Skip to content
Grow In Backyard
Fire Pits

What to Put Under a Fire Pit: Match It to the Surface

Grass, a wooden deck, pavers and gravel each need a different answer. Radiant heat and ember fall are two separate problems, and only one is obvious.

By Zakaria 3 min read Save to Pinterest — opens Pinterest in a new tab
A steel fire bowl with logs burning inside it, raised clear of the surface beneath
Photo: Tommy_Rau, Pixabay Content License, via Pixabay
On this page

There are two separate problems here and most advice only solves one. Embers fall down; radiant heat comes off the base. A mat catches the first and does nothing about the second — which is why decks char under mats that look like they are doing their job.

Match it to the surface

This table answers the question on its own
SurfaceWhat to put under itWhy
GrassPavers, or steel plate on brick spacersStops the turf being cooked and the soil sterilised
Wooden deckRaised heat shield with an air gapA flat mat conducts heat into the boards
Composite deckingRaised shield and check the maker’s limitsMany composites soften well below char temperature
Pavers / concreteSand bed or fire-rated paversSpreads heat; guards against staining and spalling
GravelNothingAlready non-combustible and free-draining
Bare soilNothing, or a sand bedFine as it is
Artificial grassDo not site a pit hereMelts from radiant heat alone

A mat lying flat on decking is the classic mistake

It stops embers and conducts radiant heat straight into the boards underneath. The products that work lift the pit clear and let air move beneath it. Use the manufacturer’s stated clearance as a minimum, not a target.

Grass: the dead circle is heat, not fire

A wood-burning pit sitting close to the ground cooks the crowns and sterilises the top inch of soil. You get a dead circle that takes weeks to recover and sometimes needs reseeding.

  • Pavers, laid level, are the simple fix
  • A steel plate on brick spacers works and packs away
  • Legs that lift the bowl clear reduce the damage a lot on their own
  • Move the pit between burns if you would rather not build anything

Decking: air gap or nothing

This is the one worth being strict about. The risk is sustained radiant heat on timber over a whole evening, not a stray spark.

  1. A purpose-made raised heat shield, or spacers that lift the pit several inches
  2. A non-combustible layer — pavers or a steel sheet — on top of that, for ember fall
  3. Check the maker’s clearance for your specific pit and respect it
  4. Composite decking needs more care, not less — many grades soften well below the point at which wood chars

Pavers and concrete: staining first, cracking second

The likelier damage is ash and dripping fat staining the surface. The less likely but more serious one is thermal cracking or spalling, where uneven expansion and trapped moisture break the surface up.

  • A 2–3 inch sand bed spreads the heat and is cheap to replace
  • Fire-rated pavers under the pit do the same job permanently
  • Avoid damp or newly laid concrete — trapped water turns to steam

Sand inside the bowl, as well as under it

A couple of inches of dry sand in the base of a metal pit spreads heat and slows the burn-through that eventually destroys thin steel. Keep it dry; saturated sand steams when the fire lights.

What not to use, and why

  • Rubber mats — melt, and smell appalling doing it
  • River stones and other rounded water-worn rock — can hold moisture internally and crack or burst when heated
  • A flat metal sheet on timber — conducts heat rather than blocking it
  • Artificial grass, plastic decking tiles, treated timber — all fail from radiant heat alone, before anything touches them

Common questions

What should you put under a fire pit?

It depends entirely on what the pit is standing on. On grass, a layer of pavers or a sheet of steel on bricks stops the turf being scorched and killed. On a wooden deck, you need a purpose-made heat shield that creates an air gap beneath the pit, not just a mat laid flat. On pavers or concrete, a mat or a bed of sand protects against staining and thermal cracking. On gravel or bare soil, usually nothing at all. The common mistake is treating heat and embers as one problem when they need different solutions.

Can you put a fire pit directly on grass?

You can, but it will kill the grass beneath it and leave a dead circle that takes weeks to recover. Radiant heat from the base cooks the crowns, and for a wood-burning pit the heat is enough to sterilise the top inch of soil. If the pit has legs that lift the bowl well clear the damage is much less. For anything sitting close to the ground, lay pavers, a steel plate on brick spacers, or move the pit between burns.

Do fire pit mats actually work on a deck?

Only if they are designed to create an air gap. A thin mat lying flat on decking blocks ember fall but conducts radiant heat straight into the boards underneath, which is the thing that actually chars them. The products that work are raised heat shields, or a mat used together with legs or spacers that lift the pit several inches clear. Check the manufacturer's stated clearance for your specific pit and treat it as a minimum rather than a suggestion.

Will a fire pit crack concrete or pavers?

It can. Rapid, uneven heating makes concrete and stone expand at different rates internally, and trapped moisture turns to steam — both can cause spalling or cracking, and the risk is highest on damp or new concrete. A bed of sand, a layer of fire-rated pavers, or a raised pit with good airflow underneath all spread and reduce the heat reaching the surface. Staining from ash and dripping fat is the other, more likely, problem.

Is sand a good base for a fire pit?

Yes, and it is one of the cheapest options. A two to three inch bed of sand under and inside a pit absorbs and spreads heat, protects whatever is beneath, and is easy to rake out and replace. It is particularly useful inside a metal bowl, where it also slows the burn-through that eventually destroys thin steel. Keep it dry if you can — saturated sand turns to steam when the fire is lit.

What should you not put under a fire pit?

Anything that traps heat without an air gap, and anything that holds water. Rubber mats melt, river stones and other rounded water-worn rocks can trap moisture and crack or burst when heated, and a sheet of metal lying flat on decking simply conducts heat into the boards. Treated timber, plastic decking tiles and artificial grass are all non-starters — artificial turf in particular melts from radiant heat long before anything touches it.

Backyard Weekly

Know what to plant, week by week

Join the weekly email for seasonal jobs, fixes for what is going wrong, and gear worth buying.

Not ready to subscribe? The planting calendar is free and needs no email.

By subscribing you agree to our privacy policy. Grow In Backyard will never sell your address.

Keep reading