Joist Notching Calculator — Where Holes & Notches Are Allowed

Match the notch to its zone, then apply the D-limit. At the very end of the joist — over a support — the notch may reach one quarter of the depth (D/4): a 2×10 at 9.25″ allows 2.31″. In the outer thirds of the span the allowance tightens to D/6 (1.54″ on the same 2×10). The middle third bans notches outright. Depth is only half the check: the length of a notch may not exceed one third of the depth, and the remaining depth sets the shear capacity that actually carries the floor into the support.

Two limits, whichever is smaller. The diameter cap is one third of depth: 3.08″ for a 2×10, 3.75″ for a 2×12. The clearance rule demands 2″ of wood between the hole edge and the top and bottom faces: for a 2×10 that caps the hole at 9.25 − 4 = 5.25″, so the D/3 rule governs. For a shallow 2×6 the clearance rule takes over: 5.5 − 4 = 1.5″ max, tighter than its 1.83″ D/3 allowance. Keep holes in the middle third of the span and at least 2″ apart from each other.

A hole removes material but leaves the grain continuous above and below — the section still works as a beam with a reduced web. A notch severs fibers across the full thickness at the worst place: at a bottom notch, it cuts exactly the tension zone (or at a support, the shear area), creating a stress concentration at its square corners. That is why codes allow a 3″ hole in a 2×10 but only a 1.54″ notch in its outer thirds. If a service run does not fit through holes, the answer is relocation or a header, never a deeper notch.

Follow the path the zones allow. Drain lines are the challenge — a 3″ PVC drain needs a 3.5″+ hole, which only fits a 2×12 at midspan and never fits a 2×10 within the rules. The standard solutions: run drains parallel to the joists in a furred or dropped ceiling, frame a dedicated chase with doubled joists and headers around the pipe, or re-route so penetrations land in the middle third. Holes go through the center of the depth, 2″ clear of top and bottom, and never within a notch zone.

No — I-joists follow the manufacturer's catalog, and the catalog is stricter and more specific than sawn lumber rules. Typical TJI-class products allow round holes up to 2″ in the web's neutral zone with minimum spacing from supports and from each other; larger openings need engineered reinforcement or a web stiffener package. Never cut into the flanges — they carry the bending the web only distributes. The location chart in the manufacturer's literature is the whole rulebook; when the chart says no, the answer is no.

End notches exist to seat joists on low walls or to align with a sill plate, and they get the most generous limit — D/4 — because they sit where the shear area still has the whole depth behind it a few inches in. Do the math on what remains: a 2.31″ notch in a 2×10 leaves 6.94″ of depth, about 75% of the shear area. Keep the notch length under D/3, avoid square inside corners (a small drilled relief hole or tapered cut stops split propagation), and never notch past D/4 no matter how the plate alignment begs.

Romex needs only a 3/4″ to 1″ hole — trivially inside every D/3 limit. The rules that matter are placement ones: center the hole vertically for the 2″ clearance, keep it in the middle third where bending stress is lowest, and use nailed-on steel nail plates anywhere the cable runs within 1¼″ of the joist face. Bundled holes for multiple runs should stay one diameter apart from each other so the remaining web between them can transfer shear.

Four repairs cover the common cases. Small overcuts: sister a full-depth board alongside, glued and nailed in two staggered rows, running at least 2 ft past the defect each way. Holes too close to an edge: fasten wood or steel side plates (½″ plywood gussets work) spanning the hole. Notches at supports: add a solid header or post beneath if access allows, or a metal bearing plate to redistribute the crush. And for anything past half the section removed, get the repair designed — a compromised joist under a finished ceiling is not a trial-and-error repair.