If you are planning an extension or new build around Northamptonshire, your foundation choice often comes down to a traditional strip footing or trench fill. They start from the same trench but finish very differently, and the right one depends on your soil, your depth and how much you want to spend on labour versus concrete.
A strip foundation is a relatively thin layer of concrete poured in the bottom of the trench, usually around 225mm to 300mm deep. The wall is then built up from that strip in brick or block below ground until it reaches floor level.
Trench fill does what the name suggests: the trench is filled almost to the top with concrete, leaving just a couple of courses of blockwork to reach ground level. You pour far more concrete but lay far fewer underground blocks.
The honest answer is that one saves concrete and the other saves labour. A traditional strip uses less concrete but needs a bricklayer working down in the trench, which is slow and weather dependent. Trench fill costs more in ready-mix but gets you out of the ground in a day, which is why it has become the default on many local jobs.
As a rough guide, deeper foundations tip the balance towards trench fill, because the cost of laying block below ground starts to outweigh the extra concrete.
Northamptonshire has a lot of shrinkable clay, and clay moves as it dries and wets. Building Control will often want foundations at 1m or deeper near trees, and the NHBC tables push depths further still depending on the species and distance of nearby trees and hedges.
Once you are down at 1m or more, deep narrow trench fill is usually the practical choice. Where there are trees close by you may also need compressible material against the side of the concrete to allow for clay heave, which your building inspector will specify.
Strip foundations still make good sense on firm, stable ground where the dig is shallow, on smaller garden walls and outbuildings, and where you have the time for bricklaying below ground. They can be the cheaper option overall when concrete volumes would otherwise be large.
On most house extensions in town, though, the speed and simplicity of trench fill tends to win once labour and trench safety are factored in. The trench spends less time open, which matters in wet Northampton winters.
It depends on depth and ground. Strip uses less concrete but more bricklaying labour, while trench fill costs more in concrete but far less in time, so the deeper the foundation the more trench fill usually wins.
That is decided by your soil and any nearby trees, and confirmed by Building Control. On Northamptonshire clay near trees it is commonly 1m or more, while firm ground with no trees may allow much shallower footings.
Yes. A Building Control inspector should check the open trench and depth before any concrete goes in, and on clay sites they may also call for compressible board to allow for ground movement.
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