25 Materials To Use in the Bottom of a Fire Pit – Non Combustible Materials & Fire-Pit Construction Details
What To Use in the Bottom of a Fire Pit
A fire pit is built outdoors for recreational purposes. It’s a spot for warming up during the winter and cold nights, barbequing, roasting, toasting and even grilling some pizza. The high temperature containment in which a fire pit functions means you have to choose your building materials carefully. Fire-resistant materials are required to build a fire-pit, but they should also be non-combustible or non-flammable. A non-combustible material does not ignite or catch flames. Note that there are fire-resistant materials which catch flames and these are not appropriate for building a fire-pit. The right materials are those which don’t ignite, burn, melt, lose strength, change state, shape nor get deformed by fires. These materials will resist flaming, structural change and damage when subjected to fire temperatures in a normal 40% oxygen environment.
According to the International Building Code Fire Safety FS5-07/08 Part I Section 202:
Noncombustible material. A material that, under the conditions anticipated, will not ignite or burn when subjected to fire or heat. Materials that pass ASTM E 136 are considered noncombustible materials.
According to the National Fire Protection Association NFPA 101 and NFPA 5000:
Noncombustible Material. A material that, in the form in which it is used and under the conditions anticipated, will not ignite, burn, support combustion, or release flammable vapors, when subjected to fire or heat. Materials that are reported as passing ASTM E 136, Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 Degrees C, shall be considered noncombustible materials.
Now that you have the fireproof requirements right, you should look for materials with these properties. Some of the most common fireproof building materials are shown below. Most of them are suitable for use in a fire-pit:
- Natural Stone (Granite, marble, quartz, sandstone, limestone, gravel, basalt, slate, onyx, travertine)
- Clay bricks
- Sand Lime bricks
- Fire bricks
- Cast-on-site concrete
- Precast concrete
- Ceramic glass
- Fire glass (Tempered glass in the form of cubes, crystals, diamonds, beads and other small bits or chunks)
- Natural volcanic lava rocks (Basalt)
Designing a Fire Pit
When designing a fire pit you should choose fire-resistant and non-combustible materials as previously stated. There are many other building codes that you have to comply with, depending on your local municipality, city or town.
Fire Pit Permit
A permit is often required for building a permanent firepit, setting up the appliance and open-air burning. Appliance certification is becoming compulsory in many states in the USA, and will likely be required before a permit is issued. Outdoor fire pits, chimneas and fire bowls are grouped under open-air burning, so you will need a permit under this purpose, as well as appliance certification if applicable.
Permits are often not required for outdoor chimney fireplaces with a front screen, portable/moveable fire pits and gas fired barbeques, pits and tables, including those fuelled by propane and charcoal.
Permanent dug-out fire pits can be built above or below ground level. Both in-ground and above-ground fire pits should take into account drainage, ventilation and containment of flames.
During a period of non-use, fire pits are susceptible to flooding during the rainy reason. The substructure of your fire-pit must be designed and built in a way that allows quick drainage of surface water. For this reason, the base will be built from granular material such as gravel, sand, pebble stones, fire glass and lava rocks in large, medium and small size. Granular material has excellent drainage qualities and permeability, creating a porous surface that allows water to drain away quickly. Note that this layer functions as both a drainage and protective fireproof layer.
Fire Pit with Slab or Paved Base
If the fire-pit base is made of a concrete slab or pavers as an architectural specification or requirement, a steel drain and grate with a circular or square face should be installed at the bottom of the pit. Steel drain pipes should be used instead of PVC or plastic. The paved floor will still need to be covered with a layer of granular material such as gravel and stones (150mm minimum height). This layer not only protects the steel drain, but it also protects the slab as well as allowing free drainage to the outlet.
Fire Pit on Subgrade
This fire pit has no pavers or slabs at the bottom. The fireproof/drainage layer is laid directly on top of compacted subgrade soil. In this case, the pit is excavated to a suitable depth, removing soil and then levelling the bottom. The ground is compacted and a layer of granular material (e.g. gravel) at least 4 inches thick (100mm thick) is poured in the bottom, spread and levelled. Both in-ground and above-ground fire pits can have their fireproof/drainage layer installed directly on top of subgrade soil.
Fire Pit with Wall Openings
This is a suitable option when you are building an outdoor above-ground fire pit on a paved surface, for example on top of concrete slab, block pavers and tiles. This method also works on a lawn and bare ground. The fire-pit walls are arranged in a way that contains flames, and at the same time allowing surface water to pass through the gaps between walls. Fireproof granular material will be laid on top of the paved surface or compacted stripped earth. If you want to build the fire-pit on your lawn without stripping away the grass, you should lay block pavers at the bottom and fill the gaps with sand. You can then put some stone filling on top of the pavers. Ideally, in order to ensure drainage of water away from the pit, the centre of the pit must be at a slightly higher elevation than the perimeter of the pit.
Ventilation is required in both wood and gas operated fire pits. In wood fired pits, ventilation aids in the combustion reaction, supplying the oxygen (air) required for the reaction to occur. Combustion is a scientific name for “burning”. In order for combustion to take place, you need three elements – heat, oxygen and fuel, as shown in the equation below:
Heat + Fuel + Oxygen > Carbon Dioxide + Water Vapour
When combustion takes place, carbon dioxide (CO2) and water vapour (H20) are produced as well as energy in the form of heat and light.
Rectangular steel fire-pit vents with a typical width of 7.9 inches to 9.75 inches, and height of 2.9 inches to 3.9 inches are installed at the bottom of walls, just above the top granular layer. This converts to a width of 200mm to 248mm, and height of 74mm to 100mm. Vents should not be obstructed or buried by granular filling, otherwise air flow will be obstructed.
Three vents are enough on a typical above-ground fire pit. Install them at equal spacing around the perimeter of the pit. So if the external diameter of your pit is 1000mm, they should be installed at 1000/3 = 333mm intervals around the pit.
In a gas operated fire-pit, vents are needed to provide an outlet for build-up of pressure. Gas released from the appliance fitting is trapped inside the pit, and if there is no outlet or vent for this gas, the enclosure may explode.
Containment of Flames, Fire-Pit Dimensions and Location
The fire pit or enclosure must be deep enough to contain flames and prevent them from escaping and spreading across the ground. In this case, we need to differentiate between an in-ground pit and above-ground fire enclosure.
Depending on your city or county building codes, in-ground fire pits shall not exceed a specified minimum and maximum depth. The City of Yellowknife recommends a maximum depth of 500mm. The City of Green Bay, Wisconsin recommends a minimum depth of 150mm and maximum of 300mm. The City of Winnipeg recommends a maximum depth of 600mm, with a projection of 150mm above the ground.
The town of Vermilion recommends a minimum depth of 400mm and the fire-pit must project 150mm above the ground. Walls must be at least 100mm thick. A fire-pit should have an opening of at least 600mm in diameter and a maximum diameter of 1200mm.
A 150mm thick ground apron or paving (surround) should be laid around the fire-pit, and this pad should be at least 1000mm wide. Concrete slabs, brick pavers, gravel and stone slates are some of the recommended non-combustible materials that can be used to build a fire-pit surround.
Ideally, the internal side of fire-pit walls should be protected with a fireproof liner/cladding such as veneer fire bricks or steel sheet. There are purpose-made steel ring liners (hollow cylinders) which can be inserted inside the fire-pit. Steel ring liners should be at least 5mm thick.
Most local building codes require the fire-pit to be at least 10 feet (3000mm) from any building, structure, property line, boundary wall/fence or combustible material. However, some local building codes such as Green Bay City in Wisconsin and state of New Hampshire recommend a distance of 25 feet (7600mm) from physical structures, installations and property line.
Above-ground fire pits should not exceed a maximum height of 600mm (2 feet) measured from ground level. Some cities require a maximum height of 500mm.
To control the risk of fire spreading into the surroundings, the height of flames is also regulated. Fire-pit flames should not exceed 3 feet (914mm) high.
Fire Pit Floor:
The fire-pit bottom or bedding should be at least 100mm thick and non-combustible. However, this depends on the municipality building codes in each town or city. Some towns recommend a minimum bedding of 250mm if the fire-pit is not built on natural rock.
If the fire-pit is built on a concrete slab or stone paving, you still need to add a non-combustible bedding on top of the slab. This bedding is usually made of gravel, sand or lava rock filling. Fire glass is not suitable for a wood-burning pit. It can only be used for gas and propane fire-pits.
A stone paving or concrete slab supporting a fire-pit must be laid on a hardcore base of compacted gravel at least 200mm thick, on which 50mm sand blinding (levelling course) has been spread.
Also, as discussed previously, the fireproof bedding must be free draining and a grated steel drain connected to an underground pipe must be installed on the bottom of the fire-pit if the fire-pit is built on a concrete slab or stone paving.
What Not To Use in the Bottom of Fire Pit
Combustible materials that catch flames, burn and give off vapours should not be used, as well as those that melt and change physical state. These include natural wood, engineered wood, plastic polymers, PVC, HDPE, rubber, gypsum, blueboard, plasterboard and many others.
Most organic materials including those containing organic compounds are flammable and should be avoided, whereas most inorganic materials are non-combustible and have higher melting points. Examples of organic materials include plant matter, compost, humus and topsoil rich in organic matter. Most of these materials contain cellulose, carbohydrates, lignin, proteins, tannin and lipids.
Gas Fire Pit
The bottom of a wood-fired pit requires special attention to the choice of bedding material. A gas fire-pit works differently from a wood-fired pit, and therefore it is built and structured differently as well. Gas fire pits come in different shapes and designs. Some look like tables, and thus named fire tables. But what you have to know is that gas fire pits have two levels – the roof and the floor. In between the roof and the floor, allowance is made for empty space, a minimum height clearance ranging from 12 inches (305mm) to more than 18 inches (457mm). This space should not be filled. It can only accommodate the gas piping which connects the burner tray to the control panel fixed on the wall.
The floor is made of non-combustible pavers (such as basalt) which must be at least 25mm thick. This layer is only needed when the existing floor is flammable (for example, on a wooden deck or grass). If the existing floor is fire-resistant and non-combustible (e.g. concrete slab, stone or brick paving), the basalt layer is not needed.
Burner Ring and Steel Tray:
The burner ring and steel tray form the top part of the gas fire pit. The tray can be filled with Lava rocks or fire glass bits for aesthetic purposes as well as to mimic a firewood flame. The burner ring is made of steel tubes which are perforated with holes (gas and flame outlets). Gas fire burner rings come in different diameters, ranging from 6 inches (152mm) to 36 inches (914mm). Well known sizes are 6,12,18,24,26,30 and 36 inches.