What to Put in the Bottom of a Fire Pit on Concrete

When installing a fire pits for outdoors on concrete, it's important to protect the surface from heat damage by choosing the right materials for the fire pit bottom. This in-depth guide will provide tips on the best materials to use, proper installation techniques, maintenance, and safety when building a fire pit on concrete.

Key Takeaways

• Protect concrete from fire pit heat using non-combustible materials like sand, gravel, fire glass, lava rock, fire bricks, or concrete pavers in the bottom.

• Allow for proper drainage and ventilation under the fire pit with gravel, holes, and vents.

• Materials must be tamped down level and compact with at least 2-6 inches of depth depending on the type.

• Perform routine maintenance by cleaning out and replacing deteriorated bottom materials.

Why Put a Protective Bottom in a Fire Pit on Concrete?

Concrete is susceptible to cracking and deterioration when exposed to the intense heat of a fire pit burner. The flames can reach temperatures of 800°F or more, which can cause the concrete to expand and contract rapidly. This results in cracks, pits, and spalling over time.

Placing protective bottom materials in a fire pit creates a buffer between the concrete and the high heat. This allows the concrete to last longer and prevents hazardous damage.

A layer of non-combustible material also:

• Insulates the concrete from thermal shock
• Absorbs and distributes heat evenly
• Allows for proper airflow and ventilation
• Creates a level and stable base for the fire pit

Best Materials to Use as a Fire Pit Base on Concrete

When choosing materials for the bottom of a fire pit on concrete, non-combustible materials are a must. Combustible materials like wood, fabric, or rubber could catch fire.

Some of the best options include:

Sand

Pure sand is an affordable and readily available choice for protecting concrete fire pit surfaces. Build up a layer at least 4 inches deep in the bottom of the pit.

Coarse builder's sand is best to allow air circulation. Fine beach sand can become too compact. River rock sand provides excellent drainage.

Regularly remove any accumulated ashes to keep the sand fresh and replace it once a year.

Benefits:

• Inexpensive and accessible
• Natural insulator
• Easy to shape and install
• Withstands high heat

Considerations:

• Can mix with ashes
• Potential moisture retention
• Needs replacement over time

Gravel

For improved airflow and drainage, pea gravel or crushed stones like lava rocks make ideal fire pit bases. Use smooth pebbles around 1/4 to 1/2 inch in diameter.

Install at least a 6-inch layer and tamp down to compact. Refractory cement can help hold rounded rocks in place.

Benefits:

• Allows ventilation
• Long-lasting
• Fun colors and textures
• Holds shape well

Considerations:

• Can shift out of place
• Requires a deeper pit
• Needs containment border
• Potential sparks

Fire Glass

For a modern, sleek look, consider using tempered fire glass. This material is designed specifically to withstand the high temperatures occurring in gas fire pits.

Fire glass is available in a rainbow of colors and different texture variations. Use a 2-inch layer for fire features.

Benefits:

• Withstands heat over 1,100°F
• Fun, sparkling appearance
• Low maintenance
• Clean burning

Considerations:

• More expensive
• Can crack over time
• Needs a gas fire system
• Requires more frequent cleaning

Lava Rocks

If you love the look of natural stone, lava rocks are a durable and decorative option for lining the bottom of your fire pit.

Use rocks around 2-4 inches in diameter and build up a layer at least 3 inches thick. Soak and rinse the lava rocks first to remove any loose debris or dust.

Benefits:

• Long lifespan
• Retains and distributes heat
• Natural, rustic appearance
• Low maintenance

Considerations:

• Can pop or explode when heated
• Potentially dusty
• Needs containment
• Heavy to install

Fire Bricks

Lining the bottom of your fire pit with fire bricks is a traditional method that withstands heat and contains the fire safely. Lay bricks at least 4 inches thick and use refractory mortar designed for heat resistance.

Benefits:

• Insulates and protects concrete
• Designed for high-heat applications
• Contain sparks and embers
• Reusable for years

Considerations:

• Higher cost
• Time-consuming to install
• Mortar also required
• Custom sizing may be needed

Concrete Pavers

For a decorative look, concrete pavers can provide a non-combustible base layer for your fire pit. Use solid concrete – not aggregates – rated for exterior use.

Arrange pavers in a design with at least 1/8-inch spacing between them for expansion. Level and tamp into a compact base.

Benefits:

• Available in shapes and textures
• Good durability
• Matches existing concrete
• Provides stability

Considerations:

• Potential for cracks
• Spacers needed between pavers
• Heavy to install
• Can be more expensive

Best Practices for Any Fire Pit Bottom

Proper installation is just as important as choosing the right material when it comes to maximizing fire pit safety and longevity. Keep these essential guidelines in mind:

Allow for Drainage and Ventilation

Stagnant moisture and lack of airflow under a fire pit accelerate the deterioration of the materials and structure.

It’s critical to allow for drainage and cross-ventilation of gasses and heat.

Tips for proper drainage and ventilation:

• Add a layer of gravel under the bottom materials
• Drill drainage holes in the bottom of the fire bowl
• Install vents at opposite sides of the pit
• Use a containment liner with an open bottom

Use Only Non-Combustible Materials

Any material going inside a fire pit must be non-combustible to prevent burning. Common combustible materials include:

• Wood
• Plastics or rubber
• Polyester or fabric
• Paper or cardboard

These materials may seem safe when cool but can ignite when exposed to direct fire heat. Choosing fire-rated materials is a must.

Level and Compact Materials

For maximum stability and efficiency, materials like sand, gravel, or pavers must be installed level and tamped down.

A small layer of sand or mortar can help level out uneven surfaces:

• Use a tamper, hand compactor, or feet to compact
• Check with a level and add or remove material
• Don’t leave loose pockets of air
• Make sure the surface is flat and flush with the pit walls

Determine Correct Depth

The needed depth of bottom materials depends on:

• Size of fire pit
• Type of material used
• Weight of fire bowl or burner
• Heat output

For example, propane pits need less depth than wood-burning ones.

Follow manufacturer guidelines, or allow:

• 2 inches for fire glass up to 6 inches for gravel
• Minimum 4 inches of sand
• At least 4-inch thick fire bricks

Use a Liner for Containment

While optional, a liner contains the bottom material and protects the concrete from heat exposure if cracks form.

Liners are available in:

• Stainless steel
• Hot rolled steel
• Fireproof masonry
• Refractory mortar

For custom sizes, line the inside with fire bricks instead.

Final Words

Fire safety begins from the ground up, so take the time to choose durable, fire-rated materials that will withstand the heat of your burning fire. By using heat-resistant materials like lava rocks or fire glass at the bottom, you can keep your fire pit burning brightly while protecting the concrete underneath. If you're looking for help selecting the perfect fire pit for your needs, be sure to check out our comprehensive buyer's guide for fire pits.

FAQs

Is it necessary to use a liner for my fire pit?

Yes, using a metal fire pit liner or a heat shield for the patio can protect your concrete from the heat of the fire. Especially for fire pits built into the ground, it is an essential material to put at the bottom of your fire pit.

Can propane fire pits be used on concrete?

Yes, a gas propane fire pit is a great feature for an outdoor fire on a concrete patio. Remember to place your propane fire pit bowl on top of a heat shield or use a layer of sand in the bottom to help protect the concrete.

Can a portable fire pit be used on concrete?

Yes, a portable fire pit can be used on a concrete surface. However, you may want to consider raising your fire pit on a platform or using a heat shield for the patio to protect the surface from high temperatures.

Are there certain materials to avoid in the bottom of my fire pit?

It is advisable not to use flammable or heat-sensitive materials that cannot withstand high temperatures at the bottom of your fire pit. This includes materials like regular glass, plastic, and certain types of rocks that can explode when heated.

Where is the best place to position my fire pit?

Ideally, your fire pit should be in an open space and not under any overhanging structures for safety reasons. Your backyard fire pit is a great way to make your outdoor setting cozy and warm. However, if your only option is your concrete patio, ensure you use heat-resistant materials in your bottom layer.

How can I ensure my fire pit doesn't damage my concrete?

Use materials that will protect the concrete from high heat at the bottom of the fire pit. Sand at the bottom will help distribute the heat from the wood fire or propane fire evenly. Another solution can be putting your outdoor fire pit on a platform made from heat-resistant materials.