Floor joists: spacing, bearing, notching and drilling
Floor joists need a minimum 90 mm end bearing on masonry (or a correctly sized and fully nailed hanger), notches no deeper than 0.125 × the joist depth and only between 0.07 and 0.25 of the span, and holes on the neutral axis within the permitted zone. Strutting is provided according to the span.
The figures on one page
| Item | Limit | Figure |
|---|---|---|
| Floor joist end bearing on masonryBearing directly onto masonry or a wall plate; joist hangers follow the hanger manufacturer's seating instead | Minimum | 90 mm |
| Permitted notches and holes in floor joistsSolid timber floor joists — engineered joists follow the manufacturer's zones instead | Range | Notches: max depth 0.125 x joist depth, only between 0.07 and 0.25 of the span. Holes: max diameter 0.25 x joist depth, on the centreline, between 0.25 and 0.4 of the span, at least 3 diameters apart |
| Joist bearing and pointingTimber joists require a minimum bearing surface, typically 90mm, on supporting masonry or timber to ensure stable load transfer | Minimum | 90 mm |
| Light steel floor joist spacing and designLight steel floor joists should typically be installed at a maximum spacing of 600mm centre-to-centre, although wider spacings may be permitted if specifically engineered | Maximum | 600 mm |
| Strutting rows by joist spanHerringbone or solid strutting, tight between joists and restrained at the ends | Minimum | One row over 2.5 m span; two rows over 4.5 m |
What this means on site
Engineered I-joists follow the manufacturer’s hole chart, not the solid timber rule — cutting the flange of an I-joist is never acceptable and cannot be repaired on site.
Where a joist has been over-notched, the member has to be replaced or repaired to a designed detail. Plating over a bad notch without a design is not a fix.
What actually gets picked up
- Plumbers and electricians notching outside the permitted zone, usually near mid-span.
- Hangers only part-nailed, or the wrong hanger for the joist depth.
- Joist ends bearing less than 90 mm because the wall was built out of line.
- Strutting omitted entirely on long spans, giving a springy floor at handover.
Questions we get asked
- How deep can you notch a floor joist?
- No deeper than 0.125 × the joist depth, and only between 0.07 and 0.25 of the span from the support.
- What is the minimum bearing for a floor joist?
- 90 mm onto masonry, or a properly sized hanger fully nailed with the specified nails.
- Can you drill an engineered I-joist?
- Only within the manufacturer’s published hole chart and never through the flange.
Where this is inspected
This is ProPlot's editorial summary of publicly available UK regulation and good practice. It is not building control approval or a warranty determination, and the designer's or manufacturer's tighter figure always takes precedence.