How many layers of plasterboard do I need for fire rating?

For 30-minute fire resistance: 1 layer of 15mm fire-rated board per side. For 60 minutes: 2 layers of 15mm per side. For 90 minutes: 2 × 15mm + 1 × 12.5mm per side. For 120 minutes: 3 × 15mm boards per side. These are based on British Gypsum and Knauf tested systems. Always use pink-coloured fire-rated boards (Type F to BS EN 520), not standard wallboard.

What screw lengths do I need for multi-layer fire boarding?

Each layer needs longer screws to penetrate through to the stud: first layer 42mm, second layer 60mm, third layer 75mm. Screws must penetrate at least 10mm into the timber or metal stud. Use bugle-head drywall screws. For metal studs, use self-drilling screws. Always stagger joints between layers by at least 600mm.

What is intumescent sealant and where do I use it?

Intumescent sealant expands when exposed to fire, sealing gaps and maintaining the fire barrier. Apply it around the full perimeter of fire-rated walls (where board meets floor, ceiling and abutting walls), around all service penetrations (pipes, cables, ducts), and at board joints. Use a mastic gun with 310ml cartridges — each tube covers approximately 6 linear metres.

Do both sides of a fire wall need boarding?

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.

What fire rating do I need for a UK domestic partition?

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.

Fire Rated Wall Calculator — Board Layers, Screws & Sealant

Calculate plasterboard layers, screw lengths, and intumescent sealant for fire-rated walls from 30 to 120 minutes per Approved Document B.

Wall Length

Total wall length

Wall Height

Floor to ceiling

Fire Rating Required

Check Approved Document B for your requirement

Board Both Sides?

Standalone partitions need both sides

Board Price (optional)

Per 15mm fire-rated board

Sealant Price (optional)

Per 310ml intumescent tube

Screw Price (optional)

Per individual screw

How We Calculate This

This calculator determines the board layers, screw types and intumescent sealant needed to achieve a specified fire resistance rating.

The formula

Boards per side = (Wall area ÷ Board area) × Waste factor × Layers

For a 4m × 2.4m wall (9.6 m²) at 60 min, both sides: boards per layer = 9.6 ÷ 2.88 × 1.1 = 4. 4 boards × 2 layers × 2 sides = 16 boards total.

Fire-rated systems

30 min: 1 × 15mm Type F board per side
60 min: 2 × 15mm Type F boards per side (staggered joints)
90 min: 2 × 15mm + 1 × 12.5mm per side
120 min: 3 × 15mm Type F boards per side

Based on tested systems from British Gypsum (White Book) and Knauf. All boards must be Type F (fire-rated) to BS EN 520.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.