Strip foundations are the simplest and cheapest form of foundation for modern detached houses. They support all load-bearing walls, including external cavity walls and some internal walls that help support floors. Foundations are formed using concrete poured into excavated trenches. Proper excavation depth is important and ensured using tools like boning rods and levels. Foundation width depends on building load and ground conditions.

1. The foundation runs under all the
loadbearing walls.
Strip Foundations
A typical modern detached house will,
where possible, sit on strip foundations.
Providing the ground is suitable, this is
the simplest and cheapest form of
foundation.
In this example this includes the external
walls……
and the internal walls
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2. The drawing on the left shows a
foundation plan superimposed on the
house plan. As stated on the previous
page, the foundation supports all the
loadbearing walls. This includes the
external cavity walls (275mm wide),
and the internal walls (100mm wide).
The internal walls are loadbearing
because they help support the 1st
floor joists. All the upstairs internal
walls are non loadbearing - this is the
norm in modern houses.
In modern construction foundations
are invariably formed in concrete.
This is usually brought onto the site
ready mixed; - but first the
foundations must be excavated.
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3. Once the the topsoil has been removed (and
levels reduced if necessary), the
foundation can be set-out by surveyors or
engineers. The graphic on the left shows
the foundation plan superimposed over the
site.
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The line of the foundations can be
highlighted using hydrated lime -
here the trench centre line is being
marked. On some sites you will find
both sides of the trench marked.
Digger buckets are available in
various widths to suit specific
trenches.

5. Machine excavation is the norm. Keeping the
trenches straight and keeping the dig the
right depth is harder than it looks. Boning rods
and various types of Level ensure the latter
(see next page).
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Part of table only
Foundation width depends on building load and
ground type. Here, the load is not more than
40 kN per metre run, and the ground is firm
clay. Hence the foundation width is 450mm.

6. Getting the right depth….
Establish site rails at known
level, say ‘X’ metres. If Boning
rod is 1.50 metres long, then
depth of trench is ‘X’ minus
1.50metres.
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Boning rod
Site rail
Site rail
Correct depth can be
checked with a Level
and staff.

7. When trenches have been excavated they
should be checked for ‘square’ etc - mistakes
can be expensive once the concrete has been
poured.
The trenches should be cleaned out and
‘bottomed-up’ (cleaned out and loose material
removed), ready for Building Control
inspection.
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Provision may have to be made for services
which need to cross the trench - this depends
on whether it is a traditional strip foundation
or trench fill. In traditional strip foundations
the services usually cross the trench above
the concrete - in trench fill they cross
through it.
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Traditional strip
Trench fill

9. This is a simple cross section at A - A. The
section shows the two external walls and one
internal wall. Note that the internal wall is a
single leaf of blockwork (100mm wide). In
theory the width of the foundation width can
sometimes be less for these internal walls. In
practice, it’s sometimes easier and cheaper to
provide a bit of extra concrete rather than
waste time changing the digger bucket.
A A
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10. This is a simple cross section at A - A. The
section shows the two external walls and one
internal wall. Note that the internal wall is a
single leaf of blockwork (100mm wide). In
theory the width of the foundation width can
sometimes be less for these internal walls. In
practice, it’s sometimes easier and cheaper to
provide a bit of extra concrete rather than
waste time changing the digger bucket.
A A
End