Timber floor joists come in standard sizes, and their safe spans are well understood - but 'what size joist do I need?' still fills builders' group chats because the honest answer depends on span, spacing, loading and timber grade together. Here's the practical picture.
Common sizes and indicative domestic spans
| Joist size (mm) | Indicative max span |
|---|---|
| 47 × 145 | ≈ 2.9m |
| 47 × 170 | ≈ 3.4m |
| 47 × 195 | ≈ 3.9m |
| 47 × 220 | ≈ 4.4m |
- Spacing: 400mm centres is standard for domestic floors; 600mm centres reduces capacity noticeably
- Grade matters: C16 spans less than C24 in the same size
- These figures assume ordinary bedroom/living loads - not baths, pianos, partitions or storage
When a floor needs engineering, not a table
- Loft conversions - existing ceiling joists are not floor joists; the new floor is a designed structure
- Openings - trimming around stairs and chimneys concentrates load into doubled or steel members
- Point loads - a partition, water cylinder or bath mid-span changes everything
- Long spans - beyond ~4.5m, engineered solutions (I-joists, metal-web joists, steels) usually beat sawn timber
- Old floors - Victorian joists were sized by rules of thumb; assessing them for new uses is judgement work
Span tables answer the simple case. The moment your floor has an opening, a point load, an awkward span or a Building Control officer asking for justification, that's timber design - quick, inexpensive engineering that we turn around routinely as part of extension and conversion packages.
Frequently asked questions
What size joists for a 4m span?
Indicatively 47×220mm C24 at 400mm centres for an ordinary domestic floor - but confirm against your actual loading and support conditions, and note anything unusual (openings, point loads) needs design.
Can I use ceiling joists as floor joists in a loft?
Almost never - ceiling joists are typically 100-125mm deep, sized to carry plasterboard, not people and furniture. Loft conversions get a new, independent floor structure designed by an engineer.