Flat roof joist size depends on the clear span, joist centres, roof dead load and timber strength class. A 6x2 or 8x2 trade name on its own is not enough to choose the joist.
Quick-reference C16 and C24 flat roof spans
These examples use the current BSW/Timber Development UK domestic flat-roof tables for access for maintenance and repair only, with dead load not exceeding 0.50kN/m². Maximum clear spans are in metres.
| Actual section | Class | 400mm centres | 450mm centres | 600mm centres |
|---|---|---|---|---|
| 45 × 145mm | C16 | 3.00 | 2.95 | 2.72 |
| 45 × 145mm | C24 | 3.43 | 3.32 | 3.03 |
| 45 × 195mm | C16 | 4.15 | 4.00 | 3.64 |
| 45 × 195mm | C24 | 4.61 | 4.45 | 4.05 |
Clear span is the unsupported distance
Measure between the inside faces of the supports, not the full timber length.
Dead load can change the answer
The roof deck, insulation, covering, ceiling and other permanent materials contribute to dead load. As dead load increases, the permitted span can reduce. Read What Is Dead Load? before selecting a row from a span table.
Use the actual timber section and strength class shown on the design
Trade descriptions such as 6x2 and 8x2 can hide differences in finished size. Match the actual section, centres, loading and strength class to the span table. If the roof falls outside the published assumptions, it needs a project-specific structural design.
Firrings create the fall; they do not replace the structural joist
The joist carries the roof structure. A firring strip sits above the joist to create the drainage fall where that build-up is used.
View the full BSW/TDUK span tables. Once the structural specification is known, browse structural timber and the Build a Flat Roof range.
