is 5372mm x 5054mm wide, and the retaining wall footings are projecting 178mm from the wall. You need to calculate the area based on the projection of foundation footings:

Length: 5372mm + 178(2) = 5728mm

Width: 5054mm + 178(2) = 5410mm

The surface area of the basement bulk excavation will be 5728 x 5410mm wide, and the cubic volume of bulk excavation will be 5728 x 5410 x 2195mm deep.

Excavate the pit using an excavator machine, and dump excavated material outside the pit in spoil heaps.

Earthwork Support to Excavated Pit:

Earthwork support also known as timbering or shoring is needed when your trench/pit excavations are 1500mm or deeper. Install earthwork support inside the pit around the perimeter. Trench boxes, raking shoring and hydraulic shoring systems will be suitable for a big and wider pit such as a basement excavation.

Foundation Trench Excavation – Second Stage:

Excavating the pit down to the bottom of RC retaining walls gives you a reference base (reduced level) from which you can start excavating foundation trenches with precision. These are the footings for the retaining wall. Excavate trenches for 380mm thick x 1220mm wide footings around the internal perimeter of the pit, dumping excavated material on spoil heaps in the centre of the pit.

Level and Compact Bottom of Trenches:

Level and compact the bottom of trenches to 95% Modified ASSHTO Density, breaking down oversized material and evenly distributing the excavated material on the surface. Apply some soil insecticide to the sides and bottoms of trenches.

Sand Blinding:

Add a layer of 50mm sand blinding on compacted surfaces of trench bottoms. If the soil is a weak type, spread a layer of weak concrete (soilcrete) of 10MPa strength or lower.

2 – BASEMENT SUBSTRUCTURE & SUPERSTRUCTURE

Formwork to Reinforced Concrete Footings:

Depending on the stability and cohesiveness of the soil, you may or may not need formwork. Install formwork for cast-in-place reinforced concrete footings. Formwork will be required on both sides of the footings and the height of the formwork should be equal to or slightly higher than the thickness of the concrete footings. Formwork should be left in place until the concrete has hardened.

Reinforced Concrete Footings

Before you pour concrete into the formwork for foundation footings, you must place some vertical reinforcement spacers on the surface, arranged in an appropriate way at suitable distances apart. Vertical spacers are used to suspend horizontal reinforcement bars or mesh wire at a suitable height above the ground or surface. This provides a clearance which will be filled by concrete cover. Concrete cover protects reinforcement against corrosion, weathering and damage. There are different types of reinforcement spacers also known as cover blocks, which among them includes plastic spacers and the commonly used concrete and wire chair spacers. Concrete block spacers are usually placed at a distance of 500, 600 or 700mm apart for RC footings and slabs.

The minimum concrete cover for RC footings is 50mm, so you should get vertical spacers that match this requirement.

Lay the Reinforcement Bars for Footings and Stub Walls:

After placing and setting up the spacers, you should lay the steel reinforcement bars over the spacers, tied at suitable points.

When laying steel reinforcement, remember to include L-shaped vertical steel dowels for concrete stub walls. These stub wall dowels will project from the footings and they must be placed in position at 16 inches o/c along the footing perimeter. For connecting the raft foundation edge thickening to the stub wall and footing, L-shaped horizontal steel dowels are positioned in place at 16 inches o/c along the footing perimeter, with a minimum lap of 24 inches into the raft slab.

Pour in Wet Cast-In-Situ Concrete for Footings:

Pour cast-in-situ ready-mixed concrete (25MPa + strength) under and over the reinforcement. The vibrated concrete should be able to flow easily filling the space under the reinforcement. Compact the wet concrete using an immersion needle vibrator. Alternatively, steel or wooden tamping rods can be used to compact the concrete, but mechanical compaction using a needle vibrator is the best. When using mechanical compaction, take precautions to avoid over-compaction. Compaction eliminates air bubbles, which is required to produce dense and impervious concrete.

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Formwork to Reinforced Concrete Walls:

Formwork To Stub Walls:

Stub walls also known as stem walls are built when the footings have been cast in place, dried and hardened. You should have stub wall steel dowels sticking out from the footings after the footings are cast in place.  To built stub walls, you must first erect formwork around the basement footing perimeter. Double-sided formwork is needed to contain the concrete filling, and the internal spacing must match the width of the concrete wall (204mm). A typical stem wall is 1.65 to 4 feet high (500 to 1200mm), so your formwork should also be about this high.

Formwork To Superstructure Walls:

Once stem walls have been built, your next step is building superstructure walls, which is literally extending the wall height. Once again, you have to erect double-sided formwork up to the height of the basement (2500mm).

You also need to erect formwork for wall openings such as windows and doors:

• Smooth Formwork To Edges of Egress Window Opening (≤ 300mm), Not Exceeding 2000m Girth.
• Smooth Formwork To Edges of Door Opening (≤ 300mm), Not Exceeding 2000m Girth.

Reinforced Concrete Stem Walls

Pour in Wet Concrete Cast in Formwork for Stem Walls:

Pour cast-in-situ ready-mixed concrete (35MPa + strength) inside the formwork for stem walls. Vibrate and compact the wet concrete around the reinforcement using an immersion vibrator.  Vibrated concrete is viscous, flows easily to fill space. Alternatively, a steel or wooden tamping rod can be used to compact the concrete, although mechanical compaction using a needle vibrator gives much better results. When using a mechanical compactor, take precautions to avoid over-compaction. Compaction eliminates air bubbles, which is required to produce dense and impervious concrete.

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Reinforced Concrete Superstructure Walls

The superstructure walls for the basement must have a minimum ground clearance of 1 foot (305mm) above the ground level.

Lay the Reinforcement Bars for Superstructure Walls:

Set up the horizontal as well as the vertical reinforcement steel bars for basement walls. Vertical steel dowels should be tied to stem wall dowels, and they should be spaced at 16 inches o/c along the wall perimeter. Horizontal steel dowels should be tied to vertical dowels and placed at 16 inches o/c along the wall height.

Pour in Wet Concrete Cast in Formwork for Superstructure Walls:

Pour cast-in-situ ready-mixed concrete (35MPa + strength) inside the formwork for basement walls. Vibrate and compact the wet concrete around the reinforcement using a poker vibrator.  Vibrated concrete is viscous, flows easily to fill space. Alternatively, a steel or wooden tamping rod can be used to compact the concrete, although a mechanical compactor is more effective. When using a poker vibrator, take precautions to avoid over-compaction. Compaction eliminates air bubbles, which is required to produce dense and impervious concrete.

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Reinforced Concrete Raft Foundation Slab

Underground basements are built on a raft foundation, an RC slab with edge thickenings (beams) around the perimeter. The minimum thickness of a concrete surface bed or slab is 4 inches (100mm) but for a raft foundation 150mm is the recommended minimum. The thickened edge (beam) should have a minimum depth of 225mm to 305mm if it is load bearing, and at least 120mm if it is non-load bearing. The raft foundation for this basement will be non-load bearing, built within the inside perimeter of the RC retaining walls and resting on retaining wall footings which are load-bearing.

Level Compact the Excavated Surfaces Under Surface Beds:

Level and compact the soil under surface beds to 95% Modified ASSHTO density with a mini vibratory roller, breaking down oversized material and evenly distributing the excavated material on the surface, including wetting the soil and adding suitable material where necessary, compacting the soil in 150mm layers until the ground is firm and stable. Spray some approved soil insecticide or termite proofing on excavated surfaces.

Excess excavated soil from spoil heaps stored on site will be used to beef up ground formations under the surface beds.

Sub-base Coarse Filling (Hardcore Material):

After the excavated surface under floors is levelled compacted, you should proceed to add a layer of 150mm thick sub-base coarse filling (i.e. G5 material, gravel, crushed stone, bricks or concrete). Spread, level and compact the coarse filling in a single layer 150mm thick to 100% Mod AASHTO density at OMC, stabilised to attain UCS > 1,0 Mpa after seven days.

Sand Filling

Add a layer of 50mm sand blinding or levelling course over the gravel filling.

DPM

Over the sand, place some moisture-resistant underlayment known as a vapour barrier or damp-proof membrane (DPM).  This can be a 10 mil PVC plastic film or one layer of 250 micron green polyethylene waterproof sheeting sealed at laps with PVC self-adhesive tape.

Reinforcement

In an above-ground surface bed on strip foundations, welded mesh wire is usually used to reinforce concrete slabs. This fabric reinforcement is rolled out and laid on top of the DPM. In underground basements and heavy-duty commercial structures, reinforcement bars (steel dowels) are laid out perpendicular to each other in a mesh pattern and tied to each other with mild steel wire ties. The steel dowels are laid crosswise, running both ways at a spacing of 16 inches (406mm) apart. One or two layers of steel bar mesh are usually installed in the surface bed before wet concrete is poured. The horizontal L-shaped dowels sticking out from the bottom of RC stem walls are tied to the slab dowels to create a rigid continuous basement structure that can withstand settlement caused by uplift, lateral and shear earth forces.

When setting up reinforcement on horizontal ground surfaces, the bars should be laid on vertical spacers as mentioned previously to allow for concrete cover as well as vertical height positioning (suspension).

Pour in Wet Cast-In-Situ Concrete for Surface Beds

Pour cast-in-situ ready-mixed concrete (30MPa + strength) under and over the reinforcement for surface beds. Vibrate and compact the wet concrete around the reinforcement using a poker vibrator.  Vibrated concrete is viscous, flows easily to fill space. Alternatively, a steel or wooden tamping rod can be used to compact the concrete, although a mechanical compactor is more effective. When using a poker vibrator, take precautions to avoid over-compaction. Compaction eliminates air bubbles, which is required to produce dense and impervious concrete.

Finishing Top Surface of Concrete Slab:

While the concrete is still wet and viscous, level the surface with a levelling board. After at least 30 to 45 minutes when the surface has stopped bleeding, you can apply a 35mm to 55mm thick cement/sand screed (1:3 mix) or topping finished with a wood float or steel trowel according to your requirements. Allow the concrete surface to cure using a water based acrylic copolymer curing compound with minimum moisture retention rate of 90% over a 72 hour period.

BASEMENT WALKOUT STEPS & STAIRWELL DRAIN

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3 – BASEMENT ROOF / UPPER FLOOR

In a multi-storey house, the first floor may be built on top of the basement whereby the first floor also functions as the upper floor and ceiling for the basement. If the basement is a single storey structure, it will have its own roof which is usually an RC slab.

Reinforced Concrete in Suspended Slab

The wall structure of a basement requires a two-way RC slab (upper floor or roof). A two-way suspended concrete slab supported on all four sides of the basement walls is going to be built. Two-way slabs transfer loads in both directions perpendicular to each other.

Smooth Formwork To Soffits of Slab

Smooth Formwork To Soffits of Slab (≤ 300mm), Propped Up at a Height Not Exceeding 3000mm.

Smooth Formwork To Edges of Suspended Slab

Smooth Formwork To Edges of Suspended Slab Not Exceeding 300mm High.

Smooth Formwork To Edges of Staircase Opening

Smooth Formwork To Edges of Staircase Opening (≤ 300mm), Not Exceeding 2000m Girth.

Reinforcement To Suspended Concrete Slab

Place some vertical reinforcement spacers on the horizontal formwork, evenly distributed on the surface at a spacing of 500, 600 or 700mm apart.

Lay the main bars (tension steel) on top of the spacers along the shorter span of the slab. Lay the distribution bars across and on top of the main bars in the longer span of the slab. Tie the top and bottom bars where they intersect with mild steel wire ties.

Pour in Wet Concrete Cast in Formwork for Suspended Slabs

Pour cast-in-situ ready-mixed concrete (30MPa + strength) under and over the reinforcement for suspended slabs in formwork. Vibrate and compact the wet concrete around the reinforcement using a poker vibrator.  Vibrated concrete is viscous, flows easily to fill space. Alternatively, a steel or wooden tamping rod can be used to compact the concrete, although a mechanical compactor is more effective. When using a poker vibrator, take precautions to avoid over-compaction. Compaction eliminates air bubbles, which is required to produce dense and impervious concrete.

Basement Below Grade / Ground Level

UNDERGROUND  BASEMENT ROOF STRUCTURE & DRAINAGE SYSTEMS

An under-garden basement is a below-grade basement which has its full height submerged below the ground level, and it has no upper floor. It’s roof is level with or slightly above the ground level. Building a roof for an under-garden basement is challenging and needs careful planning due to the fact that the roof is below the BFE (Base Flood Elevation), which exposes the basement to flooding.

The best way to plan for surface water flooding and drainage when building an under-garden basement is building it as a walk-out basement, with its external wall door leading out to an outdoor stairway made of concrete, bricks or stone. It’s not recommended to install the external door on the roof i.e. through the internal staircase opening.

Just outside the external door entrance on the bottom landing of the outdoor stairway, either a grated drainage channel (trench drain) or grated square drain is installed to collect and drain away surface runoff water to a sewer or catch pit during rainfall or flood incidence.

An under-garden basement also needs a sump pump system installed under the basement to collect excess water caused by a high water table and floods. The sump pump drains water from retaining wall weeping tiles, underfloor sub-surface drains, internal floor drains and any underground seepage. It is a good practice to install drains on the basement floor slab, especially where water leakages and condensation are more likely to happen, for example in the combustion room where home heating systems such as furnaces, boilers, heat pumps, radiators, water heaters and water tanks are installed. Internal floor drains can also quickly drain away water from the basement in case of flooding and heavy rainfall.

Waterproofing a Reinforced Concrete Flat Roof for Basement:

A modern flat roof makes use of a warm roof system instead of the traditional cold roof system. There are two types of warm roof systems, the normal warm roof and the inverted warm roof.

The normal warm roof has 4 layers excluding the ceiling and battens. These are, starting with the top layer:

• Waterproofing
• Thermal insulation
• Vapour control layer
• Concrete roof slab
• Ceiling and battens fixed under the slab

Just like the normal warm roof, an inverted warm roof has the waterproofing, thermal insulation and vapour control layer above the concrete roof slab, but the difference is the order of these layers. In an inverted warm roof, the thermal insulation is on top of the waterproofing and vapour control layer. Also, in an inverted warm roof, some ballast (heavy weights ) such as washed stones, self compacting concrete coat, concrete paving blocks, thin brick pavers (veneer bricks) or concrete tiles are placed on top of the thermal insulation to keep it in place and prevent it from being blown away by the wind. A geotextile fabric may be placed over the thermal insulation to prevent it from being damaged by the ballast.

Another important layer which may not be so obvious, but common to all types of flat concrete roofs is the cement screed or mortar which is applied on the wood floated concrete slab at an inclined / sloping angle to drain away surface runoff rainwater to a hopper head and downpipe. A scupper drain system is required for all flat roofs where a parapet wall is enclosing the roof structure.

DRAIN PIPES, BACK-FILLING AND WATERPROOFING TO RETAINING WALLS

HDPE perforated drain pipes (at least 4 inches in diameter) are laid under the surface beds and behind retaining wall foundations during construction. Drainage pipes under surface beds are laid before the RC slab is cast, and those behind retaining walls are laid near building completion before back-filling is done.

Before the HDPE perforated drain pipes are laid , a geofabric membrane or textile (filter fabric) is laid over the compacted subgrade. A layer of sand (at least 32mm thick) is placed over the filter fabric to function as levelling course or cushion to the drain pipes. The drain pipes are laid over the sand bed at a suitable slope and covered with gravel or 8mm stone sub-base filling, providing a cover of at least 6 inches (152mm) around and over the pipe. Once this is done, a filter fabric is wrapped around the gravel filling to prevent fine soil particles from clogging the free draining material.

Behind the retaining wall foundations, a granular backfill, gravel or any permeable backfill (about 12 inches wide) is placed over the geofabric textile, filling the working space up to the height of the basement at natural ground level. The sides of basement excavations must be compacted and backfilled with excavated material stored on site prior to filling in the working space with granular / permeable backfill. Compact the excavated surfaces to a minimum of 95% Mod AASHTO density, adding some excavated native soil  stored on site and compacting the backfilling until the sides are firm and stable.

Remember that the purpose of granular/permeable backfill behind the retaining walls is to intercept sub-surface water from a high water table and drain it away via the perforated HDPE drain pipes laid underground. The external back-filled sides of basement at ground level must be sloped away from the walls at a minimum incline of 3 to 5% (3 to 6 inches per 10 feet). The sides can be paved with a layer of concrete slab, block pavers, asphalt or impervious material. The edge of the pavement should empty the surface runoff rainwater into a U-shaped drainage channel that sends the rainwater to a catch pit or sewer.

Waterproofing the External Sides of Basement Walls

Basement waterproofing also known as basement tanking in the UK is whereby the walls are waterproofed with a moisture-resistant barrier. There are different types of waterproofing materials or methods, the five main types being:

• Cement based binding materials (This is a special type of cement/sand screed, mortar or slurry mixed with additives that enhance water resistance and impermeability ).
• DPM sheets or plastic films (e.g. overlapped Polypropylene or Polyethylene sheets sealed with PVC adhesive tape at joints).
• Liquid based DPM (This is applied as a liquid on concrete walls using a brush or roller. Start by applying a primer coat that improves bonding, then apply two coats of liquid DPM. Allow the coat to cure and set into a solid protective rubber film.) Examples of liquid-based damp-proof membranes include Polyurethane and Epoxy-based liquids.
• Bituminous/asphalt coating – Commonly used on flat concrete roofs, this waterproofing method can also be used on concrete walls. Spray the surfaces with a liquid bituminous coating, then place sheets of bituminous membranes on top of the coating. A bituminous coating should be covered to prevent damage by sunlight.
• Bentonite geosynthetic liners – This waterproofing method consists of a layer of water-resistant bentonite sandwiched with a geotextile membrane to withstand damage caused by mechanical and chemical corrosion/erosion under the ground. Bentonite clay liners are commonly used to provide self-healing water-tight seals in ponds, canals, lagoons, dams, reed beds, landfill liners and man-made lakes. Alternatively, a much better innovative form of bentonite like sodium bentonite can be used in place of clay liners. Hydration takes place when sodium bentonite comes in contact with water, absorbing water and swelling up to eight times its initial volume. The resulting colloidal sodium acts as an excellent underground hydraulic seal when it expands, creating an impermeable barrier that actively seals pores, holes, cracks and gaps wherever they occur.