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Fire Pits

What to Put in the Bottom of a Fire Pit: Sand, Gravel or Nothing

A metal bowl usually needs nothing. A built-in pit wants a drainage layer of gravel or lava rock — and never ordinary river stone, which can explode.

By Zakaria 2 min read Save to Pinterest — opens Pinterest in a new tab
Logs burning inside a round steel fire bowl against a dark background
Photo: Tommy_Rau, Pixabay Content License, via Pixabay
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Usually nothing, if it is a metal bowl. A built-in or dug pit wants 2–5 inches of lava rock, pea gravel or coarse sand for drainage. And never smooth river stone — it traps water and can burst.

Which pit are you filling?

This table answers the question on its own
Pit typeWhat goes inWhy
Portable steel or cast-iron bowlNothingDesigned to burn on the steel and drain
Thin-walled cheap bowlThin sand or gravelShields the floor from burn-through
Built-in masonry pitLava rock or gravel, 2–5 inDrainage
Dug pit in the groundGravel, 3–5 inDrainage into the subsoil
Gas or propane pitOnly the maker’s mediaFollow the manual exactly

Stones that can explode

River rock, flint, sandstone, limestone and slate absorb water into small pores. Heat turns that water to steam faster than it can escape and the stone cracks or bursts, sometimes throwing fragments several feet. Use lava rock, dry gravel or firebrick and the hazard is gone.

Sand or gravel?

  • Coarse builder’s sand — traditional, insulates well, spreads load. Holds water and does not drain, so in a steel bowl it can keep the metal permanently damp.
  • Pea gravel — drains, cheap, perfectly serviceable.
  • Lava rock — drains, already been through extreme heat, porous by nature so it cannot build internal steam pressure. The best of the three.

Ash is not all bad

A thin layer of ash insulates the base and helps a new fire establish. A deep accumulation holds moisture against the metal and is the usual reason a bowl rusts through from the inside. Rake it out every few fires, and always before winter.

Not the same question as what goes underneath

What goes in the bowl is about drainage and protecting the pit floor. What goes under the pit is about protecting a lawn, deck or patio from heat — a separate decision with a separate answer.

Step by step

  1. Decide whether you need a layer at all

    A portable steel or cast-iron bowl with drainage holes generally needs nothing in it. Add a base layer only in a built-in masonry pit, a dug pit, or a thin-walled bowl you want to protect from burning through.

  2. Choose the material

    Use lava rock, pea gravel or coarse builder's sand. Lava rock is the best of the three because it is already volcanic and porous, so it cannot trap water the way a dense stone can.

  3. Check the stone is safe

    Never use river rock, flint, sandstone, limestone or any smooth water-worn stone. These hold moisture in pores and can crack violently or explode when heated.

  4. Lay the layer

    Spread two to five inches across the base, levelled. Deeper than that holds heat against the pit floor without improving drainage.

  5. Keep the drainage holes clear

    If the pit has holes in the base, make sure the layer does not block them. Water sitting under a fire pit rusts the steel out and freezes in winter.

  6. Clear the ash periodically

    Rake out built-up ash every few fires. Ash holds moisture against the metal and is the main cause of a bowl rusting through from the inside.

Common questions

What should I put in the bottom of a fire pit?

In most portable metal bowls, nothing at all — they are designed to burn directly on the steel and to drain through holes in the base. In a built-in masonry pit or a dug pit, a two to five inch layer of lava rock, pea gravel or coarse sand gives you drainage and protects the base. Whatever you choose, keep any drainage holes clear and avoid smooth water-worn stone entirely.

Can you put sand in the bottom of a fire pit?

Yes, coarse builder's sand works and is the traditional choice. It insulates the base from heat and spreads the load. The drawback is that sand holds water and does not drain, so in a steel bowl it can keep the metal permanently damp and accelerate rust from the inside. Gravel or lava rock drains better and is the safer choice in a metal pit.

Is lava rock better than gravel?

Slightly, and for one reason: lava rock has already been through extreme heat and is porous by nature, so it does not trap water in sealed pores and cannot build internal steam pressure. Pea gravel is perfectly serviceable and usually cheaper. Both are far better than any smooth river stone.

Can rocks explode in a fire pit?

Yes, and this is the one genuine hazard in this whole question. Dense water-worn stones — river rock, flint, sandstone, limestone, slate — absorb water into small pores. Heat turns that water to steam faster than it can escape and the stone cracks or bursts, sometimes throwing fragments several feet. Use lava rock, dry gravel or firebrick and the risk disappears.

Do you need gravel under a fire pit or in it?

Both are discussed and they are different jobs. Gravel under a pit is about the surface the pit stands on — protecting a lawn or patio from heat and giving a non-combustible base. Gravel in a pit is about drainage and protecting the pit floor. This article covers what goes inside the bowl; the surface beneath it is a separate decision.

Should I leave the ash in the bottom?

A thin layer of ash is actually useful — it insulates the base and helps a new fire establish. A deep accumulation is not, because ash is hygroscopic and holds moisture against the metal, which is the most common way a steel bowl rusts through from the inside. Rake it out every few fires and always before the pit is left over winter.

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