NHBC Standards 2006

4.4 Strip and trench fill foundations


4.4 - S1Sitework standards
4.4 - S2Setting out foundations
4.4 - S3-S8Excavations
4.4 - S9-S10Services and drainage
4.4 - S11-S12General construction
4.4 - S13Strip and trench fill foundations

4.4 - S1
All sitework shall:
(a) meet the Technical Requirements
(b) take account of the design
(c) follow established good practice and workmanship

Sitework that complies with the design and the guidance below will be acceptable for both strip foundations and trench fill foundations.


4.4 - S2
The setting out of foundations shall take account of the design details

The accuracy of setting out should be checked by control measurements of trenches, including their location relative to site boundaries and adjacent buildings. Levels should be checked against bench marks, where appropriate.

In particular, for excavations check:

  • trench lengths
  • trench widths
  • length of diagonals between external corners.

Setting out foundations

Walls should be located centrally on the foundation, unless specifically designed otherwise.

Any discrepancy in dimensions should be reported promptly to the designer. Resulting variations should be distributed to all concerned with sitework, including NHBC, where appropriate.


4.4 - S3
Excavations for foundations shall take account of design dimensions

Excess excavation should be avoided. Inaccuracy may prevent walls and piers being located centrally and therefore result in eccentric loading of foundations and possible foundation failure.

Accurate trench digging is particularly important where the width of the foundation is only slightly wider than the wall to be supported.

Any ground condition that might cause the foundation design to be modified should be reported promptly to the designer.

4.4 - S4
Excavation shall be to a depth that gives adequate bearing and protection from frost damage

To avoid damage from frost action, the depth of foundation in frost susceptible ground should be at least 450mm below ground level. If finished ground level is to be above existing ground level then, in cold conditions when freezing is expected, the foundation depth should be taken from the existing, not finished, ground level.


4.4 - S5
Excavation in shrinkable soil shall take account of the foundation design

The design should specify the minimum foundation depth. In shrinkable soils, the minimum foundation depth should be as in the following table:

Volume change potentialMinimum depth (m)

These minimum depths may only be used where any existing or proposed trees or shrubs are outside the zone of tree influence (See Chapter 4.2 'Building near trees' (Design)).

4.4 - S6
Excavations shall take account of localised effects

Where localised changes in strata give rise to differences in bearing capacity, special precautions will be necessary and reference should be made to the designer.

At soft spots, excavations should be deepened locally to a sound bottom or, alternatively, the concrete should be reinforced.

Hard spots should be removed.

Where roots are visible on the sides or bottoms of trenches (especially in clay soils), excavations may need to be taken deeper, or special precautions determined by an Engineer in accordance with Technical Requirement R5.

On sites where there are or have been trees, foundations constructed in accordance with the guidance given in Chapter 4.2 'Building near trees' will be acceptable to NHBC.

4.4 - S7
The shape of the trench shall not impair the performance of the foundation

Unless otherwise designed by an Engineer in accordance with Technical Requirement R5, trench bottoms should be horizontal with all loose material removed. Trench sides and steps should be, as near as possible, vertical.

Shape of trench

4.4 - S8
Trench bottoms, when prepared for concreting, shall be compact, reasonably dry and even

If any part of a trench bottom is affected by rainwater, ground water or drying, it should be re-bottomed.

Trenches should be kept free of water.


4.4 - S9
Existing services shall be adequately protected

Any existing services, such as cables, water pipes or gas mains, may need to be supported and protected.

Drains which are redundant should be cut open and filled or removed.

Any existing drains should be diverted or adequately protected.

Services should not be rigidly encased in the foundations.

Ground water drains should be diverted.

Ground water drains diverted

4.4 - S10
Provision shall be made for service entries or services to safely pass through, or above, foundations

For details of underground drains and services, reference should be made to Chapters 8.1 'Internal services' (Design and Sitework) and 5.3 'Drainage below ground' (Design and Sitework). Reference should also be made to Chapter 5.1 'Substructure and ground bearing floors' (Design and Sitework).


Services should not pass through strip foundations but through the masonry above. Adequate lintels should be provided in the masonry. Reference should be made to Chapter 5.1 'Substructure and ground bearing floors' (Design and Sitework).


Where services pass through trench fill foundations, they should not affect the ability of the foundations to carry loads. Services should be either sleeved or passed through a suitably strengthened opening in the foundation. This is to ensure that differential movement will not damage services.

In the case of drains, it is important to leave sufficient space for movement to ensure that the drain is capable of maintaining line and gradient.

Maintain space for movement with drains


4.4 - S11
Concrete shall be correctly mixed, placed and cured

Concreting should be carried out, as far as possible, in one operation, taking account of weather conditions and available daylight. Concrete should be placed as soon as possible after the excavation has been checked.

Mixing, placing, testing and curing of concrete should be carried out as indicated in Chapter 2.1 'Concrete and its reinforcement' (each section), and for work carried out in cold weather, Chapter 1.4 'Cold weather working'.

The foundation thickness should be:

  • 150mm to 500mm - for strip foundation
  • not less than 500mm - for trench fill foundations.

Where trench fill foundations are in excess of 2.5m depth, they must be designed by an Engineer in accordance with Technical Requirement R5.

For trench fill, it is particularly important to check that the finished foundation level is correct and horizontal. It will be difficult to adjust for discrepancies in the small number of brick courses (possibly only 6) between foundation and dpc level.

Trench fill foundations

4.4 - S12
Strip and trench fill foundations shall be reinforced, where necessary, to suit localised ground conditions

Reinforcement, if needed, should be clean and free from loose rust and should be placed correctly. Bars, of an appropriate size, should be properly supported to ensure that they are 75mm above the base of the foundation or as indicated in the design. They should be secured at laps and crossings.

If in doubt about any soft spots, the designer's advice should be taken before placing the concrete.

Concrete trench fill


4.4 - S13
Strip and trench fill foundations shall be constructed to take account of the foundation design

Items to be taken into account include:

(a) construction joints


If construction joints are unavoidable, they should not be positioned near a return in the foundation. All shuttering should be removed before work continues beyond the construction joint. For strip foundations, construction joints may be formed by one of the methods shown below.

Strip foundation using reinforcing bars

Strip foundations expanded metal lath


It is important that concrete mix, workability and placement are maintained throughout a trench fill foundation. However, where a joint is unavoidable, it should not be positioned near a return in the foundation. Before work continues beyond the construction joint, all shuttering should be removed. Construction joints may be formed by one of the methods shown below.

Trench fill foundations corrugated metal former

Trench fill foundations expanded metal lath

(b) stepping of foundations

Sloping ground may require stepped foundations.

Where foundations are stepped, the height of the step should not exceed the thickness of the foundation, unless it forms part of a foundation designed by an Engineer in accordance with Technical Requirement R5.

Foundation bottoms should be horizontal and steps, as near as possible, vertical.


The overlap should be not less than:

  • 2 x S, or
  • T (maximum 500mm), or
  • 300mm,

whichever is the largest.

Strip foundation overlap


The overlap should be not less than:

  • 2 x S, or
  • one metre,

whichever is the larger.

Trench fill foundation overlap