Joist Load Bearing Calculator — Uniform Loads, Point Loads & Bearing Stress

Three checks decide it. Bending: the moment w·L²/8 for uniform loads (or P·L/4 for a midspan point load) may not exceed the allowable stress times the section modulus — for a 2×10 SPF No. 2 that is about 23,700 in-lb. Shear: 1.5× the support reaction divided by the cross-section must stay under the species' shear value. Bearing: the reaction pressed through the contact seat must stay under Fc⊥, roughly 425–625 psi depending on species. The weakest of the three is the joist's true capacity, which is exactly how this joist load calculator reports it.

Span capacity asks how far a joist can reach before it bends or sags too much; bearing capacity asks how hard the support can squeeze its end before the wood crushes. They fail differently: span overload cracks and sags across the middle, bearing overload squashes fibers slowly right at the support — a floor that dips near a wall or beam is usually a bearing problem, not a span problem. Codes treat them separately: minimum seat lengths exist precisely because bending math never guarantees them.

At midspan, a single 2×10 SPF No. 2 can carry roughly 560 lb of point load by bending alone (4 × 23,700 in-lb ÷ 168 in). But a real floor already carries its 40/10 psf uniform load, which consumes about 80% of the bending budget at a 14 ft span — leaving only around 100 lb of headroom for anything concentrated. That is why safes, tubs and tanks need to land over walls or beams, or need doubled joists and blocking to share the load with neighbors.

An RTU never loads one joist. Say a 800 lb unit sits on a curb: the curb spreads it to the joists it crosses, commonly three — about 267 lb per joist as a point load near midspan. Run that against a 40/10 psf floor and bending utilization jumps past 110% on a 14 ft 2×10. The fix is dedicated framing: doubled joists, a perpendicular header, or bearing walls under the unit line — which is why mechanical units on framed floors deserve a real load path, not just a curb on the sheathing.

10 psf is the standard allowance for subfloor, finishes and the ceiling below. Add for reality: ceramic tile pushes the assembly toward 15 psf, a double floor with soundproofing toward 20 psf, and a slate or concrete topping far beyond. The dead load rides in every check at once — bending, shear, deflection and bearing — so a 5 psf underestimate trims the real safety margin by roughly a tenth across the board.

Check three things before placing concentrated weight. Position: within a few inches of a bearing wall or over a beam is dramatically stronger than midspan — moving the same 500 lb safe two feet can halve the bending it adds. Spread: a tub on a full-contact base loads several joists at once; the same weight on four small feet loads two. Direction: if the load sits near midspan of a long-span joist, assume the 40 psf uniform budget is already spent and add framing rather than trusting the feel of the floor.

Start with the species and grade from the stamp, or assume the weakest common value if unstamped. Then discount for condition: split or badly checked ends cut shear capacity, KC-dry century-old lumber may actually test stiffer than modern tables, but rot at bearing points can erase them entirely. Probe the bearing seats with a screwdriver — soft, crumbling fiber means the Fc⊥ assumption is gone regardless of what the tables say. When in doubt, measure actual deflection under a known load and back-calculate an effective stiffness.

Because crushing, not breaking, is the quiet failure mode. A 14 ft joist at 100 psf pushes about 1,030 lb into each end; squeezed through a 1½-inch seat on SPF that is 456 psi against a 425 psi limit — overloaded, while the same reaction on a 3-inch seat sits at a comfortable 228 psi. Codes answer with minimums: 1½ inches on wood or metal, 3 inches on masonry. Wider seats cost nothing in lumber and turn a marginal bearing into a permanent one.