Waste and soil pipes often need to pass through structural elements — particularly where a bathroom is above a newly opened floor plan. Whether this is acceptable depends on the element type and the position of the hole.
Steel Beams
For a 50 mm waste pipe hole in a steel web, the same rules apply as for any larger opening:
– Hole centre must be within the middle third of the beam depth
– Hole centre must be within the middle third of the beam span
– Centre-to-centre distance between adjacent holes ≥ 2.5 × the diameter of the larger hole
– Hole centre at least one full beam depth away from any support or concentrated load
If these conditions are all met, the hole is generally acceptable without engineering calculations. If any condition is not met, raise a Change Request before drilling.
Timber Joists
For timber I-joists and solid timber joists, the rules differ:
– Holes should be within the middle third of the joist span (horizontally)
– Holes should be within the middle third of the joist depth (vertically) — and for a typical 200 mm deep joist, the hole should be no larger than approximately 50 mm diameter (≤0.25 × depth is a common guide)
– Holes must be the minimum diameter necessary
These are general principles; some manufacturers publish specific drilling zone diagrams for I-joists, which should always be followed in preference to general rules.
Timber Beams
Timber beams (large section glulam or engineered beams carrying significant loads) are more sensitive to holes than joists. Do not drill through a timber beam without engineer approval — the reduced section area near a support can cause the beam to fail in shear.
How to Confirm
If your proposed pipe route passes through a beam near a support, is a large diameter, or if you are uncertain about any of the criteria above, raise a Change Request with PorthouseDean. We can confirm whether the hole is acceptable or advise on alternative routing.
Submit a Change Request or contact us before drilling any beam if you are unsure.
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