TradeCalculator.co.uk
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.
Frequently Asked Questions
Related Calculators
Related Calculators
UK DPC Specifications
DPC types, widths and placement rules per Approved Document C for UK building construction.
Joist Span Tables — C16 and C24
Maximum clear spans for C16 and C24 floor joists at 400mm and 600mm centres, calculated to Eurocode 5 with UK National Annex parameters.
Ventilation Rates by Room
Extract ventilation rates by room type per Approved Document F 2021. Intermittent and continuous extract rates.
UK Screw and Fixing Sizes
UK screw gauge sizes, lengths and pilot hole diameters for wood, masonry and metal fixings.
Was this calculator helpful?
Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.