Where are cavity barriers required in UK buildings?

Cavity barriers are required at party wall junctions, around openings in external walls, at compartment wall and floor junctions, at eaves and verges, and at maximum 20m intervals both horizontally and vertically within continuous cavities. This is set out in Approved Document B Section 9 and Diagram 9.3. Additional locations include junctions between external wall cavities and internal compartment walls.

What is the maximum spacing for cavity barriers?

The maximum spacing for cavity barriers is 20 metres in any direction within a cavity, as specified in Approved Document B Table 9.1. This applies to both horizontal and vertical dimensions. In practice, cavity barriers at party walls, compartment walls and around openings will often subdivide cavities at intervals well below 20 metres.

What materials can be used for cavity barriers?

Approved Document B allows cavity barriers made from steel (minimum 0.5mm), timber (minimum 38mm), polythene-sleeved mineral wool, calcium silicate board, cement-based board, or proprietary intumescent products. For buildings above 18m, cavity barriers must be non-combustible (Class A1 or A2-s1,d0). Mineral wool sleeved in polythene is the most common choice for masonry cavity walls.

Do cavity barriers need fire testing certification?

Yes, cavity barriers should be tested to BS EN 1363-1 and classified to BS EN 13501-2. They must provide a minimum of 30 minutes fire resistance for integrity, and in some cases insulation. For buildings over 18m, barriers must meet the non-combustible requirements of Regulation 7(2). Proprietary products should have third-party certification such as BBA or Certifire accreditation.

Are cavity barriers needed around windows and doors?

Yes, cavity barriers must be provided around all openings in external cavity walls to prevent fire spreading unseen within the cavity past window or door frames. The barrier should close the cavity at the head, jambs and cill of each opening. This is particularly important in buildings with combustible insulation in the cavity, and is a requirement for all building heights under AD B.

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Cavity Barrier Calculator

Calculate cavity barrier positions, quantities and types for fire compartmentation in walls and roofs per Approved Document B.

Barrier Position

Where in the building the cavity barrier is needed

Barrier Material

Type of cavity barrier product

Cavity Width

Width of the cavity to be closed

Building Height

Affects material combustibility requirements

Wall Length

Total wall length for barrier calculation

Wall Height

Total wall height

How We Calculate This

This calculator determines the linear metres of cavity barrier required based on the barrier position, wall dimensions and opening count. It follows the requirements of Approved Document B Section 9 and BS 9991.

How it works

Position: Selects where the cavity barrier is needed — party wall junctions, around openings, at eaves, or at 20m maximum subdivisions.
Material type: Mineral wool (most common), intumescent, timber, steel or composite — each with different fire ratings and fixing methods.
Cavity width: Barrier must fill the full cavity width to prevent fire bypassing the barrier.
Building height: Above 18m, non-combustible barriers are mandatory per Regulation 7(2).

Key standards

Approved Document B: Section 9 — Cavity barriers
BS 9991: Fire safety in residential buildings
BS EN 1363-1: Fire resistance test standard

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.