A floor joist span table only works when the timber strength class, actual section, joist centres and floor loading all match the table. Measure the clear unsupported span and use the actual timber section, not just the trade name.
Quick-reference domestic floor joist spans
The table below uses the NHBC 2025 domestic floor-joist tables for a dead load not exceeding 0.50kN/m², excluding the joist self-weight. Maximum clear spans are shown in metres.
| Actual joist size | C16 | C24 | ||||
|---|---|---|---|---|---|---|
| 400mm | 450mm | 600mm | 400mm | 450mm | 600mm | |
| 47 × 95mm | 1.77 | 1.68 | 1.47 | 2.05 | 1.94 | 1.70 |
| 47 × 120mm | 2.40 | 2.28 | 2.01 | 2.67 | 2.57 | 2.32 |
| 47 × 145mm | 2.89 | 2.78 | 2.52 | 3.22 | 3.10 | 2.81 |
| 47 × 170mm | 3.38 | 3.25 | 2.95 | 3.77 | 3.62 | 3.29 |
| 47 × 195mm | 3.87 | 3.72 | 3.38 | 4.31 | 4.15 | 3.77 |
| 47 × 220mm | 4.36 | 4.19 | 3.81 | 4.85 | 4.67 | 4.24 |
Clear span is the unsupported distance
Measure between the inside faces of the supports. Do not use the full timber length.
Do not use a span table without checking the assumptions
Dead load, imposed load, joist spacing, timber strength class, service conditions and the actual finished timber section all affect the permitted span. The NHBC tables contain additional load cases and sizes; use the current table or a structural design where the build falls outside those assumptions.
View the NHBC 2025 floor-joist tables.
Once the joist size has been specified
Browse our structural timber and joist hangers & timber connectors. Check the supplied timber and connector specification is suitable for the design before installation.
