Soil Bearing Calculator

Not for typical residential decks on average soil. IRC R401.4.1 lets you use the 2,000 psf presumption without a test. You DO need a soil test when: (1) you visibly see soft clay, fill, or organic material at footing depth, (2) you're putting a hot tub or outdoor kitchen on the deck, (3) your local building department requires it for permit, (4) the calculator flags 'unsuitable soil' or 'soil test required', or (5) site slope exceeds 15-20%. A residential soil test runs $400-800 from a geotechnical engineer.

Dig a 4-foot test hole at the proposed footing location and look at the soil. Hard rock that you can't dig with a shovel = bedrock or sedimentary rock (highest bearing). Tan/yellow with gravel and stones = sandy gravel. Tan/brown mixed with stones = sand-or-gravelly-clay (the IRC default 2,000 psf). Gray that sticks to itself = clay (1,500 psf — lower bearing). Dark soft soil with roots = organic (unsuitable — must remove). Use the visual clue dropdown in this calculator for help.

Uncontrolled fill is ANY soil added to the site without documented compaction. This includes: dirt fill placed to level a slope, debris fill behind retaining walls, old septic pits, leach field areas filled in, areas where a previous structure was demolished and filled. Uncontrolled fill can settle 6-12 inches over a decade as it consolidates — which means your footings settle the same amount, causing differential settlement and deck damage. The fix: either excavate down to original undisturbed soil, or hire a geotechnical engineer to design a footing system for the fill (deeper footings, pier-and-beam, helical piles).

Significantly — especially for clay-rich soils. Clay loses 15-40% of its dry bearing capacity when fully saturated. Wet conditions (visible pooling after rain, soft underfoot for days after rain) drop typical 2,000 psf clay to 1,400-1,700 psf. Water-saturated conditions (water table at footing depth) can drop to 1,200 psf or lower. The fix is drainage improvement (French drain around the deck, regrading to direct water away) + larger footing diameter to compensate.

Expansive clay (common in Texas, Oklahoma, parts of California) deserves its own warning. The IRC R401.4.1 presumptive 1,500 psf assumes 'normal' clay behavior — expansive clay can lift footings 2-4 inches in wet seasons and drop them in dry seasons. Standard sonotube footings on expansive clay often fail in 5-10 years from this cycle. Fixes: deep footings below the active zone (depth varies by region — 5-7 ft in DFW), helical piles, or pier-and-beam with under-deck void space. Always get a soil test in expansive clay regions.

Directly proportional. A 12-inch diameter footing has 0.785 sqft bearing area. At 2,000 psf clay = 1,570 lb allowable. At 1,500 psf clay = 1,180 lb (25% less). At 3,000 psf sandy gravel = 2,355 lb (50% more). The higher your soil bearing, the smaller footing you can use for the same load. Use the Footing Diameter Calculator with the PSF value from this calc to size sonotube diameter.

Yes — much more permissive. Crystalline bedrock (granite, schist) has presumptive 12,000 psf — meaning a tiny 6-inch sonotube footing can hold 2,400 lb (most of a residential deck post load). Sedimentary rock (limestone, sandstone) gets 4,000 psf. Some jurisdictions allow direct pier-on-rock construction without a poured footing for small decks. Always confirm rock identification on-site — what looks like rock can sometimes be hard clay or cobbles.

Organic soil (topsoil + peat + decomposed plant matter) compresses 5-15% as the organic content decomposes over decades. A 6-inch layer of organic soil can lose 1-1.5 inches of thickness as it decomposes — meaning footings on top of organic soil settle that amount over the deck's life. Even small amounts of organic content reduce bearing capacity. Always excavate down to mineral soil before placing footings — the typical topsoil layer is 6-18 inches deep in residential yards.

Yes — that's the entire point of soil testing. A geotechnical engineer can document soil bearing values up to 5,000+ psf even in areas where the IRC presumptive is 2,000 psf. The test runs penetration testing (SPT) + Atterberg limits + grain size analysis + occasionally plate-load testing. Costs $400-800 for a residential test. Worth it for decks with hot tubs or outdoor kitchens because higher PSF allows smaller footings, which saves more on concrete + labor than the test costs.