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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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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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.

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.

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 bechecked with a Leveland staff.

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.

duct

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Traditional strip

Trench fill

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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