Home & Construction⏱ 5 min read
How to Calculate Floor Joist and Beam Span Sizes
Undersized floor joists bounce, vibrate, and can fail. Here is how span tables work, the key variables that determine joist size, and what to check before opening up a floor.
Floor joist sizing is governed by span tables published in BS 8103-3 and the Building Regulations Approved Document A. You do not need to do complex engineering calculations for standard domestic floors -- but you do need to read the tables correctly.
Key Variables in Joist Sizing
1. Span: the clear distance between supports (wall to wall or beam)
2. Spacing: centre-to-centre distance between joists (typically 400mm or 600mm)
3. Load: floor type determines design load
- Domestic floor (habitable room): 1.5 kN/m2 imposed load
- Domestic floor (bedroom): 1.5 kN/m2
- Roof (accessible): 1.5 kN/m2
- Storage: 7.5 kN/m2
4. Timber grade: C16 (most common) or C24 (higher grade)
5. Species: strength class varies -- most structural timber is C16
Reading the Span Tables (Simplified)
For domestic C16 timber at 400mm centres:
47mm x 97mm: spans up to 1.72m
47mm x 122mm: spans up to 2.23m
47mm x 147mm: spans up to 2.75m
47mm x 170mm: spans up to 3.21m
47mm x 195mm: spans up to 3.71m
47mm x 220mm: spans up to 4.23m
47mm x 245mm: spans up to 4.74m
At 600mm centres (same C16):
47mm x 122mm: spans up to 1.99m
47mm x 147mm: spans up to 2.47m
47mm x 195mm: spans up to 3.31m
47mm x 220mm: spans up to 3.78m
For spans over 4.8m: engineered joists (I-joists or LVL beams) required.
Deflection Check
Even within the span table limits, floor bounce is a concern.
Serviceability limit: deflection should not exceed span/333.
For a 4m span:
Maximum acceptable mid-span deflection: 4000/333 = 12mm
A 47x220mm C16 joist at 4m span and 400mm centres:
Calculated deflection: approximately 8-10mm -- within limit
"Springy floor" indicators:
- Deflection exceeds span/333
- Incorrect or missing bridging/strutting between joists
- Notches or holes made incorrectly by plumbers/electricians
(Max notch: 0.125-0.25 of joist depth, in zones 0.1-0.4 from supports)
(Max hole diameter: 0.25 of joist depth, in middle third of span)
Adding a Load-Bearing Beam (Steel or Timber)
When a wall is removed, the beam must carry:
- The floor load above (load from joists bearing on beam)
- Any walls/roof load from above
For domestic single-span scenario:
Uniformly distributed load (UDL) on beam = Load per m2 x Tributary width x Span
This calculation quickly becomes complex and should be done
by a structural engineer. For typical domestic beam replacement:
3-4m span, 2-storey house: typically 127-152mm UB steel (universal beam)
Cost of structural engineer report: £200-£600 -- always worth it
Building regulations: any structural alteration requires
building control notification and sign-off.
Use of a building notice or full plans application required.