How the Joist Hole & Notch Checker works

The limits come from one paragraph of the 2021 International Residential Code, Section R502.8.1, and the figure that goes with it (Figure R502.8). They cover solid sawn-lumber joists. Every size limit is a fraction of the member's actual depth D (a 2×10 is 9 1/4" deep, not 10") or a fixed 2 inches.

The hole rules interact. A hole has to leave 2" above it and 2" below it, so the widest hole that fits is the smaller of D/3 and D − 4". On a 2×6 the edge rule wins: one-third of the depth is 1.833", but only a 1 1/2" hole leaves 2" on both sides. A member 4" deep or less has no room for a hole at all.

The table is rounded down to the nearest 1/8", the same way the International Code Council prints these fractions in its CodeNotes table, so a figure read from it is never over the limit. Actual depths are the dry dressed sizes in the American Softwood Lumber Standard (PS 20-20, Table 3). Measure your own joists: old and rough-sawn lumber may be a different depth.

Where the middle third is

The code says "the middle one-third of the span" and does not define span. The two drawings ICC publishes disagree. Figure R502.8 in the code itself dimensions L, and each L/3, from the outer ends of the joist, over the bearings. Figure 1 in ICC's CodeNotes draws the span between the inside faces of the supports. This tool does not pick one. It asks how much of the joist rests on each support and closes the stretch that is in the middle third under either reading:

On a 12' 0" clear span with the joist resting 3 1/2" on each support, the joist is 12' 7" long. The middle third of the clear span runs from 48" to 96" from the face of the support; the middle third of the joist runs from 46.83" to 97.17". No notch is passed between 46.83" and 97.17". With equal bearings b at both ends, that is one-third of the bearing length wider on each side than the clear-span reading. A notch that stops exactly on the boundary passes; any part of it inside fails.

Why the middle third and the edges

A joist carrying a floor bends. Its top edge is squeezed and its bottom edge is stretched, and those forces are strongest at the two edges and much lower near the middle of the depth. Bending is also greatest at the centre of the span. That is the explanation Glenn Mathewson gives in Fine Homebuilding for the shape of the rules: notches, which remove wood from an edge, are kept out of the middle third, and holes are kept in the body of the joist, away from both edges. The same article says the rules are written for simple spans, a joist running from one support to the next with nothing in between, and cannot be used at or near the support of a cantilevered joist.

Where the wording leaves room, this tool takes the stricter reading

• The no-notch zone covers the middle third of both the clear span and the full joist length, as set out above.
• The 2" distances are measured from the rim of the hole, not its centre. ICC's CodeNotes figure dimensions them that way.
• The distance between a hole and a notch is measured along the length of the joist. A hole directly above or below a notch counts as zero inches away, however much wood is left between them.
• The deeper D/4 limit is given only to a notch that starts at the very end of the joist and stays over the bearing. The code gives no position or length for a notch "at the ends"; Figure R502.8 draws its end cuts over the bearing only. A notch that runs past the face of the support is held to D/6 like any notch in the span. One consequence is stricter than the sentence in the code: where a joist end hangs in a metal hanger and has no bearing length, this tool gives no D/4 allowance at all, so an end notch deeper than D/6 fails there. The code says only "at the ends"; your building department may allow D/4 at a hanger.
• The length limit of D/3 is applied to end notches too. The sentence that sets it is written for notches in general, and the sentence about end notches relaxes only the depth. Figure R502.8 shows no length for end cuts, so this is a reading, not a stated limit, and it fails an ordinary bearing notch longer than D/3. Your building department may read it differently.
• For lumber 4" nominal or thicker the code bars notches on "the tension side" except at the ends, without saying which edge that is. It depends on how the member is supported and loaded, which a web page cannot verify. So for such lumber this tool passes only an end notch over the bearing, and fails a notch anywhere else on either edge.

What this tool does not cover

• Anything but one simple span. A joist that runs continuously over a middle support, or cantilevers past its support, is not checked: choose that option on the form and the tool says so instead of giving a verdict.
• Engineered wood: I-joists, trusses, LVL and other structural composite lumber, glulam and cross-laminated timber. Sections R502.8.2 and R802.7.2 prohibit cutting, notching or boring them unless the manufacturer's recommendations permit it or a registered design professional has accounted for it in the design. No numbers in this tool apply to them.
• Rafters. Section R802.7.1 applies the same paragraph to rafters and ceiling joists, and R802.4.1 measures rafter span along the horizontal projection, but nothing read for this tool says how to lay out the middle third or the positions of cuts on a sloped member. Notches in a rafter tail (R802.7.1.1) and taper cuts at the ends of ceiling joists (R802.7.1.2) have their own rules as well.
• Square or oblong holes. The code speaks of the diameter of a hole; the Fine Homebuilding article notes that square holes are not addressed by it.
• More than one notch. The tool checks a single notch. The same article reads the code as allowing only one notch on each side of the middle third.
• Built-up beams made of several pieces nailed together, where the code does not say how nominal thickness counts.
• Wall studs and top plates, which are limited by a different section (R602.6).
• Whether the joist is the right size for its span and load to begin with.

What the tool assumes until you say otherwise

The form opens on an example. Its numbers (a 12 ft span, 3 1/2" of bearing at each support) are repeated back in the "Read as" line above the checks; replace them with your own. One pre-selected answer is a question of scope, not a number: "one simple span". Until you have tapped your support type, the lines that would pass are tagged "Assumed" instead of "Pass", the verdict is amber, and its headline reads "if this is one simple span".

Input guards

The span is entered in feet and everything else in inches. To catch a figure typed in the wrong unit, the tool refuses a span over 30 ft or under 2 ft, a joist depth outside 3" to 24", a bearing over 12", a hole as wide as the joist is deep, a notch longer than 24", and any position past the far support. It repeats back how it read every value, in feet and inches, above the list of checks. A link carrying an option the form does not offer is refused rather than read as a default. These bounds are plausibility checks of this tool, not code limits.

Editions

The numbers above are from the 2021 edition, read in the Texas IHB Residential Code 2021 on UpCodes, which marks no amendment on this section. The Texas Windstorm Insurance Association Residential Code 2024, also on UpCodes and linked under Sources, carries the same wording for R502.8.1. One older difference is worth knowing: a jurisdiction tip sheet written for the 2012 codes also allows a notch up to D/3 deep in the top of a rafter or ceiling joist, within one member depth of the face of the support. That allowance is not in the 2021 text read for this tool, so it is not offered here. Check which edition and amendments your jurisdiction has adopted.

This tool cannot be published unless it reproduces every example below. 4 are copied from a published source. 18 were worked by hand from the cited rule.