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Notching and Drilling Rules for Joists
Permitted notch depths and hole diameters for timber floor joists per Approved Document A and BS 8103-3. Avoid weakening structural timbers.
Notching Rules (Top Edge Only)
Reference: Approved Document A, Diagram 7 / BS 8103-3. D = joist depth, L = span.
| Rule | Requirement | Example (200mm joist, 4m span) |
|---|---|---|
| Maximum notch depth | D / 8 (one-eighth of joist depth) | 200 / 8 = 25mm max |
| Permitted zone (from support) | 0.07L to 0.25L from face of support | 280mm to 1000mm from wall face |
| Notch location | Top edge only — never bottom edge | — |
| At bearings | Notches at the bearing point are permitted | e.g., notch to clear wall plate |
Drilling Rules (Holes Through Joist)
| Rule | Requirement | Example (200mm joist, 4m span) |
|---|---|---|
| Maximum hole diameter | D / 4 (one-quarter of joist depth) | 200 / 4 = 50mm max |
| Permitted zone | 0.25L to 0.4L from face of support | 1000mm to 1600mm from wall face |
| Hole position (vertical) | On the neutral axis (centre of depth) | Centre of 200mm = 100mm from top/bottom |
| Minimum spacing between holes | 3 x diameter of largest hole (centre to centre) | 3 x 50 = 150mm min between holes |
| Distance from notch | Not in the same cross-section as a notch | — |
Quick Reference — Common Joist Sizes
| Joist Depth (mm) | Max Notch (D/8, mm) | Max Hole (D/4, mm) |
|---|---|---|
| 150 | 18.7 | 37.5 |
| 175 | 21.9 | 43.8 |
| 200 | 25.0 | 50.0 |
| 225 | 28.1 | 56.3 |
Common Pipe Sizes vs Max Hole Diameters
| Service | Pipe OD (mm) | Hole Needed (mm) | Min Joist Depth (mm) |
|---|---|---|---|
| 15mm copper pipe | 15 | 20 | 80 |
| 22mm copper pipe | 22 | 28 | 112 |
| 28mm copper pipe | 28 | 35 | 140 |
| 40mm waste pipe | 40 | 45 | 180 |
| 110mm soil pipe | 110 | — | Cannot drill — use joist hangers or notch at bearing |
Note: Always refer to the current edition of Approved Document A and BS 8103-3 for definitive requirements. These rules apply to solid sawn timber joists only — engineered timber products have their own manufacturer-specific rules.
Related Pages
See the Joist Span Tables, the UK Timber Sizes reference, and the UK Pipe Sizes.
Next up: related pages
Frequently Asked Questions
Notches and holes weaken joists by removing material from the cross-section. Notches in the wrong position (mid-span) can cause the joist to split or fail in shear. Holes that are too large or too close together reduce the effective depth and bending strength. The rules in Approved Document A (referencing BS 8103-3) define safe zones and maximum sizes to ensure the joist retains adequate structural capacity.
If you need to cut or drill a joist outside the permitted zones, or exceed the maximum sizes, a structural engineer must assess the specific situation. Options include using joist reinforcement plates (galvanised steel plates bolted either side of the notch), using deeper joists to compensate, or rerouting the pipe/cable to avoid the conflict. Never notch or drill engineered joists (I-joists, glulam) without following the manufacturer's specific guidelines.
No. Engineered timber joists (I-joists, metal web joists, glulam beams) have their own manufacturer-specific rules for service holes. I-joists typically have designated knockout holes in the web and prohibit notching or cutting the flanges. Metal web joists allow services to pass between the webs without any cutting. Always follow the manufacturer's installation guide for engineered products.
Notches should only be cut in the top edge of a joist. The bottom edge is in tension under load and cutting a notch there would create a stress concentration that could cause the joist to crack and fail. All notch rules assume top-edge notching only. If a pipe must pass below the joist, use a deeper joist or route the pipe along the joist rather than through it.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.