Snawfus Hearth — soil bearing pressure

The build: MM's premier thermal masonry heater — double-bell burn chamber, bread oven, serpentine flue with thunderbird flue channels, cast melon-head/moon-eye geopolymer brick, integral clean-out channels. Always installed on slab (not an elevated wood floor).

The problem: does the slab/soil beneath it actually carry the weight? Thermal/draft design (the bell chamber, flue serpentine) is specialist masonry-heater-designer territory, outside this guide's scope per the Method rule — this chapter stays on the standard structural question: bearing.

Concepts pulled in — new, closes Ch. 4's open bearing item with the real method: soil bearing pressure. A different question than anything so far — not "does it bend" or "does it slide," but "does the ground under a concentrated point load compress or settle unevenly."

Assumed: total heater weight ≈ 5,000 lb (typical mid-size double-bell masonry heater; geopolymer may run differently than the clay-firebrick figures this is usually based on — confirm once fully designed), footprint ≈ 3×4 ft (12 sq ft).

Calculation

Bearing pressure = weight ÷ footprint = 5,000 ÷ 12 ≈ 417 psf. Compared to a typical default allowable soil bearing (1,500 psf minimum assumption for untested soil): 417 psf is ~28% of allowable — comfortably clears.

Answer (the real governing factor, not the bearing number)

Raw soil bearing passes easily. The actual risk with a heavy masonry structure like this isn't the soil — it's a standard 4" slab with light wire mesh flexing slightly under one heavy concentrated footprint. Because the heater itself is rigid, mortared masonry, even minor uneven settlement cracks joints in a way framed construction shrugs off. Standard practice: a locally thickened, reinforced pad (6–8"+, real rebar, not mesh) under the heater's exact footprint, independent of what the bearing-pressure math says.

Refinement — wider, reinforced pad (Zach's call)

Instead of relying on a standard residential slab, give the heater its own wider reinforced pad. It helps in two separate ways.

• Wider footprint lowers soil pressure. Example: 3×4 ft heater on a 5×6 ft pad (1 ft past the heater on every side), 6" thick. Pad weight = 30 sq ft × 0.5 ft × 150 pcf ≈ 2,250 lb. Total 5,000 + 2,250 = 7,250 lb ÷ 30 sq ft ≈ 242 psf, down from 417 psf.
• Thickness + rebar is what prevents cracking. Soil pressure was never the weak link; a wide but thin slab still dishes under the heater. A stiff, reinforced pad settles as one piece, which is what mortared masonry needs.
• Rebar vs. mesh: mesh is shrinkage crack control and tends to end up on the bottom of the pour unless lifted. A rebar grid does the structural work — commonly #4 bars about 12" apart each way, held on chairs roughly 3" above the soil rather than lying on it. Confirm size and spacing against local requirements or an engineer before pouring; this is the concept, not a stamped spec.
• Practice note: the pad is commonly separated from the surrounding house slab by an isolation joint so it can move independently without cracking the floor around it.

Open items for a later pass

Actual heater weight once the double-bell/serpentine design is finalized (geopolymer density vs. traditional firebrick), and the thermal/flue-draft design itself — outside this guide, belongs with a masonry heater specialist.