Detail Post – Foundation Details
The following post includes excerpts from our book Understanding Architectural Details – Residential, 4th Edition.
01 UAD Resi Banner

Understanding Foundation Details

The main role of foundations is to structurally support the building by transferring the loads of the building through the walls into the surrounding soil. In terms of a timber frame structure, the foundations must also protect the timber from moisture ingress by lifting the members above the ground.

The foundation must transmit the combined, dead and imposed loads on a building to the ground safely. 

The Building Regulations in the UK provide guidance on foundation design. You can find the Building Regulations here.

Types of foundation

There are four main types of foundations:

Strip foundation – the preferred and most common choice for low rise housing. Strip of concrete under all load bearing walls.

Pile foundation – Long concrete members take the load of the building through weak soils to load bearing strata.

Pad foundation – More commonly used under point loads, such as columns, but can be used under ground beams to transfer loads.

Raft foundation – Concrete raft which spreads the loads over the whole ground floor, 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.

02 Foundation types

Strip Foundations

Strip foundations are the common foundation design in residential construction. They consist of a strip of reinforced concrete formed centrally under a load bearing wall. 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.

03 Strip foundations

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 trench depth of 450mm is the minimum set out in the Building Regulation requirements, however, this is increased to 700mm if there is a danger of frost heave.

Given that most sands and gravels can support low rise housing with a concrete strip width of only 400- 500mm a cost effective alternative option is a trench fill method. This avoids the need for working space and can be a more economic option. The trench fill is a deeper concrete fill bringing it closer to ground level, where the wall is then constructed.

04 Simple strip foundation deatil 3dG01B

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 driven into the ground in the correct position, 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.

05 Pile foundation detail example G09

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.

06 Pad foundation detail example GC06

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 results in little if any 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.

07 Raft Foundation Example MG2A

Key Components of Foundation Detailing

A well detailed foundation comprises several critical components.

 

Site Investigation and Soil Analysis

Thorough site investigations and soil analyses are essential for assessing soil bearing capacity, settlement potential, and groundwater conditions. Geotechnical data informs foundation design decisions and ensures that the chosen foundation type is suitable for the site’s specific conditions.

 

Foundation Design and Engineer Input

Foundation design involves calculating structural loads, analysing bearing capacity, and determining appropriate foundation depths and dimensions. A structural engineer will need to calculate suitable foundation solutions based on the specific project requirements. Compliance with relevant UK building regulations and standards is essential to ensure structural stability, durability, and safety.

 

Foundation Type

Selecting the appropriate foundation type based on site-specific conditions, building loads, and construction methods is critical. Factors such as soil type, groundwater levels, and environmental considerations influence the choice of foundation type, with options ranging from shallow foundations for stable soils to deep foundations for challenging ground conditions.

 

Waterproofing and Damp Proofing

Incorporating effective waterproofing and damp proofing measures into foundation designs is essential to prevent moisture ingress, mould growth, and structural deterioration. Installing membrane systems, drainage solutions, and protective coatings mitigates the risk of water damage and enhances the durability and performance of the foundation.

Foundation Detail Examples

The following images provide examples of foundation details from the book Understanding Architectural Details – Residential [4th Edition].

08 Strip Foundation Detail MG1A
09 Strip Foundation Detail MG1A 3D

Strip Foundation Detail (MG1A)

10 Concrete Raft Foundation Detail MG2A
11 Concrete Raft Foundation Detail MG2A 3D

Concrete Raft Foundation Detail (MG2A)

12 Strip Foundation with Beam and Block Floor Detail MG4
13 Strip Foundation with Beam and Block Floor Detail MG4 3D

Strip Foundation with Beam and Block Floor Detail (MG4)

Regulatory Considerations for Foundation Details

Compliance with building regulations is a primary consideration in the design and construction of foundations in the UK. Key regulatory considerations include:

 

Building Regulations

UK building regulations govern various aspects of foundation design and construction, including structural stability, ground movement, waterproofing, and thermal insulation. Compliance with these regulations is mandatory to obtain building control approval and ensure the safety and integrity of the building.

 

Planning Permission

Obtaining planning permission for foundation works may be necessary, especially in sensitive or environmentally protected areas. Engaging with local planning authorities and stakeholders early in the design process can help navigate planning requirements and ensure compliance with local regulations.

Get the book today!

Understanding Architectural Details – Residential, is dedicated to residential construction. The book is packed full of over 120 construction details in 2D, and 120 construction details in 3D. We look at the principles behind construction detailing, and delve deep into each area of the building to make sure you have a full understanding of construction design.

More Foundation Detail Examples

If you are looking for reference details to use as a starting point in your own projects, our partner website Detail Library has a huge database of details ready for you to download and use. 

Below we share a few foundation detail examples from the Detail Library.

14 Strip Foundation with Timber Frame Detail DL71
15 Strip Foundation with Timber Frame Detail DL71 3D

Strip Foundation with Timber Frame Detail (DL71)

16 Strip Foundation Slab above insulation CLT construction DL207
17 DL207 CLT foundation detail concrete ground floor 3D

Strip Foundation, Slab above insulation, CLT construction (DL207)

18 Strip Foundation to 150mm cavity wall beam and block floor DL315
19 DL315A Partial fill 150mm masonry cavity wall, trench foundation with beam and block floor 3D

Strip Foundation to 150mm cavity wall, beam and block floor (DL315)

If you are interested in joining our community, head over to the Detail Library to learn more about our platform and get yourself signed up.

The Detail Library provides hundreds of construction details for you to download and use adjust for your own projects. The details are available in 2D Revit, CAD dwg, and 3D SketchUp. You can start downloading straight away and build your own detail reference library. Not like other BIM libraries, the Detail Library contains fully resolved details rather than individual components.

Summary

In conclusion, foundation detailing is a critical aspect of residential and small-scale construction projects in the UK, serving as the bedrock upon which buildings stand. Selection of the type of foundation depends on many factors, a good understanding of the site and conditions, along with structural requirements of your project is key.

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.

You might be interested in

We have loads of other incredible architecture content. Be sure to check it out.

Floor Details FI
Floor Details FI

Other recent posts…

Introduction to Beam and Block Floors

Introduction to Beam and Block Floors

What are Beam and Block Floors? There are two types of concrete floor system commonly used in residential construction; ground bearing and suspended. In this article we will be focusing on the suspended concrete floor, particularly beam and block floors. The suspended...

Detail Library – New Details May 2024

Detail Library – New Details May 2024

New Details Something a little different this month. We have been talking about a template of construction build ups for some time now, and it is finally here! The construction build up template provides a large range of different construction assemblies that can be...

Building Regulations for House Extensions

Building Regulations for House Extensions

Introduction to Building Regulations for House Extensions House extensions have become increasingly popular across the UK as homeowners seek to create more space and enhance the functionality of their properties. Whether it's to accommodate a growing family, create a...

3 Comments

  1. thanks Emma, Always a very clear description and visual treat.

    Reply
  2. I was looking for the detail at doorways and how the inner floor transfers out to the external leaf of the wall. That is passes over the gap between the leaves of the foundation wall if the gap is not fully filled. And in particular the treatment of the DPC at that point, including the door DPC. Everyone seems to avoid this detail.

    Reply
    • Hi Frank, threshold details are covered in our Residential Book 4th edition. We also have a number of threshold details on the detail library which you can see here:
      https://detail-library.co.uk/details/?s=threshold

      Hope this helps.

      Reply

Submit a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.