Yes, but only in specific ways. I-joist manufacturers allow round — and sometimes rectangular — holes cut through the web, the thin vertical section running down the joist's center, for ductwork, plumbing, and wiring, as long as the hole's size, shape, and location fall within that manufacturer's published hole chart for the exact joist depth, span, and product series. The top and bottom flanges, however, can never be cut, notched, drilled, or shaved under any circumstances, because they carry the bending stresses that keep the joist from failing. There is no single hole size or spacing number that applies to every I-joist — the correct chart is determined by the brand and model stamped on the joist itself, not by rules used for solid-sawn lumber or a different manufacturer's product.
Why I-Joist Rules Are Different From Solid Lumber
An I-joist is an engineered wood product built from three parts: a top flange, a bottom flange, and a thin web that connects them, forming a cross-section shaped like the letter I — similar to a steel I-beam. The flanges are typically made from laminated veneer lumber (LVL), laminated strand lumber (LSL), or machine-graded solid sawn lumber, and they run continuously along the entire length of the joist. The web, usually oriented strand board (OSB) or plywood, holds the flanges apart and transfers shear forces between them.
Each part has a distinct job. The flanges resist the tension and compression stresses created by bending — the top flange compresses while the bottom flange stretches as the joist carries a load. The web resists shear, the sliding force that builds up between the top and bottom of the joist, especially near supports. Because the work is split between two different materials in two different shapes, an I-joist doesn't behave like a solid 2x10 or 2x12, where the wood fibers run continuously through the entire cross-section and traditional notch-and-bore allowances apply.
Model residential building codes generally address cutting and boring of solid-sawn joists directly, but engineered wood products like I-joists are typically required to be installed according to the manufacturer's code-recognized specifications rather than the sawn-lumber allowances. In practice, that means the building department, the framer, and the trades all defer to the same document: the hole chart published for that specific joist product — not a generic framing rule of thumb.
The Flange Rule: Why Cutting or Notching Is Never Allowed
Across manufacturers, one rule stays constant: the top and bottom flanges of an I-joist may never be cut, notched, drilled, routed, or otherwise reduced — regardless of the size of the cut, the location along the span, or how "minor" it looks. No published hole chart includes a flange allowance, because the flange is doing the structural work that a solid joist's outer fibers would normally do, concentrated into a narrower, precisely sized piece of material.
Removing even a small notch from a flange — to clear a duct boot, seat a strap, or route a cable — cuts directly into the fibers carrying the highest bending stress in that section of the joist. Unlike a web hole, which can sometimes be compensated for with careful placement, there is no field method for restoring a flange's capacity once it has been cut. The joist's rated span and load capacity assumes an intact, continuous flange from end to end.
Web Holes: What Manufacturer Specifications Actually Permit
The web is where I-joists genuinely allow field modification for ductwork, plumbing, and wiring — but "allowed" is defined narrowly. Manufacturers pre-engineer two categories of openings:
- Factory knockouts — round, perforated openings punched into the web at set intervals during manufacturing. These are pre-approved, but they are not unlimited; charts still restrict how many additional holes can be added nearby.
- Field-cut holes — openings the installer cuts on site, sized and located according to the manufacturer's chart for that specific product, depth, and span.
Round holes almost universally qualify for the largest allowable diameters, because a circular opening distributes stress evenly around its edge. Square and rectangular cutouts concentrate stress at their corners, so charts typically restrict them to smaller sizes, specific locations, or require additional reinforcement — some manufacturers do not permit rectangular web cutouts at all without an engineered detail.
Where a hole falls along the joist's length matters as much as its size. Shear forces are highest near supports — walls, beams, girders, or posts — and taper toward the center of the span. Because of this, hole charts almost always prohibit any field-cut opening within a stated distance of a support or a concentrated ("point") load, and become more permissive moving toward mid-span.
Reading a Hole Chart: The Parameters That Control Every Hole
Every manufacturer's hole chart is built around the same handful of concepts, even though the specific numbers differ by brand, series, and depth. Understanding these parameters helps you read any chart correctly — and understand why a number from one product can't be borrowed for another.
| Parameter | What It Controls | Why It Matters |
|---|---|---|
| Hole shape | Whether a round or rectangular chart applies | Round holes generally qualify for larger diameters; corners on rectangular cutouts concentrate stress |
| Distance from support or point load | How close a hole may be cut to a bearing wall, beam, header, post, or concentrated load | Shear is highest near supports, so the "no-hole zone" is widest there and narrows toward mid-span |
| Hole diameter vs. joist depth | Maximum size of a single hole at a given point along the span | Deeper joists generally tolerate proportionally larger holes, but the exact ratio is set by the tested product, not a fixed universal fraction |
| Spacing between holes | Minimum center-to-center distance between two separate openings | Enough solid web must remain between holes to transfer shear around each opening |
| Group or cluster limits | How many holes are allowed within a given length of joist | Individually compliant holes can still overload the web if too many are clustered close together |
| Flange clearance | Any cut, hole, or notch touching the top or bottom flange | Absolute prohibition — this zone has no size-based allowance in any chart |
Because the diameter-to-depth ratio, the no-hole distance from supports, and the spacing rules are all specific to the tested design of one manufacturer's product line, a chart printed for one brand's joist — or even a different depth within the same brand — cannot be applied to a different product. Matching the exact stamped designation to the exact chart is not a formality; it's the only way the hole layout reflects how that joist was actually engineered and tested.
How to Find the Correct Hole Chart for Your Joist
Every I-joist carries an ink stamp near the top flange identifying the manufacturer and the product series or model designation. That stamp — not a generic search for "I-joist hole chart" — is where verification starts.
- Photograph the stamp on the joist, capturing the manufacturer name, product series, and depth clearly.
- Check whether the original framing plans or truss/joist layout package from the builder or supplier already include an approved hole layout — custom home projects often receive this directly from the manufacturer's engineering department.
- If no layout exists, contact the manufacturer, an authorized distributor, or the original lumber supplier and request the current hole chart for that exact series and depth.
- Confirm the chart you receive lists the correct depth and, if shown, the correct span range — hole allowances can change even between depths within the same product family.
Step-by-Step: Verifying a Hole Location Before You Drill
- Identify the joist. Record the manufacturer, product series, and depth from the stamp on the joist you plan to modify.
- Get the matching chart. Obtain the current hole chart for that exact model and depth from the manufacturer or an authorized distributor — not a chart saved from a previous project with a different joist.
- Measure to the nearest support. Measure from the center of the proposed hole to the face of the nearest bearing wall, beam, girder, post, or concentrated load.
- Compare against the chart. Check that distance, along with the proposed hole diameter and shape, against what the chart allows at that specific point along the span.
- Check for nearby holes. Confirm no other hole — factory knockout or field-cut — already exists close enough to violate the chart's spacing or group-hole limits.
- Confirm flange clearance. Verify the cut stays entirely within the web and doesn't touch, notch, or come within the chart's required clearance of either flange.
- Document it. Keep a marked-up copy of the chart, a photo of the verified layout, or a note in the project file before cutting — useful for your own records and for inspection.
When Ductwork or Plumbing Won't Fit the Allowed Zone
HVAC trunk ducts are the most common reason homeowners ask about notching I-joists — a deep, wide rectangular duct often doesn't fit within the web's allowed hole size or its distance-from-support restrictions, especially in shallower joists. Plumbing lines, particularly smaller supply lines, more often fit within permitted zones near mid-span, but larger drain, waste, and vent (DWV) pipes can run into the same size limits as ductwork.
When a duct or pipe genuinely doesn't fit the allowed opening, the answer is never to enlarge the hole past the chart or cut into a flange to make room. Realistic alternatives include:
- Switching to a shallower, wider, or oval duct section sized to fit within a chart-compliant hole
- Rerouting the line through a section of the joist bay where the chart permits a larger opening, typically closer to mid-span
- Dropping a soffit or bulkhead below the framing so the duct or pipe runs beneath the joists instead of through them
- Specifying a deeper I-joist at the design stage, sized to both the structural load and the largest opening the mechanical plan requires
- Involving a structural engineer to review an oversized-opening detail — some manufacturers publish or approve specific reinforcement methods for openings larger than the standard chart allows, but only when engineered and documented for that exact product
What Happens When Charts Are Ignored — and Common Mistakes to Avoid
Because engineered joists are designed with less built-in reserve capacity than an oversized piece of solid lumber, damage from an improper cut doesn't always show an obvious warning sign before it becomes a problem. Symptoms that do show up over time include sagging or springy floors, cracked drywall or tile above the affected joist, popping fasteners, or in more serious cases, sudden localized failure under a concentrated load such as a bathtub, water heater, or heavy furniture.
Most improper cuts trace back to a handful of avoidable mistakes:
- Assuming solid-lumber notching and boring rules apply to I-joists
- Using a hole chart from a different manufacturer, or from a different depth within the same brand
- Treating a factory knockout as an unrestricted opening rather than checking spacing to nearby holes
- Cutting one oversized hole to avoid a second pass for a later trade, rather than verifying both against the chart
- Enlarging an already-approved hole during a remodel to fit a newly added duct or pipe
Your Checklist Before Drilling or Routing Through an I-Joist
- Confirm the exact manufacturer and product series stamped on each joist you plan to modify
- Obtain the current hole chart directly from that manufacturer for the exact depth and span involved
- Measure and mark the proposed hole location before cutting, referencing the distance to the nearest support or point load
- Verify the hole diameter and shape are within the chart's limit for that specific location along the span
- Check spacing against any existing or planned holes in the same joist
- Confirm no part of the cut touches, notches, or comes within the chart's clearance of either flange
- Keep the verified chart and photos of the marked layout with your project records
- Ask your local building department whether documentation of manufacturer compliance is required at inspection
- If ductwork or pipe size won't fit the allowed zone, contact the manufacturer's technical support or a structural engineer rather than oversizing the hole
Frequently Asked Questions
Can you ever cut or notch the top or bottom flange of an I-joist?
No. No manufacturer publishes an allowance for cutting, notching, drilling, or shaving the top or bottom flange under any circumstances. The flanges carry the bending stresses that keep the joist from failing, and removing any material from them can create a structural defect. A flange that's already been cut typically needs a manufacturer- or engineer-approved repair, or joist replacement — not a field patch.
How do I find the correct hole chart for my specific I-joist?
Look for the manufacturer's ink stamp near the top flange, which identifies the brand and product series. Use that exact designation and joist depth to request the current hole chart from the manufacturer or an authorized distributor, or check whether your builder retained the original stamped joist layout drawings, which often show pre-approved hole locations for HVAC and plumbing trades.
Is there a standard hole size that works for every I-joist?
No. Allowable hole size, shape, spacing, and distance from supports vary by manufacturer, product series, joist depth, span, and design load. A chart from one brand or one depth cannot be applied to a different product. The manufacturer's own published chart is the only reliable source for a specific joist.
Can I use factory-cut knockouts in the web for anything I want?
Factory knockouts are pre-approved openings, but they still have limits. Manufacturer charts commonly restrict how many additional field-cut holes can be added near an existing knockout and how close two openings can be to each other, because clustered openings can overload the web even when each individual hole would otherwise qualify.
What should I do if the duct or pipe I need won't fit within the allowed web hole zone?
Do not enlarge the hole or cut into the flange to make it fit. Options typically include switching to a shallower or oval duct section, rerouting the line through a section of the joist where the chart allows a larger opening, dropping a soffit below the framing, or having a structural engineer specify an approved reinforcement detail or a deeper joist sized for the required opening.