How Deep Should a Footing Be? | Foundation Depths of Various Outdoor Structures

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How Deep Should a Footing Be? | Foundation Depths of Various Outdoor Structures

How deep should a footing for an outdoor structure be? There are many types of outdoor structures that you can build to enhance the value of your home. External works like fencing and screen walls improve the security of your home. Outdoor ornamental structures like outdoor ponds, showers, lamp posts, trees, pergolas and garden sculptures add aesthetic appeal to your yard, estate and residence. A third category of outdoor projects that you can have on your private property are utility additions like curbside mailboxes, septic tank, manhole and inspection chambers. Another outdoor home improvement idea that homeowners would love to have on their private property is a hybrid entertainment/utility addition like a barbeque island.

Before you embark on building any of the above structures, you have to know the foundation depth that you have to dig or excavate, as well as the width. This detail will be provided on the building drawings done by your engineer, architect or draftsman. Depending on your state or municipality, you might be required to obtain a building permit before building the structure. There may be additional permits required for digging trenches that exceed a certain minimum depth. Deeper trenches pose a hazard, therefore you will be required to meet OSHA regulations such as earthwork support, protective trench boxes and safe access in and out of the trench. Prior to digging any hole, excavation or trench on your property, one of the preliminary requirements that you have to meet is contacting the 811 Safe Dig number to notify the utility operators so that they can come locate and mark the position of utility services in your yard.

The following is the minimum depth of excavation for various outdoor projects:

Excavation / Foundation Depths of Various Outdoor Projects

Let’s look at the depth of excavations required for different types of outdoor projects:

Footing Size Chart – Foundation Depths of Various Outdoor Projects

Foundations for Screen Wall with Rails, Posts, Concrete Block Bottom and Concrete Base.

The size of concrete foundations will be 450mm wide x 900mm deep from level ground.

Foundations for Pergola Posts

The size of concrete foundation bases will be 400x400x500mm deep from level ground.

Foundations for High Tensile Wire Fence

The depth of holes for posts will be about 1093mm (43 inches) or 36 to 60 inches depending on the height of the post.

Foundations for Chain-Link Fence with Top Rail

For a post which is 48 inches (1,220mm) high from the ground level, the depth of the foundation base will be 30 inches (762mm) from level ground.

Foundations for Brick Boundary Wall

For a brick boundary wall or screen wall, the depth depends on soil conditions (the load bearing capacity of the soil) as well as the load of the superstructure. The depth can be anywhere from 500mm to 2500mm. 500 is usually the minimum depth, but the thickness of the concrete footing should be a minimum of 200mm. The earth cover (backfilling to the sides of trenches) should be a minimum of 300mm, and the plinth level should be level with DPC (Damp proof course) laid on top of substructure brickwork. The minimum height of the plinth from ground level is 150mm.

The minimum width of a brickwall concrete footing is 600mm. As a rough guide, the width of a concrete footing is generally 3 times that of a half-brick width.

Foundations for Steel Palisade Fencing

A steel palisade fence can be as low as 700mm and as high as 3,600mm above ground level. The minimum depth of the excavated hole for posts should be 600mm from ground level.

The diameter of excavation holes depends on the height of the fence above ground level. For posts less than or equal to 2,400mm high, the size of holes can be 350x350mm or 450mm in diameter. Higher posts which are 3,000mm or above need holes of size 450x450mm or 600mm in diameter.

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Foundation Depth for Water Fountain

The overall foundation depth of a water fountain or water feature from the ground level includes space for the reservoir and re-circulating pump, which is usually built and installed at the bottom of the water foundation floor. Alternatively, the reservoir (and pump) may be built on the extreme sides of the water foundation floor. There are fountains with a single pond which also functions as the reservoir, and fountains which have a separate reservoir and pond. The typical minimum depth of the excavation required for a water fountain is 14 to 30 inches, depending on the size, location and design of the reservoir.

Foundations for Excavated Pond

An excavated pond is built by digging a pit in the ground which will contain the body of water. The typical depth of excavation to subgrade level is about 919mm.

Foundations for Raised Pond

A raised pond has the body of water contained above ground level rather than in the excavated hole or pit. The raised container walls are built from bricks, stones, concrete blocks or reinforced concrete which is lined with waterproofing.

The typical foundation depth of a decorative raised pond ranges from a minimum of 100 or 150mm, up to 500mm deep from ground level. The height above ground is about 500mm. The height of a decorative pond shouldn’t be too high because it has to provide a perfect uninterrupted view from above.

If you are into ornamental fish breeding for commercial purposes, the approximate size of a big Japanese Koi pond is about 3.96m long x 1.83m wide x 0.91m high above ground level. Strip excavation to reduced level typically ranges from 150mm to 500mm deep from natural ground level.

Foundations for Stone/Metal Garden Sculpture

An art landscape sculpture is usually made of stone, metal or wood. The horizontal area of the foundation base (its spread) depends on the size of the sculpture and its weight. The depth of the foundation excavation from ground level can be anywhere from 100mm to 500mm.

Foundations for Pathway Lamp Posts

A Victorian garden lamp post can be a nice addition on your private property. It can also be used as a driveway lamp. Solar light lamp posts can be as high as 3.25m or as short as 0.60m.

Depending on the height of the lamp post, the typical depth of excavation for the foundation of a short lamp post (e.g.48 inches high above ground level) will be about 22 inches deep and 12 inches wide (559mm deep x 305mm diameter). The total depth of excavation includes a 6 inch thick bottom layer of compacted gravel and a 16 inch high unreinforced concrete base.

A higher solar lamp post such as the one used in the streets will have a much deeper foundation, from 600mm to 1100mm. A street light pole which is 6.0m high will need a foundation that is 1000mm to 1100mm deep. A 1500mm deep foundation is suitable for a 6.5m high street light column. A 3m high street light pole, measured from ground level requires a foundation that is 700mm deep.

Foundations for Brick Letterbox

A free-standing letterbox situated near the front yard gate close to the street sidewalk is called a curbside mailbox in the United States. Curbside mailboxes can be built from various materials such as brick, stone, metal, wet concrete or precast concrete. According to USPS rules and regulations, the height of a curbside mailbox should not be outside the 41 to 45 inches range. The standard and legal average height is 42 inches (1067mm). The minimum distance of the letterbox from the edge of the adjacent road should be 6 to 8 inches.

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As a rule, a mailbox adjacent to a street with no curb (sidewalk) should be located at least 2 feet (610mm) from the edge of the road.

A mailbox facing a street with curb (sidewalk) should be located at least 6 to 8 inches (152 to 203mm) from the edge of the sidewalk.

The foundation for a brick mailbox is similar to the foundation of a brick column. The size of the hole excavation depends on the size of the column base. A typical brick mailbox has four sides which are two-brick long. A standard brick size is 230mm long x110mm wide x 76mm high, with a mortar joint of 10mm. Each side will therefore be (230×2)+10 = 470mm. A 470 x 470 x 1067mm high brick column requires a foundation base of approximately 690x690mm.

USPS regulations specify a maximum depth of 24 inches (610mm) for mailbox foundation excavations, so that’s a volume of 690x690x610mm of excavated earth.

Foundations for Steel Letterbox and Post

In the United States, the Post Office (USPS) requires a home curbside letterbox to be mounted on a steel or aluminium post with a minimum diameter of 2 inches (51mm). Alternatively, letter boxes can be mounted on timber posts of minimum thickness 4×4 inches.

According to USPS, the maximum depth of excavated holes for a letterbox post foundation should be 24 inches (610mm). The depth of foundation shouldn’t exceed 24 inches. This allows the post to yield when it’s hit by a moving vehicle. The width of the foundations will be about 300x300mm.

Depth of Excavation for Planting Trees

Planting a tree successfully in your backyard is not as easy as most people think. Incorrectly planting a tree can lead to withering or death even if the tree was healthy. Planting a tree too deep or too shallow causes complications that will reduce its life span. A very large portion of the tree’s roots (80%) grow and spread horizontally like the spokes of a flat bicycle wheel and these roots occupy the topsoil at a strip level of up to 18 to 24 inches (457 to 610mm) deep.

A qualified and experienced horticulturalist will plan ahead before planting a tree. He or she will take into account the information mentioned above, the maximum horizontal spread of the roots as well as the overall depth occupied by the roots. To these parameters, the estimator should add an allowance of 250mm to the horizontal spread and another 250mm to the root depth. So in this case, the maximum overall depth of the excavation will be  610mm + 250mm = 860mm.

To find the right width for your excavated pit, you will have to measure the horizontal spread of the roots, then add an allowance of 250mm on both sides of the pit diameter. If it’s a square pit, add an allowance of 500mm (250mmx2) to all four sides.

Foundations for Timber Pergola

A timber pergola is made from four wood posts and coverless roof with only rafters and joists to support vines and climbing plants. The holes for posts are 500mm deep from ground level, and 400x400mm wide.

Foundations for Outdoor Shower

An outdoor backyard shower is an interesting structure which allows you to take a shower in the open air environment, decongesting and preventing the use of internal showers. It’s ideal for guests living in the outbuildings and social functions such as home parties. Building an outdoor shower includes excavating the topsoil to a depth of about 150mm, compacting the ground and pouring in a concrete slab over a waterproofing membrane. The shower floor can be built like a normal residential shower floor, slightly sloping towards a drain built in the centre or at the extreme end of the base.

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Alternatively, the outdoor shower can be built like a tennis court, slightly sloping, with precast concrete drainage channels or French drains around the perimeter of the base.

The shower base can be enclosed with timber, glass, PVC, stone or brick walls. If you are building an enclosure, you will need to dig foundations for the walls to lay reinforced concrete ground beams. The walls can also rest on the shower concrete pad if they are lightweight. A typical raft foundation has a 200x200mm edge thickening and 100mm slab.

Foundations for Septic Tank

A septic tank is a sewage storage and disposal system for a household or small community which is not connected to the main municipality sewage disposal system. The septic tank system consists of a tank and drainage pipes connected to a distribution box (pump box) which sends the wastewater into a drain field. The drain field (also known as a leach field) has rows of perforated pipes from which the effluent can leach into the ground.

A septic tank is buried completely under the ground with the exception of access risers (inspection pipes) that will protrude to the surface. Septic tanks come in different designs such as one compartment tank, and two or three compartment tanks. The size depends on the holding capacity.

Most septic tanks are made of reinforced concrete, precast concrete or brick walls. A typical two-compartment septic tank with a holding capacity of 1600 gallons is 68 inches wide (1700mm),and 145 inches long (3,680mm). It includes a 150mm floor slab, 1500mm high walls, and 100mm thick R.C. top slab with two or three 450mm diameter manhole covers and frame.

The topsoil backfill cover for a buried tank is 300mm, therefore your overall excavation depth will be about 1800mm to 2000mm. The size of the excavated pit will be about 1700 x 3680 x 1800mm deep.

Foundations for Manhole or Inspection Chamber

A manhole and inspection chamber is the same thing. This is an underground compartment built from brick or precast concrete walls, concrete slab and top iron cover and frame access. The compartment is shaped like a rectangular box or hollow cylinder, and its main purpose is to house junctions and bends for drainage pipes. Inspection chambers are also used for housing access points for underground electrical/telecommunication cables as well as water supply junctions, meters and valves. It is common to refer to these compartments as junction boxes.

The depth of a manhole that you want to build depends on the diameter of the sewer pipe, as well as the width of the chamber. For a 1050mm diameter chamber with 150mm sewer pipes, the maximum depth of the manhole is 3.10 metres. The in-situ base is 1.50 metres in diameter and its nominal height is 400mm.

Foundations for Outdoor Barbeque Area

A backyard barbeque area is a nice addition on your private residential property. Basically, it looks like a kitchen fitting with a built-in grill, cupboard and granite countertop. The only exception is that the compartment is built from bricks, concrete blocks or stone walls. A stainless steel barbeque grill with burners is then fitted onto the compartment.

Outdoor BBQ islands are built on a patio floor with a concrete slab or pavers. To build a brick or stone barbeque compartment, you will need to excavate a wide but shallow trench for its foundation pad, about 350 to 450mm deep. A small compartment for a 36-inch gas grill is 10 feet long (3050mm), 2.5 feet wide (760mm) and 3 feet high (914mm).

The concrete foundation pad will be 235mm wider than the brick compartment on all sides. Therefore, you will need to dig a trench that is 3520mm long x 1230mm wide x 450mm deep.

Before digging, call 811 in your state to locate underground utility services. You also have to meet OSHA requirements for trench excavations.


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