🏗️ 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.

Site Investigation For New Works

the basic objective of this form of site investigation is to collect systematically and record all the necessary data which will be needed or will help in the design and construction processes of the proposed work. The collected data should be presented in the form of fully annotated and dimensioned plans and sections. Anything on adjacent sites which may affect the proposed works or conversely anything appertaining to the proposed works which may affect an adjacent site should also be recorded. Procedures 1 . Desk study 2. Field study or walk-over survey 3. Laboratory analysis Desk Study ~ collection of known data, to include: • Ordnance Survey maps † historical and modern, note grid reference. • Geological maps † subsoil types, radon risk. • Site history † green-field/brown-field. • Previous planning applications/approvals. • Current planning applications in the area. • Development restrictions † conservation orders. • Utilities † location of services on and near the site. • Aerial photographs. • Ecology factors † protected wildlife. • Local knowledge † anecdotal information/rights of way. • Proximity of local land fill sites † methane risk. Field Study ~ intrusive visual and physical activity to: • Establish site characteristics from the desk study. • Assess potential hazards to health and safety. • Appraise surface conditions: * Trees † preservation orders. * Topography and geomorphological mapping. • Appraise ground conditions: * Water table. * Flood potential † local water courses and springs. * Soil types. * Contamination † vegetation die-back. * Engineering risks † ground subsidence, mining, old fuel tanks. * Financial risks † potential for the unforeseen. • Take subsoil samples and conduct in-situ tests. • Consider the need for subsoil exploration, trial pits and bore holes. • Appraise existing structures: * Potential for re-use/refurbishment. * Archaeological value/preservation orders. * Demolition † costs, health issues e.g. asbestos.

Site Analysis

Prior to purchasing a building site it is essential to conduct a thorough survey to ascertain whether the site characteristics suit the development concept. The following guidance forms a basic checklist: * Refer to Ordnance Survey maps to determine adjacent features, location,roads,facilities,footpaths and rights of way. * Conduct a measurement survey to establish site dimensions and levels. * Observe surface characteristics, i.e. trees, steep slopes, existing buildings, rock outcrops, wells. * Inquire of local authority whether preservation orders affect the site and if it forms part of a conservation area. * Investigate subsoil. Use trial holes and boring to determine soil quality and water table level. * Consider flood potential, possibilities for drainage of water table, capping of springs, filling of ponds, diversion of streams and rivers. * Consult local utilities providers for underground and overhead services, proximity to site and whether they cross the site. * Note suspicious factors such as filled ground, cracks in the ground, subsidence due to mining and any cracks in existing buildings. * Regard neighbour scale and character of buildings with respect to proposed new development. * Decide on best location for building (if space permits) with regard to `cut and fill', land slope, exposure to sun and prevailing conditions, practical use and access.

Advantages of reinforced concrete

1. As compression is mainly taken by concrete and tension by steel,so the combination is very economical. 2. It can can be easily made since materials are easily available almost everywhere. 3. It can be moulded to any shape and size. 4. It is water resistant,when rich and well proportioned mixes are used. 5. It is impermeable to moisture. 6. It does not require any painting. 7. It is not attacked by termites. 8. It is fire resisting and also earthquake resisting. 9. Its maintenance cost is very low.

10 things you should know about plaster

What thickness are plasters applied? Undercoat plasters are generally applied 11mm thick (for walls), or 8mm (for ceilings) and finish coat plasters at 2mm thick. Thistle Universal One Coat plaster is applied at 13mm thickness (for walls), or 10mm (for ceilings). Thistle Magnetic Plaster is applied at a minimum of 3mm. What plaster to apply to sand & cement undercoats? The best finish to apply depends on suction level of the background material. Thistle Multi-Finish or Thistle Durafinish would both be suitable. What plaster to use after DPC installed? Thistle Dri-Coat is cement based plaster for re-plastering after a damp proof course. Please refer to the plaster section of the White Book for further information if required. How to plaster an existing painted wall? If the paint is in very good condition in terms of quality, strength and adhesion etc, it may be possible to wire brush the surface using a suitable detergent and apply ThistleBond-it prior to the application of Thistle Multi-Finish. We suggest carrying out this procedure on a trial area to ascertain the suitability. Where would you use Thistle GypPrime? Thistle GypPrime is a suction control primer used to reduce suction on very dry backgrounds. It is normally diluted (up to 5 parts water to 1 part Thistle GypPrime) or undiluted if severe suction control is required. Plaster is applied after Thistle GyPrime has soaked into the background. How can plaster assist in reducing air leakage of a building? Using Thistle plasters to plug gaps and cracks is a quick and easy way of maintaining airtightness. Gyproc SoundCoat Plus is used as a parge coat when using a dot and dab method with plasterboard. Does plastering help increase thermal mass? Thistle plaster is ideal for use where thermal mass is an integral part of the design of a building. Plaster provides the desired decorative finish whilst also enabling efficient heat transfer between the air and fabric of the building. Can I plaster a damp wall? Not if the wall is subject to rising or penetrating damp. Yes, if there is just residual dampness after successful DPC treatment. The wall needs to treated to eradicate any damp issues such as a damp-proof course. Thistle Dri-Coat could be used following a successful DPC installation. Please refer to the plaster section of the White Book for further information if required. How should salt contamination on a background be dealt with? Any salts brought to the surface of the background during drying should be carefully removed. The background must be clean, sound and free from dust. Heavy salt contamination can cause persistent damp problems, so it is important to establish whether the salts are purely from the drying process or whether a more serious damp problem exists. See the White Book for more information. How much plaster is made in a typical week? Barrow-upon-Soar, the home of Thistle Plasters, produce around 440,000 bags of plaster a week – that’s enough to plaster 29,000 homes.

DEFECTS IN BRICKS

Common defects in brick work and cement mortar Raw materials like brick, sand, cement and water are of sub standard quality without qualifying any test. Bricks are not soaked in water properly. The joints in brick works are thicker, unfilled properly. Raking of mortar is not done when the mortar is green. Bricks bats are used in masonry work. Mortar is not mixed properly on platform. There is improper mixing of cement mortar There is excessive water content in mortar Brick work is not in plumb, level and straight line. Mortar is not according to the structural requirements. There are gaps between door/window frame and masonry. The holes of scaffolding are not filled in with mortar properly. Vertical joints in brick work are hollow. There is high suction of brick and less water retention of mortar There are uneven joints in brick works There are voids in vertical joints There are disturbances of brick works just after the layout Over thick joints reducing the strength of brick work.

Brick work according to cement mortar ratio

1. First class brick work in cement sand mortar 1:4 Brick walls constructed with cement sand mortar 1:4 ratio means 1 part of cement and 4 parts of sand. This mortar is of high strength and is recommended for following construction works. Multi storey buildings Heavy weight /load bearing walls In earth quake zones Brick pillars Where there is more vibration due to heavy traffic or factory. Boundary wall, courtyard wall parapet wall and other free end walls Partition or 4.5 inch thick walls Brick works used for drainage 2. First Class Brick Work in cement sand mortar 1:6 Brick walls constructed with cement sand mortar 1:6 ratio means 1 part of cement and 6 parts of sand. This mortar is of medium strength and is recommended for following construction works. Three storey buildings Load bearing walls Heavy and frequent rainy zones Foundation of multi storey buildings 3. First Class Brick Work 1:7 Brick walls constructed with cement sand mortar in the ratio of 1:7 means 1 part of cement and 7 parts of sand. This mortar is low in strength and is recommended for following works. Toe walls Single storey buildings Temporary structures Light weight walls having no load of beams etc.