Avoid Costly Mistakes: The Correct Method for Cutting I-Joists

A guide on how to cut service holes in engineered I-joists the right way (Australia)

Cutting DindasJoists or any I-Joists for plumbing, electrical, or HVAC should never put your floor system at risk. With engineered I-joists, the rules are straightforward. When you follow the Dindas Australia manufacturer’s hole charts and basic do’s and don’ts. One wrong cut can trigger redesigns, propping, delays and potential replacement. Here’s how to keep the job moving and compliant.

1) Know your joist anatomy (and the one rule you never break)

 

An I-joist has flanges (top and bottom) and a web (middle).

 

• Never cut, notch or drill the flanges. All penetrations go in the web, with a small clearance to the flanges as specified by Dindas.

Why this matters: The flanges carry the bending forces. Nick a flange and you’ve compromised the member; that’s where “quick fix” turns into costly replacement.

 

2) Plan services before you cut

 

The biggest cause of mistakes is cutting “where the pipe wants to go” instead of laying out services to suit the hole chart.

• Map out plumbing and cabling runs as soon as joists are down.
• Walk the run with the hole chart for your joist size/series, spacing and design span.
• Modify the route so all penetrations land on approved hole locations.

 

Tip: Keep the hole chart on your phone and a laminated copy in the site shed. If the design span changes, re-check—shorter spans can relax minimum distances, but you must still follow the table rules.

 

3) Read the Dindas Australia hole chart like a pro

Dindas Australia gives a minimum distance x from the inside face of the nearest support to the centre of the hole. Here’s the workflow:

1.    Pick your hole size (or the next bigger size shown).
2.    Find your joist series and depth.
3.    Read off x and mark it from the inside face of the bearing/hanger.

Worked example: Need a Ø 140 mm round hole in a 240 × 52 mm Dindas I-joist with 4.0 m design span? Use the 150 mm column: x = 1.1 m, so the hole centre must be ≥ 1.1 m from the inside face of the nearest support.

 

4) The golden site rules (that prevent expensive mistakes)

 

• Holes go in the web only. Maintain the specified minimum web-to-flange clearance (no flange cuts).

 

• Hole spacing: between two holes, keep at least 2× the larger diameter (round) or 2× the longest side (square/rectangular).

 

• Maximum holes per span: three (any shapes), excluding knockouts. Don’t turn a span into Swiss cheese.

 

• Cantilevers: only Ø 40 mm round holes allowed; rectangular/square holes aren’t permitted in cantilevers.

 

• Square/rectangular holes: place at mid-web; rectangular options are restricted (single span, one per span, and more rules in the table). Avoid over-cut corners—drill corner starters first.

 

• Grouped round holes: may be treated as one larger equivalent hole if they fit inside a circumscribed circle per the chart rules.

Stick to these and you’ll avoid the classic miscuts that lead to strengthening, plating or full replacement.

 

5) Step-by-step: cutting a clean, compliant hole

1.    Measure from the right reference. Mark x from the inside face of the nearest support to the hole centre. Scribe the outline at mid-web (or as required for round holes).

2.    Support the web. Clamp a backing block if you’re worried about tear-out.

3.    Cut, don’t over-cut.

• Round: pilot drill, then holesaw/jigsaw on the line.

 

• Square/rectangular: drill ~25 mm corner holes first, then cut between them; lightly radius corners to reduce stress.

 

4.    Verify clearances and spacing. Double-check the web–flange margin and the 2× rule to other holes.

5.    Protect the frame. Keep cuts clean and dry; if anything is outside the table, stop and escalate.

 

6) The “never again” list: common mistakes to avoid

 

• Measuring from the wrong place. The hole distance x always starts at the inside face of the nearest support, not the edge of the flooring or the outside of the bearer.

 

• Nicking the flange. Even a small nick can be a big problem. There are no flange holes or notches. Ever.

 

• Overcut corners on square/rectangular holes. Sharp over-cuts create stress raisers. Drill corners, then cut. Don’t swing past the line.

 

• Too many holes in one span. You’re capped at three (knockouts don’t count). Plan runs so you don’t stack up holes near each other.

 

• Wrong hole near a cantilever. Cantilevers are strict: Ø 40 mm round only.

 

• Rectangular hole in the wrong scenario. Rectangular “duct” holes are single-span only and one per span. If that doesn’t work, talk to the engineer/NDC before cutting.

 

7) What to do if someone has already cut it wrong

 

 

1.    Stop work on that joist area. Don’t load the floor above the mistake.

2.    Record the facts: joist series/depth, actual span, joist spacing, hole size/shape/location (distance from support), and photos.

3.    Contact Dindas Australia National Design Centre (NDC). They’ll advise on whether repair is possible or a replacement is required for your specific joist and loading. Don’t invent a fix on site.

 

8) Make it easy for your crews (and clients)

 

• Put the chart in their hands. Print the Dindas Australia Web Holes Technical Bulletin for your joist series and keep a copy with the foreman and plumber/sparkie.

 

• Brief subs at pre-start. Five minutes with the hole chart saves five hours later.

 

• Mark approved hole centres. On tricky runs (kitchens, bathrooms), pre-mark compliant hole centres before services roll in.

 

• Direct clients to our resources. Our Technical Resources hub keeps the latest bulletins and guides in one place. (dindas.com.au)

 

9) Quick checklist before you drill

• Using the correct Dindas Australia I-Joists web-holes reference chart for your joist series/depth, spacing and design span.

 

• Measured x from the inside face of the nearest support.

 

• The hole is in the web only, with the specified clearance to each flange.

 

• Spacing to any other hole is ≥ 2× the larger hole’s size.

 

• You’re within the max holes per span (three, knockouts excluded).

 

• For square/rectangular holes: mid-web, corners drilled, no over-cuts, and rectangular rules (single span/one per span; no cantilevers) are met.
   

 

10) Bottom line

If you follow the Dindas Australia web-hole tables and basic cutting discipline, you’ll keep floors strong, compliant and on schedule, without the drama of costly repairs. When a run doesn’t fit the chart, don’t guess: bring it to the project engineer or the Dindas Australia National Design Centre for job-specific guidance.
 

 

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Helpful links (for your site pack)

 

•  Dindas Australia I-Joists Web Holes Reference Chart: Hole rules, examples and charts for Dindas I-joists.

 

• Dindas Australia Interactive I-Joist Span Tables: Online and interactive resource to calculate precisely your floor joist system span distances.

 

• Dindas Australia Technical Resources: Latest guides, span tables and bulletins.