Structure and support · Foundations & Ground
Strip Foundation System
A reference account of how a strip foundation is organised as an assembly - wall line, spreading element, bearing formation and the damp and thermal continuity where the wall reaches ground level - and what an owner should raise with a structural engineer.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What strip foundation is
A strip foundation is the shallow foundation used where load arrives along a line rather than at a point. The wall above is continuous, so the element beneath it is continuous too: a band of concrete, reinforced where the design calls for it, running under the wall and wider than it, so that the pressure the wall carries is handed to the ground over a broader footprint.
The boundary against its nearest sibling, the pad foundation, is settled by looking at what stands on it. Take the wall away and a strip has nothing left to do, because spreading a line of load is its only purpose; take the column away from a pad and there is still a base connection cast into it waiting for something. Line of load means strip; point of load means pad.
Traditional strip and trench fill are variants of the same idea rather than different systems. They differ in how much of the excavation ends up occupied by concrete and how much is rebuilt in masonry up to ground level, which changes buildability and the below-ground wall, not the load path or the reason the element exists.
A strip is also the place where a wall crosses from buried to exposed, so it carries duties that have nothing to do with structure. The damp-proof course in the wall has to relate to the finished external ground level, the floor damp and insulation layers have to arrive somewhere definite, and the ground returned against each face has to be reinstated without disturbing what it leans on.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- strip footing
- trench fill foundation
- wall footing
- continuous footing
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Receive the continuous line of load from a load-bearing wall and hand it to the ground over a band wider than the wall itself.
- Hold the wall at a founding level chosen for what the ground does, rather than for what is convenient to dig.
- Present a level, continuous surface from which the wall above can be set out and built accurately.
- Give the damp-control and insulation layers of the wall and the ground floor a defined place to meet at ground level.
- Give the whole wall line one continuous bearing element to stand on; whether that element can also span local variation in the ground depends on how it has been designed and reinforced.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportLoad-spreading strip
The continuous concrete band under the wall. It converts a narrow, heavily loaded wall base into a wider bearing footprint, and its width, depth and reinforcement are decided together with the ground beneath rather than from the wall above alone.
- SubstrateFormation and bearing stratum
The prepared ground surface the strip is cast against, and the material below it that actually resists the load. It is a component of the assembly even though nobody supplies it, because what the strip can do is governed by what it is standing on.
- Primary supportReinforcement, where the design calls for it
Steel placed within the strip so it can work across its width or along its length, typically where the ground is variable, where the strip is stepped, or where the wall cannot sit centrally. Whether any is needed is an engineering judgement rather than a default.
- SubstrateWall bearing face
The upper surface the wall is built off. Its level and flatness govern how accurately the first courses can be set out, and its position relative to the centre of the strip decides whether load arrives where the strip was designed to receive it.
- Control layerDamp-proof course junction
The place where the damp-proof course in the wall sits and where it has to be joined to the membrane in the floor. This is a control-layer duty carried by a structural element, and once the trench is backfilled it cannot be inspected again.
- Thermal layerPerimeter insulation continuity
The point where floor insulation, wall insulation and any edge insulation have to meet around the base of the wall. Where they do not meet, the strip becomes a path through the insulation line, and what that means in a given building is for the designer.
- ProtectionBackfill and reinstatement zone
The ground replaced against each face of the below-ground wall once the strip is built. It restrains the wall, carries surface water towards or away from it depending on how it is graded, and is where later excavation for services does its damage.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Where the wall sits across the strip is as much a part of the design as how wide the strip is. Centre the wall and pressure under the strip is broadly even; push it towards an edge and the strip is asked to work as a short cantilever in a direction it may not be reinforced for. Setting out and sizing are therefore a single decision, and a wall built off-position is a structural question rather than a tolerance to absorb.
The strip and the formation behave as a pair, and only one of them is specified. Concrete placed against a formation softened by rain, churned by plant or left open to weather is bearing on something other than what was investigated, and strength in the concrete does not recover it. Where the strip steps down a slope, the steps carry load between levels, so they belong to the load path rather than to the excavation.
Several continuity layers have to arrive at the same junction: the damp-proof course in the wall, the membrane in the ground floor, and the insulation line of each. If course and membrane are not joined into one another, the gap between them becomes invisible once backfilled but stays open for the life of the building. If insulation stops short of the wall base, the strip offers a path around it, and only a designer can say what follows.
Anything crossing the foundation line interrupts the element doing the spreading. A drain passing beneath the strip, a service entering through it, or a trench dug alongside it later all change the ground the strip was relying on, and how much that matters depends on where the opening sits relative to the wall above. Backfill returned unevenly to each face then loads the below-ground wall from one side, which is why reinstatement belongs to the design.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
These families are commonly encountered in the spreading element and its reinforcement. Which of them belongs in a particular strip, and in what arrangement, is a matter for the structural designer and the supplier documentation rather than for a reference entry.
Substrate
Masonry built off the strip up to ground level is commonly encountered in these families. Whether any of them belongs below ground in a given exposure and ground chemistry is for the designer to establish from the manufacturer information.
Control layer
These families are usually encountered where the damp control of the wall meets that of the floor. No single course or sheet makes a building resistant to ground moisture; that is a property of the completed, correctly joined and correctly installed assembly.
Thermal layer
Boards from these families are commonly encountered at the perimeter where floor and wall insulation meet the foundation. Whether a given board can be used below ground or beneath load depends on its product literature and on the designer assessment.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
The wall the strip exists for
The strip has no purpose without the wall, and the wall position, thickness and openings decide how load arrives. Concentrations under piers, at returns and beside wide openings are where a uniform line of load stops being uniform.
Read about Load-Bearing Masonry →Junction with the ground floor
Where the floor is ground-bearing rather than suspended, slab and strip meet at the wall base and their damp and insulation layers have to be joined there. Whether the slab bears on the strip or is kept clear of it is a design decision with consequences for movement.
Read about Ground-Bearing Slab →Ground-level damp continuity
This is the junction where buried and exposed parts of the wall meet, and where the relationship between the damp-proof course and the finished external ground level is set. Raising external levels later can defeat a detail that was sound when it was built.
Read about Damp-Proof Continuity →Drainage crossing the foundation line
Drain runs frequently pass beneath or through the foundation line, and the position of a crossing relative to the loaded wall above is what makes it routine or significant. Later trenching alongside a strip removes support the strip was relying on.
Read about Buried Drainage Network →Perimeter thermal continuity
The base of the wall is where the insulation lines of floor and wall have to be reconciled around a structural element running straight through them. What that junction should look like belongs with the designer and depends on the whole build-up.
Read about Thermal Bridging Control →Neighbouring foundations, boundaries and trees
Existing footings alongside a new strip, a shared boundary wall, or the roots and moisture demand of nearby trees all change the ground the strip depends on. These are site-specific matters for a structural engineer and, where boundaries are involved, for the relevant legal process.
Floor bearing and underfloor void at the wall base
Bears at or near the wall base and requires the underfloor space to be considered with the foundation.
Read about Suspended Timber Ground Floor →Ground Improvement
Continuous footings on treated ground bear across inclusions and the soil between them, so the relationship between footing width and inclusion pattern matters. A footing narrower than the pattern assumed can sit almost entirely on untreated soil.
Read about Ground Improvement →Precast Panel Structure
The lowest joint sets the geometry for everything above, because deviation there is inherited by every storey. Levelling, packing and the accuracy of the starter connections are decided before the first unit is craned into place.
Read about Precast Panel Structure →Beam and Block Floor
At ground level the units usually bear on foundation walls, so floor level, ventilation openings and the damp course are all settled by decisions taken in the foundation work long before any beam arrives on site.
Read about Beam and Block Floor →Gravity Retaining Wall
The footing under a mass wall is loaded eccentrically because the wall leans, so pressure is far from uniform across it. Its depth also has to sit below ground that changes with season and vegetation, which is a different criterion from that governing a building footing.
Read about Gravity Retaining Wall →Twin-Leaf Cavity Wall
Both leaves and the void between them land on the foundation, so the void's lowest exit and the ground outside are settled together.
Read about Twin-Leaf Cavity Wall →Masonry Veneer Wall
The veneer's weight goes into a foundation or a ledge projecting from it, and its lowest outlets sit just above whatever surface is outside. Ground, paving or planting raised against that line closes the drainage the wall depends on.
Read about Masonry Veneer Wall →Infiltration Drainage
An infiltration structure deliberately introduces water into the ground, and foundations, buried structures and slopes nearby respond to that. Its position relative to them is agreed with the structural and geotechnical advisers, not decided by where there happens to be room in a garden.
Read about Infiltration Drainage →Freestanding Wall
Wall and foundation are settled as a pair, because wind on the face reaches the foundation as overturning rather than as weight. Trees, made ground, seasonal shrinkage in clay and services crossing the line all belong to this interface, and they are usually what an owner is actually being asked about.
Read about Freestanding Wall →Underpinning
A strip footing is the element most commonly worked beneath, and its own entry describes it as cast before anything stands on it. Everything that makes it straightforward when new - open formation, no load, continuous pour - is absent here, and the differences are the reason this is a separate arrangement.
Read about Underpinning →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Sleeper Wall to Wall Plate
Where the intermediate bearing inside a suspended floor void carries the plate the floor members sit on, in a space that depends on air moving through it.
Structural transfer · Moisture · Movement · Buildability
Wall Base Damp-Proofing Junction
Where a wall's damp-proof course and a floor's damp-proof membrane must be joined so neither has an open end.
Moisture · Thermal continuity · Buildability · Structural transfer
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Ground water and soil moisture reach the strip from below and from each side, and the wall crossing it carries that exposure upward. Whether a given build-up manages it depends on the ground, on drainage around the building and on detailing, and is assessed by a qualified professional.
- Movement
- Shrinkable soils, nearby trees, made ground and seasonal wetting and drying all move the ground beneath a strip, and rarely by the same amount along its whole length. Founding level and reinforcement are the usual responses, and both are engineering decisions rather than defaults.
- Thermal
- The strip runs through the line where floor and wall insulation would otherwise be continuous. How that is handled, and whether edge insulation or a different foundation arrangement is preferable, belongs to whole-envelope design rather than to a foundation choice made alone.
- Buildability
- Trench stability, groundwater, access for concrete delivery and the weather an open formation has to survive all shape what is achievable. A strip that is straightforward on a dry, level plot can be the harder option on a sloping, wet or tightly bounded one.
- Interfaces
- Most of what goes wrong at a strip happens at its junctions: with the floor, with the drainage, with a neighbouring structure and with the reinstated ground. The field of the strip is comparatively simple; the ends, steps and crossings are not.
- Documentation
- Ground investigation records, the engineer foundation drawings and any record of the open excavation are the evidence of what the strip is bearing on. Once backfilled none of it can be seen again, so what was recorded is what is known.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Has the ground beneath the whole footprint been investigated, rather than assumed from a neighbouring plot or an earlier project?
- Is the wall centred on the strip everywhere, including at returns, piers and beside the widest openings?
- Where the ground falls, how are steps in the strip arranged, and what happens to the load path through them?
- How will the damp-proof course in the wall and the membrane in the ground floor be joined, and who is drawing that junction?
- Where do drains and incoming services cross the foundation line, and has that been agreed before anything is dug?
- What is the intended finished external ground level against each wall, and does the detail still work if it changes?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A deeper trench is not automatically a better foundation, because founding level is chosen for the stratum and the influences around it rather than as a margin.
- Concrete strength is not a substitute for a sound formation, since the strip can only deliver load into whatever it was cast against.
- Trench fill is not a different foundation system from a traditional strip; it is the same load path with more of the excavation occupied by concrete.
- A strip is not a damp-proofing product, because resistance to ground moisture is a property of the completed assembly around it and not of the concrete.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What ground investigation informed the founding level, and what did it find beneath each part of the footprint?
- Is reinforcement proposed anywhere in the strip, and what condition or risk is it there to address?
- How is the strip arranged where it steps, changes direction, or runs up against an existing foundation?
- Which drawing shows the junction between the wall damp-proof course, the floor membrane and the insulation line?
- What influence do the trees, hedges or recently removed vegetation on and around the site have on this design?
- What inspection of the open excavation is expected before concrete is placed, and who records the result?
What this page does not do
- Foundation design is site-specific and belongs to a qualified structural engineer working from ground investigation carried out on that plot.
- Nothing here indicates a founding level, a width, a reinforcement arrangement or a bearing pressure, all of which depend on ground that has been investigated.
- Excavating alongside an existing foundation, or building close to a boundary, can involve legal process and structural risk that needs to be resolved before work starts.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Substructure · Substructure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Foundation and Ground Interface Systems
Assemblies where a building first meets undisturbed ground — strip, pad, raft and piled foundations, pile caps, ground improvement and ground gas protection.
Browse all foundations & ground entries →