Strip foundation design
A strip foundation carries a line load, the weight of a wall, and spreads it into a band of ground. We size the width, set the depth for the soil and detail any reinforcement, for homeowners, builders and developers across the UK.
How wide should a strip foundation be?
You divide the load carried by each metre of wall by the safe bearing pressure of the soil. For a normal two-storey house on firm ground that is usually 450 to 600mm, but soft ground can need a metre or more. We calculate it from your actual loads and a check of the soil rather than a standard width.
What a strip foundation carries
A strip foundation sits under a wall and turns a concentrated line of load into a pressure the ground can hold. The width is fixed by dividing the load per metre run of wall by the allowable bearing pressure of the soil, so a heavier wall or a weaker soil needs a wider strip. For a typical two-storey masonry house on firm ground that comes out around 450 to 600mm wide, but on soft clay the same wall can need a metre or more, at which point a plain strip may need reinforcement or a different foundation altogether.
The thickness of the concrete matters as much as the width. In an unreinforced strip the load spreads out from the wall face at roughly forty-five degrees, and if the projection either side is longer than the depth of concrete the strip will try to bend and crack. We keep the projection within the depth, or add reinforcement across the bottom, so the concrete works in shear and bending as intended rather than snapping off at the wall line.
Depth, clay and trench-fill
Depth is set by the ground, not the load. The foundation has to reach below topsoil, made ground, frost action and, on cohesive clay, below the zone where seasonal wetting and drying moves the soil. On a low or medium shrinkage clay that is often around 900mm to 1.2 metres, but a high shrinkage clay with mature trees nearby can push the required depth beyond two metres. We use the NHBC tables to tie depth to the clay plasticity and the tree influence rather than guessing.
Once a strip has to go deep, trench-fill usually wins. Instead of a thin strip at the bottom with block work built up through the trench, the whole trench is filled with concrete to just below ground level. It uses more concrete but far less labour, keeps operatives out of a deep excavation, and is quicker and safer in unstable ground. We advise which option is cheaper overall once the depth and ground are known.
Reinforcement, steps and detailing
Most domestic strips are mass concrete and need no steel, but reinforcement earns its place where a wide strip would otherwise crack in bending, where ground is variable and the foundation must bridge soft spots, or where heave protection creates a suspended condition. We add bottom bars to Eurocode 2, and on sloping sites we detail the strip in steps, keeping the overlap between levels at least the depth of the concrete or 300mm so load is transferred across the step without a weak point.
Detailing decides whether the design actually works on site. We set the concrete strength and cover for the ground aggressiveness, coordinate the foundation with drainage runs so nothing undermines it, and where a strip runs close to a neighbouring foundation or an existing drain we check the effect of one on the other. The output is a set of calculations and drawings to Eurocode 7 and Approved Document A that a groundworker can set out anywhere in the UK or Europe.
What we check.
The points our calculations resolve for a project like this.
- Strip width from the wall load per metre and the soil bearing pressure
- Concrete depth kept within the outstand so the strip does not crack in bending
- Foundation depth for clay shrinkage, frost and tree influence
- Choice between traditional strip and trench-fill on cost and safety
- Bottom reinforcement where the strip is wide or the ground is variable
- Stepped foundations on sloping ground with adequate lap between levels
Issued drawings, redacted.
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
How wide should a strip foundation be?
You divide the load carried by each metre of wall by the safe bearing pressure of the soil. For a normal two-storey house on firm ground that is usually 450 to 600mm, but soft ground can need a metre or more. We calculate it from your actual loads and a check of the soil rather than a standard width.
What is trench-fill and when is it better?
Trench-fill means filling the whole trench with concrete to near ground level instead of laying a thin strip and building masonry up through the trench. It uses more concrete but is faster, safer and better in deep or unstable ground, so it is often cheaper overall once foundations go beyond about a metre deep.
Does a strip foundation need reinforcement?
Usually not on firm ground with normal loads, where mass concrete is enough. We add bottom reinforcement when the strip has to be wide, when the ground is variable and the foundation must bridge soft patches, or where heave protection leaves the strip spanning. We design any steel to Eurocode 2.
How do you deal with a sloping site?
We step the foundation down the slope in level runs rather than pouring it on an incline. Each step overlaps the one below by at least the depth of the concrete or 300mm, whichever is greater, so the load carries across the change in level without a weak joint.
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