Sawn joist and rafter notches and holes

Identify the member before cutting

A plumber, electrician, or HVAC installer may need a hole through a floor joist or rafter. The first step is identifying the member: solid-sawn dimensional lumber, I-joist, truss, structural composite lumber, or another engineered product. The code gives different rules for these materials. The prescriptive limits for a sawn-lumber joist do not authorize cutting a flange, chord, or web in an engineered member.

Under the 2021 IRC, solid-sawn joists, rafters, blocking, and beams are subject to limits on notch location, notch size, hole diameter, and distance from edges and other holes. The rules protect the member’s bending and shear capacity. Cutting removes material from the tension or compression zones, and clusters of penetrations can create a weak section even when each opening seems small.

Notches in solid-sawn members

For the cited IRC provision, a notch in a solid-sawn joist, rafter, or beam cannot exceed one-sixth of the member’s depth, cannot be longer than one-third of its depth, and cannot be located in the middle one-third of the span. An end notch has a separate limit: it cannot exceed one-fourth of the member’s depth. Members 4 inches or greater in nominal thickness have additional restriction on notching the tension side except at the ends.

These ratios are based on actual member depth and span location. A notch cut near midspan is especially concerning because bending moment is commonly high there; the code prohibits notches in the middle third. A deep notch at the top or bottom can weaken the member’s compression or tension zone. Avoid making a notch longer or deeper to fit a pipe after the first cut. If the framing does not match the prescriptive rule, obtain an approved repair or redesign.

Bored holes have different limits

A hole bored or cut into a solid-sawn member cannot exceed one-third of its depth in diameter under the cited IRC section. It must be at least 2 inches from the top or bottom edge and at least 2 inches from another hole. If the member is also notched, the hole must be at least 2 inches from the notch. This allows some holes in regions where notches would not be allowed, but it does not mean any location is acceptable for any size hole.

Lay out holes before drilling and keep them in a straight, documented band when multiple trades share the framing. A hole near a support may intersect a notch or hanger; a cluster can reduce the remaining wood. Check for knots, splits, existing fasteners, and the member’s orientation. Use a hole saw or approved boring tool that leaves smooth edges. Do not enlarge an opening by cutting through the bottom or top edge.

Engineered wood is a separate case

I-joists, trusses, structural composite lumber, and glulam have manufacturer-specific or engineered limits. Holes through an I-joist web may be allowed only in designated regions and within published size limits. Flanges and truss chords are structural components and should not be cut or notched unless the manufacturer or design professional specifically approves the alteration. A field crew cannot apply the one-third-diameter sawn-lumber rule to an engineered member.

If a penetration is needed in an engineered product, locate the product stamp and installation guide, identify the exact series and span, and use the manufacturer’s hole chart. If the member has already been cut, stop loading the affected area and request a repair detail from the manufacturer or engineer. Do not conceal the damage with blocking or plywood without an approved repair; the load path must be restored.

Example: plumbing through a floor joist

A 2-by-10 nominal solid-sawn joist has an actual depth of 9.25 inches. The one-third-depth limit makes the maximum hole diameter approximately 3.08 inches, subject to the required 2-inch edge distances and other conditions. A notch is governed by different size and location limits. Before drilling, the plumber measures the actual member, checks its span position, and verifies that the hole is at least 2 inches from the top and bottom and from other holes or notches.

If the framing turns out to be an I-joist rather than solid sawn lumber, the calculation above no longer applies. The plumber stops and checks the manufacturer’s chart. A hole through the web may be allowed at that location, but a cut in the flange is not a routine field adjustment. Accurate member identification prevents a code-compliant-looking opening from becoming a structural defect.

Common mistakes and exam cues

Common errors include using nominal depth instead of actual depth, placing a notch in the middle third, confusing hole diameter with notch depth, putting a hole too close to an edge or another opening, and applying sawn-lumber rules to engineered members. Another mistake is assuming a small hole is harmless when several are clustered. The code addresses each opening and the remaining section.

For exam questions, classify the member first. For sawn lumber, remember notch depth, notch length, location, end-notch limit, and hole diameter/edge distance. For engineered products, use the manufacturer or engineered detail. When field conditions do not satisfy the prescriptive limits, stop and obtain a repair design before concealment.

Quick review

• Use the sawn-lumber limits only for solid-sawn members.
• Notches: maximum one-sixth depth, one-third-depth length, and none in the middle third; end notches have a separate one-fourth-depth limit.
• Holes: maximum one-third member depth, at least 2 inches from edges, other holes, and notches.
• Do not cut engineered flanges, chords, or webs outside approved details.
• Use manufacturer charts or a designed repair when a member is altered.