🏗️ Foundation Design for Cavity Walls: What You Need to Know

When it comes to building a solid and long-lasting structure, the foundation is everything—literally. If you’re working with cavity walls, getting the foundation design right is crucial for strength, moisture resistance, and thermal performance. Whether you’re building a new home, an extension, or managing a renovation, understanding how to design the right foundation for cavity walls can save you serious time, money, and hassle. Let’s break down the essentials of foundation design for cavity walls in a clear, practical way. ⸻ 🧱 What Is a Cavity Wall? A cavity wall consists of two parallel walls (called “leaves”) with a gap—or cavity—between them. This cavity is usually filled with insulation and acts as a barrier against moisture and heat loss. The outer leaf is typically made of brickwork, while the inner leaf is often constructed with concrete blocks or masonry. This wall type is popular in modern construction for its excellent thermal and moisture performance—but it needs the right foundation to work effectively. ⸻ 📐 What Type of Foundation Should You Use? The foundation must support both leaves of the cavity wall and distribute the load evenly into the ground. Here are the most common options: 1. Strip Foundation • Most common for cavity walls in residential and low-rise buildings. • A continuous strip of concrete beneath the wall. • Ideal for stable soil with good bearing capacity. • Typical width: 600–1000 mm • Depth: Minimum 450 mm below ground level. 2. Trench Fill Foundation • Fast and practical: fill most of the trench with concrete. • Reduces need for brickwork below ground. • Good for weak or unstable soil conditions. 3. Raft Foundation • Used in poor soil conditions where loads need to be spread. • A reinforced concrete slab covers the entire footprint. • Less common but very effective for larger or unstable builds. ⸻ 🏗️ Foundation Layout for Cavity Walls Here’s what a basic cross-section of a cavity wall foundation looks like: [ Outer Brick Leaf ] ← Supported on outer edge of foundation || || ← Cavity with insulation (usually 50–100 mm) || [ Inner Block Leaf ] ← Supported on inner edge of foundation [ Concrete Foundation ] ← Spanning under both leaves To ensure stability, the foundation should be wider than the total wall thickness by at least 300 mm—150 mm on each side. ⸻ 🔍 Key Construction Details to Remember ✅ Cavity Width • Typically 50–100 mm. • Common choice: 75 mm cavity with rigid insulation boards. ✅ Foundation Width & Depth • Base should be at least 300 mm wider than the wall. • Depth depends on soil type, but 450 mm minimum is standard. ✅ Concrete Grade • Use C25/30 concrete (or as per structural engineer’s spec). ✅ Damp-Proofing • DPC (Damp Proof Course) must be laid 150 mm above ground level. • Ensure DPM (Damp Proof Membrane) is connected properly to the DPC. ✅ Wall Ties • Install stainless steel wall ties at 450 mm vertically and 750 mm horizontally. • Important for structural integrity and load sharing. ⸻ 🧮 Soil and Load Considerations Before building, always carry out a soil investigation to determine the safe bearing capacity. This ensures your foundation can safely support the weight of the structure. If the ground has poor load-bearing capacity, you may need deeper footings or reinforced solutions. ⸻ 🏁 Final Thoughts: Build Strong from the Ground Up Cavity walls are a smart choice for modern construction—but they’re only as effective as the foundation they sit on. A properly designed foundation ensures durability, insulation, and resistance to damp and structural movement. Whether you’re a builder, homeowner, or DIY enthusiast, taking time to get the foundation right is always worth it.