How do I calculate bricks for a cavity wall?

Calculate each leaf separately. The outer leaf (facing bricks) uses 60 bricks per m2. The inner leaf typically uses 100mm concrete blocks at 10.76 blocks per m2. Run the calculator twice - once for bricks, once for blocks - using the same wall dimensions.

What is the standard cavity wall thickness?

A standard UK cavity wall is approximately 270-300mm total thickness: 102.5mm outer brick leaf + 100mm cavity (with insulation) + 100mm inner block leaf. Wider cavities (up to 150mm) are used for higher insulation requirements.

What blocks should I use for the inner leaf?

Use 100mm thick concrete blocks (440mm x 215mm) for the inner leaf. Aircrete blocks (like Celcon or Thermalite) provide better thermal performance. Dense blocks are used below DPC. Calculate at 10.76 blocks per square metre.

How many wall ties do I need?

Install wall ties at 900mm horizontal centres and 450mm vertical centres - this equates to approximately 2.5 ties per square metre. Increase density around openings (300mm centres within 225mm of reveals). Use stainless steel ties for longevity.

What insulation goes in a cavity wall?

Common options include: partial-fill rigid boards (PIR/phenolic) with a 50mm residual cavity, or full-fill mineral wool batts. Partial-fill provides easier installation; full-fill offers better thermal performance. Building Regs Part L specifies minimum U-values.

Do cavity walls need weep holes?

Yes, weep holes are required above DPC and above all lintels to allow moisture to escape from the cavity. Leave vertical joints open (no mortar) at maximum 900mm centres. Use weep hole vents to keep insects out while allowing drainage.

What is the minimum cavity width?

The minimum clear cavity (unfilled by insulation) should be 50mm to prevent moisture bridging. For partial-fill insulation, maintain this 50mm clear cavity. Full-fill cavities can be fully insulated but require careful workmanship to prevent debris bridges.

How much does cavity wall construction cost?

Materials for cavity wall construction cost approximately 100-150 pounds per square metre including bricks, blocks, insulation, ties, and mortar. Professional labour adds 80-120 pounds per m2. A 40m2 garage wall costs roughly 4,000-5,000 pounds complete.

What mortar mix for cavity walls?

Use 4:1 mortar (4 parts soft sand to 1 part cement) for general work above DPC. Below DPC, strengthen to 3:1 with waterproofer. Keep cavities clear of mortar droppings - use a cavity batten during construction to catch excess mortar.

Do I need Building Regulations for cavity walls?

Yes, all new cavity wall construction requires Building Regulations approval. This ensures compliance with structural requirements, thermal performance (Part L), and moisture resistance. Building Control will inspect foundations, DPC, insulation, and wall tie installation.

Back to Brick Calculator

Brick Calculator for Cavity Walls

Building cavity walls for a house extension, garage, or new build? Calculate exactly how many bricks and blocks you need for both inner and outer leaves. Our calculator helps you estimate materials for UK Building Regulations compliant cavity wall construction.

Bricklaying Materials Calculator

Calculate cement, sand and mortar using standard 4:1 ratio

1. Material Type

2. Wall Length (metres)

3. Wall Height (metres)

4. Waste Factor: 10%

Typical: 10% standard, 5% simple work, 15-20% complex patterns

Tips for Cavity Wall Construction

Cavity walls require two calculations - outer brick leaf and inner block leaf
Standard cavity width is 100mm with 50-75mm insulation filling part of the cavity
Wall ties must be installed at 900mm horizontal x 450mm vertical centres
Full-fill or partial-fill insulation choice affects cavity width requirements

Understanding Cavity Wall Construction

Cavity walls are the standard construction method for UK buildings since the 1920s. They consist of two leaves (outer brick, inner block) separated by a cavity containing insulation. This design prevents moisture penetration and provides excellent thermal performance.

Cavity Wall Components

Outer leaf: Facing bricks - 60 per m2
Cavity: 100mm typical, with insulation
Inner leaf: Concrete blocks - 10.76 per m2
Wall ties: Stainless steel - 2.5 per m2
Insulation: Partial or full-fill

How to Calculate Both Leaves

Run the calculator twice with the same wall dimensions:

Example: 10m x 2.4m Cavity Wall

Wall area: 10m x 2.4m = 24m2

Outer leaf (bricks):
24 x 60 = 1,440 bricks (+ 10% = 1,584)

Inner leaf (blocks):
24 x 10.76 = 258 blocks (+ 10% = 284)

Wall ties: 24 x 2.5 = 60 ties
Insulation: 24m2 (+ 5% = 25.2m2)

Cavity Wall Material Guide

Outer Leaf - Bricks

Facing bricks for appearance

- 60 bricks per m2
- Engineering below DPC
- Match existing if extending

Inner Leaf - Blocks

100mm concrete blocks

- 10.76 blocks per m2
- Aircrete for thermal performance
- Dense blocks below DPC

Wall Ties

Connect both leaves

- 2.5 ties per m2
- Stainless steel type 4
- Extra around openings

Insulation

Thermal performance

- Area equals wall area
- PIR boards or mineral wool
- Meet Part L requirements

Frequently Asked Questions

Find Local Suppliers

Enter your postcode to find local builders merchants

Related Calculators

House Extensions Garages

Need Professional Help?

Get quotes from trusted builders in your area.

Need a Custom Solution for Your Business?

Whether you're a contractor needing bulk calculations, a merchant wanting to embed our tools, or a business with specific requirements - we'd love to hear from you.

Or email us directly at [email protected]