Fire
Fire-rated build-up: boards, partition thickness and screw lengths
- where
- partition thickness (mm)
- stud or channel width (mm)
- board layers each side
- board thickness (mm)
- number of boards
- partition area, one face (m²)
- waste allowance (%)
- area of one board (m²)
- minimum outer screw length into steel (mm)
- minimum outer screw length into timber (mm)
Explain this equation
Once you have chosen a fire-rated partition from the manufacturer's table, this gives its finished thickness, the board count and the minimum screw length for the outer layer.
d is the stud or channel width. Each face carries n layers of board of thickness t, so the wall is d + 2nt thick. Nb counts boards for both faces and all layers over area A, plus waste w, divided by one board area Ab. Lm and Lt are the minimum outer screw lengths: the total board thickness the screw passes through plus 10 mm into steel or 25 mm into timber.
The screw rule comes from British Gypsum: enough thread must bite into the frame to hold the boards, and timber needs more embedment than thin steel.
Sense check: two layers of 15 mm FireLine on 63 mm timber gives 123 mm and needs 55 mm outer screws, matching British Gypsum's published specification.
Watch out: this does not give a fire rating; only the tested system does.
Once you have picked a fire-rated system from the manufacturer's table, this gives the finished thickness, the board count and the minimum screw length for the outer layer.
Enter your values to see the result.
Good to know
The fire rating depends on the whole tested system (board type, layers, stud, insulation, screws, joints, head and base details), so the number of layers must come from the manufacturer's table, for example British Gypsum A026030: 63 × 38 mm timber studs with two layers of 15 mm FireLine each side, 123 mm thick, 40 mm screws for the inner layer and 55 mm for the outer. British Gypsum screw rule: board thickness plus at least 10 mm into Gypframe steel or 25 mm into timber. Inner layers are fixed at wider centres; outer layers at 300 mm on walls. Stagger joints between layers by at least 600 mm.
Common mistake
Mixing boards or screws from different manufacturers, or using standard board where the test used fire board. Any change takes the partition outside its tested rating.
Limits
Informational: this does not give a fire rating. Always use the exact tested specification and its head, base and penetration details. Screw lengths are minimums; use the next stock length up.
Sources
- British Gypsum, Timber stud partition A026030 (63 × 38 mm studs at 600 mm centres, head and sole plates, noggings; 2 × 15 mm FireLine each side, 40 mm inner and 55 mm outer screws, 123 mm thick)
- British Gypsum, High Performance Screws data sheet (screw length = board thickness + minimum 10 mm penetration into Gypframe steel, or + minimum 25 mm into timber)
- British Gypsum, White Book Specification Selector (fire, acoustic and height performance of each partition and ceiling system)
- NHBC Standards 9.2.4 Dry lining (Table 5 wall support centres, Table 6 ceiling support centres and noggings, Table 7 screw centres, Table 8 screw lengths)
- British Gypsum, GypWall Single Frame A206002 technical specification (48 mm C studs at 600 mm centres, 1 × 15 mm WallBoard each side, 80 mm thick, maximum height 2800 mm based on fire limit or deflection L/240 at 200 Pa, 25 mm screws)
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.