5.4 Common Traps in Floor Construction

Notching and Boring Solid-Sawn Joists (R502.8.1)

This is the single most-tested topic in the chapter, and the numbers are precise:

Note the common confusion the exam exploits: a general notch is limited to 1/6 of the depth, but a hole (and the maximum notch length) is limited to 1/3 of the depth. They are not the same fraction. Additionally, the tension side of a member 4 in or greater in nominal thickness may not be notched except at the very ends. The reasoning is structural: notches in the bottom (tension) fibers at mid-span concentrate stress where bending is greatest, which is exactly why the middle-third ban exists.

Worked Notch/Bore Example

Take a 2x10 joist with an actual depth of 9-1/4 in:

• Max general notch depth = 9-1/4 ÷ 6 ≈ 1.54 in (about 1-1/2 in).
• Max notch length = 9-1/4 ÷ 3 ≈ 3.08 in.
• Max end notch depth = 9-1/4 ÷ 4 ≈ 2.31 in.
• Max bored hole diameter = 9-1/4 ÷ 3 ≈ 3.08 in (about 3 in), kept at least 2 in clear of the top and bottom edges.

So a plumber boring a clean 2-1/2 in hole centered in the joist height is fine; a 4-in hole, or a hole crowding the bottom edge, is a violation. Inspection tip: a notch on the top of a joist near a support (where the joist is in compression) is far less critical than a notch on the bottom at mid-span. The middle-third rule and the tension-side rule together catch the dangerous cases. If you see a joist hacked at mid-span to route a duct, that is a cite — the fix is a header/box-out or an engineered repair, not enlarging the notch.

Engineered Lumber and Trusses — Do Not Cut (R502.8.2, R502.11)

The notch/bore allowances above apply only to solid-sawn lumber. For I-joists, wood trusses, structural composite lumber (LVL/LSL/PSL), glued-laminated members, and cross-laminated timber, the rule flips: cuts, notches, and bored holes are prohibited except where specifically permitted by the manufacturer's instructions or designed by a registered design professional (R502.8.2). I-joists do have factory knockouts and manufacturer hole charts — but the field crew must follow that chart exactly, and never cut the flanges.

Trusses (R502.11):

• R502.11.2 (Alterations): truss members and components shall not be cut, notched, drilled, spliced, or otherwise altered without a registered design professional's approval. A single cut web can drop the truss rating.
• R502.11.4 (Truss design drawings): must be on site and available to the inspector; permanent and temporary bracing must follow the design.

Draftstopping (R502.12): when there is usable space both above and below a floor/ceiling assembly, draftstopping divides concealed spaces so that no concealed area exceeds 1,000 sf, with the dividers roughly equal. Inspectors look for draftstopping where floor trusses or open-web joists create large interconnected cavities.

More Traps the Exam Loves

Beyond notch/bore numbers, several recurring distractors catch test-takers:

• Wrong fraction pairing. The single most common trap is swapping 1/6 (general notch depth), 1/4 (end notch depth), and 1/3 (notch length and hole diameter). Memorize them as a set and verify in R502.8.1.
• Hole too close to an edge. A hole sized correctly at 1/3 depth still fails if it is less than 2 in from the top/bottom edge — the exam likes a "correct diameter, wrong location" stem.
• Notch in the middle third. A shallow notch is still a violation if it lands in the middle third of the span. Location can fail an otherwise legal cut.
• Treating I-joists like 2x lumber. Applying the 1/3 rule to an I-joist or LVL is wrong — those defer to the manufacturer (R502.8.2).
• Cut truss web. Any field cut to a truss member without a registered design professional is an automatic violation (R502.11.2), regardless of how minor it looks.
• "Sistering" as a cure-all. Adding a sister joist may repair an over-notched solid-sawn member, but it does not legitimize a cut I-joist flange or a cut truss chord.

Inspector judgment

When a stem combines a deficiency with a proposed fix, ask whether the fix matches the member type. The right answer for a hacked I-joist is "stop work / manufacturer or engineer repair detail," never "it is within 1/3 so it is fine." Matching the remedy to the correct code path is what separates a passing answer from a plausible-sounding trap.

Finally, watch the units and member depth in calculation stems. Notch and hole limits are fractions of the actual depth, not the nominal name: a 2x10 is 9-1/4 in deep and a 2x12 is 11-1/4 in, so the math uses 9.25 and 11.25, not 10 and 12. An answer computed from the nominal size will be wrong by enough to land on a distractor — verify the actual dressed dimension before dividing.