Fire Rated Wall Calculator — Board Layers, Screws & Sealant

For 30-minute fire resistance: 1 layer of 12.5mm fire-rated board per side. For 60 minutes: 1 layer of 15mm per side. For 90 and 120 minutes: 2 layers of 15mm per side (staggered joints). These are based on British Gypsum, Knauf and Siniat tested non-loadbearing stud partition systems to BS EN 1364-1. Always use pink-coloured fire-rated boards (Type F to BS EN 520), not standard wallboard. The rating refers to the complete tested assembly — board, stud and fixings — so follow the manufacturer's specific system sheet.

Screws must penetrate at least 10mm into the timber or metal stud. For a single layer use 38mm screws on 12.5mm board or 42mm on 15mm board. For a two-layer system, fix the first layer with 42mm screws and the outer layer with 60mm so it bites through both boards into the stud. Use bugle-head drywall screws (self-drilling for metal studs) at 300mm centres. Stagger the joints between layers by at least 600mm so they do not coincide.

Intumescent sealant expands when exposed to fire, sealing gaps and maintaining the fire barrier. Apply it at the perimeter junctions of fire-rated walls (where board meets the floor, ceiling/deflection head and abutting walls) and around all service penetrations (pipes, cables, ducts). Board-to-board joints across the face are NOT sealed — they are reinforced with paper or scrim tape and filled with jointing compound. Use a mastic gun with 310ml cartridges — each tube covers approximately 6 linear metres.

For a standalone fire-rated partition (e.g. between a garage and house), both sides must be boarded to achieve the stated fire resistance. If one side is an existing masonry wall, you only need to board the exposed side. The fire rating of a system refers to the complete assembly including both faces, studs and any insulation.

Under Approved Document B: party walls between houses need 60 minutes. Walls between a house and attached garage need 30 minutes. Walls enclosing a staircase (means of escape) typically need 30 minutes in 2-storey houses or 60 minutes in 3+ storeys. Loft conversion walls to habitable rooms need 30 minutes. Always check the specific requirements with your building control officer.