2021 IRC Chapter 5
A floor framing inspection mostly asks whether the lumber that went into the floor is what the plans and span tables assumed, and whether anyone has cut it up afterward in ways the code does not allow. Add how the joists bear and connect at openings and supports, and whether the floor cavity needs dividing against fire spread, and you have covered nearly everything a GCHS inspector checks at a floor framing visit, whether it is new infill construction or a rehab job where the crew opened up an existing floor. Every number below carries its IRC section so it can be checked against the edition your jurisdiction has adopted.
Joists, girders, and other structural floor lumber must carry a grade mark, or an accompanying certificate of inspection, from an accredited grading or inspection agency (IRC R502.1.1). That stamp ties a specific piece of wood to the allowable-span tables the design relied on: species, grade, size, spacing, and design load all feed into the allowed span, and an inspector mainly confirms the framing on site matches what the approved plans and table allow. One distinction matters: the code publishes separate span tables for sleeping rooms and other habitable spaces, since a bedroom is designed for 30 psf live load while other habitable rooms use 40 psf (Table R301.5): Table R502.3.1(1) covers sleeping areas at 30 psf and Table R502.3.1(2) covers other areas at 40 psf (R502.3.1, R502.3.2). Using the lighter table for a room that could later be furnished as a living area over-spans the joists, so when a room's use is uncertain, the heavier table is the safe read.
Joist and girder ends need a minimum bearing length wherever they land directly on a support: 1-1/2 inches on wood or metal, 3 inches on masonry or concrete (IRC R502.6), or an approved hanger installed with the exact fasteners the manufacturer specifies as the alternative. Masonry takes more bearing length because it is weaker in localized crushing than steel or wood. Where joists frame from opposite sides of a beam and lap over the support instead of running continuously, R502.6.1 requires a minimum 3-inch lap, face-nailed together with at least three 10d nails, or an equivalent wood or metal splice, giving the two joists a continuous tension tie across the support rather than added bearing area. Joists framing into the side of a wood girder must be carried by approved framing anchors or by ledger strips not less than nominal 2x2 (R502.6.2).
Joist ends need lateral restraint so a joist cannot rotate off its bearing under load (IRC R502.7). Full-depth solid blocking, a continuous rim (band) joist, a full-depth header, or attachment to an adjoining stud all satisfy that; a single toenail does not, since it resists almost no twisting. Deep, narrow joists also want to roll over sideways along their length, not just at the ends. Where solid-sawn joists exceed a nominal 2x12, R502.7.1 requires intermediate lateral support, solid blocking, diagonal bridging, or a continuous 1x3 strip across the bottom of the joists, at intervals not exceeding 8 feet. Neither blocking nor bridging adds span capacity; both exist to hold slender members upright and share concentrated loads between neighboring joists.
Cutting into a joist for plumbing, wiring, or ductwork is where floor framing inspections most often find a defect, and IRC R502.8 sets firm limits for sawn lumber. A notch in the top or bottom edge is limited to one-sixth of the joist depth, may not be longer than one-third of the depth, and is prohibited outright in the middle third of the span, where bending stress peaks. An end notch can go deeper, up to one-fourth the depth, since a simple span's end carries shear rather than peak bending stress. A bored hole cannot exceed one-third of the depth in diameter, and must stay at least 2 inches clear of the top and bottom edges and of any other hole or notch, since the material between close holes is too thin to transfer load around them.
| Cut in a sawn joist (D = actual joist depth) | Limit |
|---|---|
| Notch, top or bottom edge | Max D/6 deep and D/3 long; never in the middle third of the span |
| Notch at the end of the joist | Max D/4 |
| Bored hole diameter | Max D/3 |
| Hole edge distance | At least 2 in. from top and bottom edges, and 2 in. from any other hole or notch |
The D/6, D/4, and D/3 rules apply to sawn lumber only. I-joists, LVL, and floor trusses may be cut, notched, or drilled only where the manufacturer's recommendations allow it, or a registered design professional has designed for the cut (R502.8.2). An I-joist's flanges carry essentially all its bending capacity, so flange cuts are prohibited, and web holes are permitted only in the manufacturer's hole zones, tighter near a bearing point where shear peaks. Web stiffeners keep the thin web from buckling under a concentrated load. Squash blocks, cut slightly taller than the joist and set alongside it under a post or wall above, carry that point load in direct bearing to the support, bypassing the flange's weak crushing capacity. A field-cut truss chord notch is a red flag regardless of size: truss members may not be cut, notched, spliced, or otherwise altered without a registered design professional's approval (R502.11.3), and the truss design drawings, not a span table, govern bearing points and bracing.
In the field
A common rehab find: a plumber has notched the top edge of an existing 2x10 joist near mid-span for a drain line, and the cut looks shallow enough to seem harmless. Depth is only half the check; even a shallow notch is prohibited outright in the middle third of the span, where the joist is doing the most work resisting bending. The fix is almost never "leave it," it is a sistered joist, an engineered repair bracket, or rerouting the line through an adjacent bay's web, not a judgment call about how deep the cut looks.
Where a floor opening for a stairway or chimney chase needs a header joist to carry the ends of interrupted joists, R502.10 keys everything to the header span. At 4 feet or less, the header can be a single member the same size as the floor joists, and a single trimmer can carry a single header located within 3 feet of the trimmer bearing. Past a 4-foot header span, both the header and the trimmer joists must be doubled and sized to support the joists framing into the header, since the trimmers now pick up the tail joists' reactions on top of their own load.
Worked example. Bore a 3-inch hole through a 2x10 joist, actual depth 9-1/4 inches. Diameter limit D/3 = 9.25 ÷ 3 = 3.08 inches, so 3 inches passes. Centered on the depth, that hole leaves (9.25 − 3) ÷ 2 = 3.1 inches clear each side, past the 2-inch minimum. The same hole pushed low, leaving only 1-1/2 inches below it, fails edge distance even though the diameter is fine, exactly the hole an inspector rejects on site.
Wood structural panel subflooring is installed with its long dimension (the strength axis) perpendicular to the joists, since the span tables assume panels run continuously over two or more spans in that direction (Table R503.2.1.1(1)). Panels are attached to wood framing per Table R602.3(1) (R503.2.3). For 19/32 to 3/4 inch panels with 8d common nails, that schedule calls for fasteners 6 inches on center at panel edges and 12 inches at intermediate supports, tighter at the edges where a panel wants to lift. Unsupported panel edges must have tongue-and-groove joints or blocking, unless an underlayment or finish flooring alternative listed in the table footnotes is used; an unsupported butt edge deflects independently under foot traffic and shows up later as a ridge or a cracked tile line.
In combustible construction, where a floor-ceiling assembly has usable space both above and below its concealed space, draftstops must be installed so the concealed area does not exceed 1,000 square feet, dividing it into approximately equal areas (IRC R302.12). Where the assembly is enclosed by a floor membrane above and a ceiling membrane below, the code names two circumstances that call for draftstopping: a ceiling suspended under the floor framing, and floor framing of truss-type open-web or perforated members. Acceptable material is not less than 1/2-inch gypsum board or 3/8-inch wood structural panel, or other approved material, adequately supported, and it is installed parallel to the floor framing members unless the building official approves otherwise (R302.12.1). An ordinary solid-sawn joist bay with drywall fastened directly to the underside is already compartmented bay by bay and is not one of those two circumstances, while a dropped ceiling or open-web trusses open up one large continuous space, exactly the condition the rule targets.
1. What minimum bearing length does IRC R502.6 require for a floor joist end on a masonry foundation wall?
2. Joists lapped over a girder instead of running continuously must, per R502.6.1:
3. Which of these satisfies the lateral restraint requirement at a floor joist end under R502.7?
4. Solid-sawn joists deeper than a nominal 2x12 need intermediate blocking or bridging at intervals not exceeding:
5. A notch is cut into the top edge of a sawn floor joist at the exact center of its span. Per R502.8, this notch is:
6. Why is an end notch in a sawn joist allowed to be deeper (D/4) than an edge notch elsewhere on the span (D/6)?
7. The maximum diameter of a bored hole in a sawn floor joist, per R502.8, is:
8. A 2x10 joist has an actual depth of 9-1/4 inches. The largest bored hole diameter permitted is approximately:
9. Which practice is acceptable on a factory-built floor truss without an engineered repair?
10. An I-joist needs a hole drilled through its web for a supply line directly over an interior bearing wall. The correct guidance is:
11. A floor opening has a header joist spanning 5 feet. Per R502.10, the header and trimmer joists must be:
12. A header joist framing a floor opening spans 3 feet. Per R502.10, the header may be:
13. For a 3/4-inch wood structural panel subfloor on wood joists, Table R602.3(1) calls for fasteners spaced at:
14. Draftstopping is required in a floor-ceiling assembly, per R302.12, when: