Holes or openings in a wooden beam or floor joist are sometimes unavoidable — for example for a pipe, cable or duct — but every hole weakens the beam. Where and how large you may drill or saw depends on the position in the beam, because a beam is loaded very differently in different places. Drilling in the wrong place can lead to deflection or even breakage.
Rules of thumb
These rules give a safe direction; the final dimensions should be determined by a structural engineer.
- Preferably drill in the neutral zone — around the middle of the beam height, where the tensile and compressive stresses cancel each other out
- Avoid the underside and topside — that is where the greatest tensile and compressive forces are; a notch on the underside is the most dangerous
- Stay away from the supports — near the bearing the shear force is large; drill rather in the middle region of the span
- Keep holes small — as a rule of thumb no larger than about one third of the beam height, and with ample distance between multiple holes
- No saw cuts or slots on the underside — these cut through the load-bearing fibres
Points to note
Are you unsure whether a beam is load-bearing, or is it a larger opening? Always have it checked. Weakening can be compensated by strengthening the beam, making a trimmed opening or rerouting the pipe.
Related terms
- Beam
- Joist floor
- Timber
- Cross section
- Trimmer
- Wood movement
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