Strip Foundation: Design, Sizing and When to Use One

A load-bearing wall carries its load continuously along its length, so the ground beneath it needs to receive that load continuously too. A strip foundation does exactly that: a continuous band of concrete, wider than the wall it supports, running underneath it and spreading the load over enough soil to carry it safely. It is one of the oldest and simplest forms of foundation, and in the right ground it remains one of the most economical.

This article explains how strip foundations work, how their width is sized, a worked example in plain numbers, the common variants, where they suit and where they do not, and the mistakes that lead to cracking and settlement. It is written for students and homeowners who want to understand what lies under a wall and why.

How a strip foundation works

The intuition is the same as wearing snowshoes. Your weight does not change, but spreading it over a larger area lowers the pressure on the snow, so you do not sink. A wall bearing directly on soil would concentrate its load on a narrow line and press into the ground. Widening the base reduces the bearing pressure to a level the soil can sustain without failing or settling excessively.

A strip foundation is a type of shallow foundation, meaning it transfers load to soil near the surface rather than to deeper strata. Our article on deep vs shallow foundations explains the distinction, and the wider overview in foundations explained places strips alongside pads, rafts and piles.

Sizing the width

The governing principle is that the pressure under the foundation must not exceed what the ground can safely carry. Working per metre run of wall:

Required width B = (wall load per metre + foundation self-weight per metre) / allowable bearing pressure

The allowable bearing pressure depends on the soil, its strength and compressibility, the water table, and how much settlement the building can tolerate. It should come from site investigation and an engineer’s assessment, not a generic table, because two sites a short distance apart can differ greatly. Our article on soil bearing capacity explains how it is established.

A worked example

Suppose a masonry wall delivers 60 kN per metre run at foundation level, the strip is to be 300 mm thick, and the ground investigation supports an allowable bearing pressure of 150 kN/m² (150 kPa) for this case.

Initial estimate ignoring self-weight: 60 / 150 = 0.40 m.
Self-weight, assuming a trial width of 0.6 m for a 0.3 m thick concrete strip at about 24 kN/m³: 0.6 × 0.3 × 24 ≈ 4.3 kN/m.
Revised width: (60 + 4.3) / 150 ≈ 0.43 m.

The calculated width is small, but practical considerations often govern. The strip should be wide enough for the wall to sit centrally with a projection either side, wide enough to excavate and construct properly, and thick enough for the load to spread through it without the projecting edges cracking. In practice a width of around 0.6 m might be adopted for this wall, subject to the engineer’s checks.

Thickness and reinforcement

Load spreads through concrete at an angle, so the projection of the strip beyond the wall face is generally kept modest relative to its thickness. A traditional rule of thumb for plain concrete strips is that the projection should not exceed the thickness, so that the base does not bend and crack. Where strips are wide, carry heavy loads or must span soft spots, they are reinforced with longitudinal and transverse bars, and the design checks bending and shear across the strip just like a small beam.

Depth and the ground

The base of the strip must sit on competent, consistent ground. That means below topsoil, made ground, roots and organic material, and deep enough that seasonal moisture change, nearby excavation or surface erosion will not disturb the bearing layer. In shrinkable clays, depth is also influenced by nearby trees, whose roots dry and shrink the soil.

Ground conditions decide whether a strip foundation is suitable at all. They work well on firm clays, dense sands, gravels and weathered rock close to the surface. In Singapore, many sites have soft marine clay or deep fill, and on such ground shallow strips can settle excessively; piled foundations or rafts are often used instead. Every site must be judged on its investigation. Where settlement has already occurred, our article on foundation settlement describes the signs and causes.

Types of strip foundation

Traditional plain concrete strip. A mass concrete band with a masonry wall built off it. Common for low-rise masonry housing on good ground.
Reinforced strip. Used where loads are heavier, ground is more variable, or the strip must bridge local weaknesses.
Wide strip. A wider, reinforced strip for weaker soils, spreading load over a larger area while staying shallow.
Deep or trench-fill strip. The trench is filled with concrete almost to ground level. It avoids building masonry below ground and can reach a deeper bearing layer, at the cost of more concrete.
Stepped strip. On sloping sites, the strip steps down in increments, with overlaps at each step so the foundation remains continuous.

Strip foundations compared with alternatives

A strip foundation is matched to linear loads: load-bearing walls. Where the structure is a frame of columns, isolated pad foundations are usually more efficient. Where strips or pads become so wide that they nearly touch, or the ground is weak and variable, a raft that covers the whole footprint may be better. Where competent ground is deep, piles take the load down to it. The decision follows the loads, the ground and the tolerance of the superstructure to movement.

Common mistakes

Assuming a bearing pressure without investigation. Generic values are no substitute for knowing the soil on the site.
Founding on fill or topsoil. Made ground and organic layers compress unpredictably.
Mixing foundation types under one wall. Joining a new strip to an existing piled or deeper foundation, as in an extension, invites differential settlement and cracking at the junction unless designed for.
Ignoring water. Leaking drains, soakaways and changing water levels can soften the bearing layer.
Leaving trenches open. Exposed formation softens in rain or dries and cracks; concrete should be placed promptly on a clean, inspected base.
Too thin a projection. Wide unreinforced strips crack along the wall face.

Frequently asked questions

What is a strip foundation used for?

A strip foundation supports a continuous linear load, most commonly a load-bearing wall. It spreads the wall’s load across a wider band of soil so that the bearing pressure stays within what the ground can safely carry without excessive settlement.

How wide should a strip foundation be?

The width is found by dividing the load per metre run, including the foundation’s own weight, by the allowable bearing pressure of the soil. Practical construction needs and a sensible projection either side of the wall often lead to a wider strip than the calculation alone suggests.

How deep does a strip foundation need to be?

It must reach firm, consistent ground below topsoil, fill and the zone affected by seasonal moisture change, roots or nearby excavation. The appropriate depth depends on the soil and site conditions and should be set by the engineer from investigation.

Are strip foundations common in Singapore?

They are used where competent ground is near the surface, such as for light structures, boundary walls and some low-rise buildings. Many Singapore sites have soft clay or fill, however, and on such ground piles or rafts are often more suitable to control settlement.

What is the difference between a strip and a pad foundation?

A strip foundation is continuous and supports a wall along its length. A pad foundation is an isolated block, usually square or rectangular, that supports an individual column. The choice follows whether the structure delivers its load in lines or at points.

Planning an extension, boundary wall or new structure and unsure what foundation the ground can support? Talk to our consultancy or get in touch.