How to Build a Concrete Fire Pit

Building a Concrete Fire Pit: Essential Guide

A concrete fire pit is a permanent, durable addition to your backyard that provides warmth, ambiance, and a gathering place for years to come. This guide covers the essential steps: forming with Sonotubes, proper reinforcement, using 4000 PSI concrete, and the critical drainage holes that prevent failure.

Why 4000 PSI Concrete is Essential

Fire pits experience extreme temperature cycles—hot fires followed by cooling. This thermal stress can crack weaker concrete. 4000 PSI concrete is denser and more resistant to thermal cycling than standard 3000 PSI mix.

Don't use 3000 PSI: It may work initially, but repeated heat cycles will cause cracking. The small cost difference (approximately $20-$40+ per yard) is worth it for durability.

Materials You'll Need

For a typical 36-inch diameter fire pit:

Sonotubes: 2 tubes (one for inner wall, one for outer wall)
Concrete: approximately 0.5-0.75 cubic yards of 4000 PSI mix
Wire lath or rebar: for reinforcement
Gravel: approximately 0.5 cubic yards for base
Tools: circular saw, level, mixing container, trowel

Example cost (for reference only): Materials typically range from approximately $200-$500+ depending on size and material choices. Actual costs vary.

Step 1: Forming with Sonotubes

Sonotubes (cardboard concrete forms) create perfect circles for fire pit walls.

Outer Form

Use a Sonotube sized for your desired outer diameter (typically 36-48 inches). Cut it to your desired height (typically 16-18 inches) with a circular saw.

Inner Form

Use a smaller Sonotube for the inner wall. The space between inner and outer forms should be 3-6 inches (this is your wall thickness). For a 36-inch outer diameter, use a 24-30 inch inner form.

Positioning

Center both forms on your prepared base. Use stakes or rebar to hold them in place. Ensure they're level and plumb (vertical). The space between them should be even all around.

Step 2: Reinforcement with Wire Lath

Reinforcement is critical for fire pits. They experience extreme heat cycles that cause expansion and contraction.

Wire Lath Option

Wire lath (expanded metal mesh) is easier to work with than rebar for curved walls. Wrap it around the space between forms, creating a continuous reinforcement cage.

Rebar Option

For maximum strength, use #3 or #4 rebar in a grid pattern. Place vertical pieces every 6-8 inches around the circumference, then tie horizontal pieces to create a grid.

Placement

Keep reinforcement 2 inches away from both inner and outer forms. This ensures concrete completely surrounds the steel, providing proper protection and bond.

Step 3: Pouring 4000 PSI Concrete

Order 4000 PSI concrete from your ready-mix supplier. Don't use bagged mix—it's typically only 3000 PSI and not suitable for fire pits.

Mixing

If you must mix yourself (not recommended for fire pits), use high-strength mix and follow directions exactly. But ready-mix is better—consistent quality and proper PSI rating.

Pouring

Pour concrete into the space between forms in layers. Fill about 1/3, then consolidate (remove air bubbles) with a stick or rebar. Continue filling in layers, consolidating each.

Finishing

Level the top with a trowel. You can create a rounded edge with an edging tool, or leave it flat. Finish while the concrete is still workable (first 30-60 minutes).

Step 4: Drainage Holes (Critical)

This is the step most people forget, and it causes failure. Fire pits MUST have drainage.

Why Drainage is Essential

Water will collect in the fire pit from rain, snow, or cleaning. Without drainage, this water:

Freezes in winter, expanding and cracking the concrete
Creates a breeding ground for mosquitoes
Makes the fire pit unusable until it dries
Can cause the base to become saturated and unstable

How to Create Drainage

Option 1: Holes in the wall

Before the concrete fully sets (but after it's firm enough to hold shape), drill or create 4-6 drainage holes in the bottom of the inner wall. These should be 1-2 inches in diameter, positioned near the base.

Option 2: Open bottom

Don't form the bottom of the inner wall. Leave it open so water drains naturally into the gravel base. This is simpler and often more effective.

Option 3: Drain pipe

Install a drain pipe through the wall before pouring. This provides the most reliable drainage but requires planning ahead.

Step 5: Curing

Cover the fire pit with plastic sheeting to prevent moisture loss. Keep it covered for at least 3-7 days, misting with water daily if it's hot and dry.

Don't remove forms too early: Wait at least 48-72 hours. The concrete needs time to gain strength.

Step 6: First Fire

After 7 days, you can start with small fires. Build gradually:

Week 1-2: Small fires only
Week 3-4: Medium fires
After 28 days: Full fires

This allows any remaining moisture to escape slowly, preventing steam explosions.

Common Mistakes

1. Using 3000 PSI concrete: Will crack under thermal stress. Use 4000 PSI minimum.

2. No reinforcement: Fire pits need reinforcement to handle heat cycles.

3. Forgetting drainage: Water will collect and cause problems. Always include drainage.

4. Too thin walls: Walls less than 3 inches thick are prone to cracking. 4-6 inches is ideal.

5. Building fires too soon: Wait at least 7 days. Large fires before full cure can cause cracking.

The Bottom Line

Building a concrete fire pit requires: proper forms (Sonotubes), adequate reinforcement (wire lath or rebar), 4000 PSI concrete, and drainage holes. Skip any of these, and the fire pit will fail.

The drainage holes are especially critical—they're easy to forget but essential for preventing water damage and freeze-thaw failure. Plan for drainage from the start, and your fire pit will last for decades.

Follow these steps, use quality materials, and be patient with curing. A properly built concrete fire pit is a permanent addition that will provide years of enjoyment.

Need help building your fire pit? Contact Local Concrete Contractor. We specialize in concrete fire pits and can help with design, installation, or guidance on your DIY project.