Partitions
Deflection head allowance under a slab
- where
- deflection to allow for (mm)
- slab or beam span (m)
- deflection limit ratio (span/k)
- flange depth needed (mm)
- minimum stud engagement (mm)
- largest deflection the channel takes (mm)
- head channel flange depth (mm)
- stud length (mm)
- floor to soffit height (mm)
Explain this equation
Works out how much movement to leave at the head of a partition built under a slab or beam that will deflect, and whether the head channel is deep enough.
δ is the expected deflection of the structure: span L in metres turned to millimetres and divided by the limit ratio k. e is the minimum length the stud must stay inside the head channel, 20 mm for British Gypsum. Fr is the flange depth you need; Xmax is the most deflection a channel of flange Fl can take. Ls is the stud length for a downward-only head set open, which is the floor to soffit height H less the gap δ.
The stud must still be gripped by the flange when the gap is fully open, so the flange has to cover the gap plus the engagement: Fr = δ + e.
Sense check: a 7.2 m span at span/500 deflects about 14 mm, needing a 35 mm flange, so a 50 mm channel is fine. A 70 mm flange takes 50 mm of movement.
Watch out: use the structural engineer's figure where there is one; long-span or post-tensioned slabs can move more than span/500.
Works out how much movement to leave at the head of a partition under a slab or beam, and whether the head channel flange is deep enough.
Enter your values to see the result.
Good to know
Studs sit loose in a deep-flange head channel and must stay engaged by at least 20 mm at all times (British Gypsum BPG002). For a downward-only head the gap is set open, so the stud is cut short by the full deflection. Use the structural engineer's long-term deflection if you have it; span/500 is the Eurocode 2 limit for deflection after construction where partitions can be damaged, so it is a sensible default. British Gypsum DC channel has 50 mm flanges and EDC 70 mm; its standard components take up to 50 mm total movement. Do not screw boards into the head channel or the movement zone.
Common mistake
Cutting studs to fit tight to the soffit and screwing boards into the head channel, so the slab bears on the partition when it deflects.
Limits
For ± movement heads set in the neutral position the total range must still fit within flange − 20 mm. Fire-rated deflection heads must follow the tested detail exactly (cloaking strips, board layers). Channel steel thickness is ignored in the stud length.
Sources
- British Gypsum, Best Practice Guide BPG002 Deflection heads (minimum 20 mm stud engagement in the head channel at all times; DC channel 50 mm flanges, EDC 70 mm; up to 50 mm total movement)
- BS EN 1992-1-1 Eurocode 2, clause 7.4.1: deflection limits span/250 (appearance) and span/500 (after construction, where adjacent parts such as partitions may be damaged), summarised by Structville
- The Concrete Centre: Eurocode 2 deflection
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.