Introduction

Foundations are one of those topics that can feel surprisingly difficult to get to grips with partly because they’re underground and invisible once built, and partly because the choice of foundation type depends on ground conditions that aren’t always obvious until you start looking into them properly.

If you’re working on a residential project and trying to understand how foundations work, what the different types are, when each one is used, and how the detail goes together at ground level, this post will walk you through the main options clearly.

We’ll cover the four main foundation types used in UK residential construction, the key considerations that inform foundation design, and some detail examples from our book Understanding Architectural Details – Residential to give you something practical to look at alongside the explanations.

In this post:

• Types of foundations: strip, trench fill, pile, pad and raft
• When each type is used and why
• Key components of foundation detailing (site investigation, waterproofing, damp proofing and thermal performance)
• Foundation detail examples
• Regulatory considerations, including radon protection and the Party Wall Act
• 2D and 3D detail examples from the book and Detail Library

Want the full set of foundation details alongside this post? 

You can download a free sample of Understanding Architectural Details – Residential to see how the construction details are set out and what the book covers in full. 

Types of foundation

In domestic construction, strip, trench fill, raft, pad and piled foundations are the most common foundation solutions.

Strip foundation 

This is the preferred and most common choice for low rise housing. It is essentially a strip of concrete under all load bearing walls.

Trench fill foundation

A variation on the strip foundation, where the trench is filled almost to the surface with concrete rather than building masonry below ground.

Pile foundation

In this type, long concrete members called piles take the load of the building through weak soils to load bearing strata.

Pad foundation

This is more commonly used under point loads, such as columns, but can also be used underground beams to transfer loads.

Raft foundation 

This involves constructing a concrete raft which spreads the loads over the whole ground floor and is used where building loads are high, or ground conditions poor.

We will briefly look at each type of foundation in turn and provide examples of foundation details.

Strip Foundations

Strip foundations are the common foundation design in residential construction. They generally consist of a strip of mass concrete beneath load-bearing walls, although reinforcement may be required in some circumstances where the loads, a wide strip, or weaker ground call for it. The width of the concrete strip is determined by the structure it will be supporting, along with the bearing capacity of the soil in order for the load to be safely transmitted to the ground. 

The greater the bearing capacity of the soil, the less the width of the foundation. The following table (based on Approved Document Part A) suggests recommended minimum width of concrete strip foundations. 

The concrete strip also needs to be thick enough to spread the load without failing in shear. As a rule of thumb, the thickness should be at least equal to the projection of the strip beyond the face of the wall, and in any case not less than 150mm.

Trenches are excavated to a level of undisturbed compacted soil, where the concrete strip will be spread. The width of the excavated trench must allow for the bricklayer to build the wall off the strip, usually a minimum of 600mm. A depth of about 450mm to the underside of the strip is the usual practical minimum, to keep the foundation below the level affected by frost. On shrinkable clay soils the depth normally has to be greater, commonly 750mm or more, and deeper still (often over 1m) where trees are, or were recently, nearby, to stay below the zone of seasonal ground movement.

Trench Fill Foundations

Most sands and gravels can support low rise housing with a strip only 400-500mm wide, and on this kind of ground trench fill is often the more economical choice. It is a variation on the strip foundation and is now one of the most common approaches on UK housing sites. 

Instead of pouring a shallow strip and building masonry up through the trench, the trench is excavated narrow and deep and filled almost to the surface with concrete, with only a few courses of masonry below ground. This removes the need for a bricklayer to work in the trench, speeds up the build, and suits the deeper foundations that shrinkable clay and nearby trees often require. 

On shrinkable clay, seasonal wetting and drying made worse by nearby trees, or by trees that have recently been removed causes the ground to shrink and swell. NHBC Standards Chapter 4.2 sets out how foundation depth should increase with the soil’s shrinkability and the size and proximity of any trees. On cleared sites, a compressible material or void former (a proprietary ‘clayboard’) is often placed against trench fill foundations to accommodate ground heave.

Pile Foundations

When strip foundations are not suitable, a pile foundation system can be used. Pile foundations can be seen where trees or shrubs are in close proximity to the proposed structure, or where a site has been cleared of existing trees and could be susceptible to volume changes. Pile foundations transfer the load of the building to a depth that is capable of supporting the full loads of the structure. It is an expensive process, due to the requirement of specialist subcontractors.

Once the piles have been installed (either driven, or bored and cast in place) the tops are cut to the required level in order to have a concrete beam cast over them forming the base for the walls. The beam spans from pile to pile, therefore not requiring support from the soil below.

Pad Foundations

Pad foundations generally consist of a concrete square pad which supports ground beams onto which the structural walls can be built. Pad foundations differ from pile foundations as they do not extend to such depths as piles, and the width of the pad varies in order to distribute the loads to a greater area. Pits are excavated to the required depth, where the concrete is then cast. Brick or concrete piers are then built or cast on the pad foundations up to the underside of the concrete beams that support the walls.

Raft Foundations

Raft foundations may be used where soils consist of very soft clay, or other situations where strip, pad or pile foundations are not suitable. A reinforced concrete raft is designed to distribute the loads of the building over the whole area under the raft, which helps to reduce differential settlement. Two main types of raft foundation, the flat slab raft and the wide toe raft.

The flat slab raft is used for smaller buildings where the loads are comparatively small. It is of uniform thickness and consists of reinforcing to the top and bottom of the slab. Where loads on the foundations would require a thick slab, the wide toe raft is used. The wide toe has a reinforced stiffening edge beam, with a toe at the edge to provide a base for the outer leaf of a cavity wall.

Foundation Detail Examples

The following images provide examples of foundation details from our book Understanding Architectural Details – Residential.

The examples below also show different ways the ground floor sits on the foundations. A ground-bearing slab is cast directly on compacted hardcore over the ground, whereas a beam and block (suspended) floor spans between supports and is preferred where fill is deep or the ground is poor.

The detail examples above are taken from Understanding Architectural Details – Residential. The book covers foundation construction in more depth including additional detail types, full annotation, and downloadable 2D CAD and 3D SketchUp files. If you’d like to see what’s included, the free sample is a good place to start.

Understanding Architectural Details – Residential (4th Edition)

If this post has been useful, the book takes everything here much further.

Understanding Architectural Details – Residential covers foundation construction alongside every other key area of residential building — walls, floors, roofs, windows, and more. It contains over 120 construction details in 2D and 120 in 3D, all updated to reflect the current Building Regulations.

It’s the kind of resource that makes the construction side of a project feel more manageable — whether you’re drawing your first ground floor detail for a university project, or working out how a trench fill foundation connects to a timber frame wall in practice.

You can download a free sample to see how it’s put together before you decide.

What to do next

This post covers the main foundation types used in UK residential construction, the factors that influence which type you’d use, and the key considerations for foundation detailing. It’s a solid starting point.

If you want to build on it, Understanding Architectural Details – Residential goes into much more depth across the whole building — from foundations right through to roof level. It’s been used by architecture students and early-career architects across the UK as a practical reference they come back to on real projects.

Author

Written by Emma Walshaw, Architectural Technologist and founder of First In Architecture and Detail Library. Emma has written a number of books about construction and architectural detailing.

Connect with Emma on LinkedIn

You might also be interested in

You might also find these posts useful.