Engineered I-Joists and the Hole Chart

Why this matters

An I-joist floor invites holes. The web is thin, it cuts easily, and it often has round knockouts already scored into it, so it looks like the one framing member designed to be drilled. It is, but only in the places and at the sizes its manufacturer published, and those rules have nothing in common with the one-third-of-depth rule for sawn joists. A tech who applies the sawn-lumber rule to an I-joist, or who trims a flange to slide a duct past, has cut a member whose whole strength depends on the part they removed.

This card is about reading the manufacturer's hole chart: why it is the only rule, what it is actually telling you, and where to go when your hole is not on it. For sawn lumber, see related: Notching and Boring Limits in Wood Framing.

Why the chart is the only rule

The residential code steps aside for engineered wood. The IRC (R502.8.2 in the 2021 edition, as adopted by your jurisdiction) prohibits cuts, notches and holes in engineered wood products, I-joists included, except where the manufacturer's recommendations permit them or a registered design professional designs them. So the legal limit for a hole in an I-joist is whatever that manufacturer's published literature for that product series says, and nothing else.

And the charts differ. Each manufacturer publishes its own hole charts, and within one manufacturer they differ by series, because series differ in flange material and size, web thickness and depth. A chart for one product applied to another is not a close approximation. Two joists of the same depth from different series can have different allowable holes at the same location.

How an I-joist carries load

The shape explains every rule on the chart.

• The flanges carry the bending. Top flange in compression, bottom flange in tension, both made of solid sawn lumber or LVL. The flanges are where the joist's strength in bending lives, and they are sized close to what the design needs.
• The web carries the shear. The thin OSB or plywood web ties the flanges together and transfers shear between them. Shear in a uniformly loaded joist is highest at the supports and falls to about zero at mid-span.

Two rules follow directly.

Never cut, notch or drill a flange. Not a notch for a pipe, not a kerf to seat a duct, not a hole for a cable. A cut flange is a cut through the part carrying the bending, and there is no field rule that makes it acceptable. Only the manufacturer or an engineer can specify a repair, and that repair usually means added material bonded and fastened to the joist by their detail, or a new joist.

Big holes go near mid-span, not near supports. A hole in the web removes shear capacity. Near a support, where shear is high, even a moderate hole can take out capacity the joist needs. Near mid-span, where shear is low, a much larger hole costs little. That is why every chart sets a minimum distance from the support that grows with hole size.

What the chart tells you

A typical round-hole chart is a grid. You enter it with three things and read one number out.

Read the chart's notes before the grid, because the notes carry the rules that the grid assumes:

• Whether the distance is measured to the hole's edge or its center. This changes the answer by half a hole diameter, and charts differ.
• The spacing between holes. Charts commonly require clear distance between adjacent holes of at least twice the diameter of the larger hole, measured edge to edge. Check yours.
• Rectangular holes. These are much more restrictive than round holes of similar area because their corners concentrate stress. Charts usually treat them separately, measure by the longest side, and require them farther from the support.
• Knockouts. The pre-scored holes in the web can usually be knocked out anywhere the manufacturer says, often treated as allowable throughout most of the span. They are usually small, on the order of an inch and a half on many products, and they count toward hole spacing.
• Where no hole is allowed at all. Over a support, in a cantilever, in the zone of a web stiffener or a hanger, and anywhere the joist carries a concentrated load from above, unless the chart or an engineer says otherwise.
• Cutting method. Holes are cut with a hole saw or a fine saw, never hammered out except at a knockout, and never with an overcut that runs into the flange.

And the maximum hole. The web depth sets an upper limit: the hole cannot touch either flange, and most charts require a margin of web left between the hole and each flange. A hole that clears the support-distance requirement but crowds a flange is still out.

A worked example: a 6 in supply through an I-joist floor

A tech is running a new 6 in round supply from a basement trunk to a first-floor register. The floor above is I-joists. The stamp on the web gives the manufacturer, the series and the depth: 11-7/8 in. The joists are at 16 in on center, and the measured clear span between the inside faces of the foundation wall and the center beam is 15 ft 9 in.

The hole. A 6 in duct needs a hole slightly larger than its outside diameter to pass the duct with its insulation removed at the joist, so call it a 6-1/4 in hole. The web depth is the joist depth less both flanges; on a joist with 1-1/2 in deep flanges that is 11-7/8 minus 3, or 8-7/8 in. A 6-1/4 in hole leaves 2-5/8 in of web, about 1-5/16 in above and below. Whether that margin is enough is a question the chart answers, not the tech.

The chart. The values that follow are illustrative, to show the method; read the real figure off your product's chart. The tech finds the 11-7/8 in row for that series, the column for a span of 16 ft (the next span listed above 15 ft 9 in, the conservative direction), and the 6-1/4 in hole size. Say the chart gives a minimum distance of 5 ft 6 in from the inside face of the support to the center of the hole.

The plan versus the chart. The register is near an exterior wall, and the straight route would put the hole 2 ft 6 in from the foundation wall. That is 3 ft short of the 5 ft 6 in minimum. The hole cannot go there.

The options, in the order the tech worked them:

1. Move the hole. Run the duct along the bay, parallel to the joists, from 5 ft 6 in or more out from the wall, and turn up into the register boot inside the same bay. That needs no hole at all if the trunk branch can enter the bay from below. In this house it could, so the duct enters the joist bay from the open basement side, runs parallel to the joists to the boot, and never passes through a web.
2. Use a smaller hole farther in. Had the route needed to cross joists, smaller holes are allowed closer to the support, which tempts a split into two 4 in runs. Two 4 in rounds give about 25 sq in against the 6 in duct's 28.3, roughly 89 percent of the area, and a small duct moves less air per square inch at the same friction, so the pair carries noticeably less than the single run. Splitting a run is a duct-design call, not a hole-chart workaround.
3. Ask the manufacturer or an engineer. If neither works, the manufacturer's technical support or an engineer can say whether a hole outside the chart is possible with reinforcement, and what that reinforcement is. Walk in with the stamp information, the measured span, the joist spacing, what sits on the floor above, the hole size and its distance from the support.

Field mistakes that are specific to I-joists

Treating a knockout as permission for a bigger hole. A knockout marks one allowed small hole. Enlarging it to pass a 3 in drain is a new hole, and it has to meet the chart's rules for a 3 in hole at that location, including the support distance and spacing from neighboring holes.

Measuring span from the wrong points. The chart's span is the clear span between supports. Measuring from the end of the joist, or from a wall that is not actually a bearing support, makes every distance wrong.

Missing the support that is not at the end. An I-joist that runs continuous over a center beam has a support in the middle of its length, and holes near that beam are holes near a support, with the same restriction. A hole that would be at mid-span on a simple-span joist can be right over the high-shear zone of a continuous one.

Losing the stamp. Without the stamp, nobody can tell which chart applies. If the web is covered, look along the joist for a section where the print is visible, check the builder's plans, or ask the customer for the original framing package. A joist that cannot be identified cannot be drilled under any chart.

Repairing an existing bad hole by eye. If you find an oversized hole or a cut flange from an earlier trade, photograph it and tell the customer. The fix is the manufacturer's or an engineer's detail, often a reinforcement attached to the web by their specification, sometimes a sister joist. A piece of plywood screwed over the hole by eye is not a repair.

What changes the answer

A concentrated load above. A point load from a post, a bearing wall or a heavy fixture over the joist changes the shear along it, and the standard chart assumes uniform load. Ask.

Wider spacing or a longer span than the house was designed for. Charts are built for the product's published spans. A joist already at the edge of its span table has less spare capacity for holes than the same joist on a shorter span, which the chart reflects; reading the right span column matters more than it seems.

Rim board and blocking. The rim board around an I-joist floor and the blocking panels have their own rules for holes, also published by the manufacturer, and they are not the same as the joist's.

References

• International Residential Code, 2021 edition, R502.8.2 (engineered wood products), as adopted by your jurisdiction
• The I-joist manufacturer's installation guide and hole chart for the specific product series, identified from the web stamp
• See related: Notching and Boring Limits in Wood Framing
• See related: Before You Cut: The Four Things a Hole Can Break
• See related: The Structural Cut Approval SOP