Foundations for a Single Storey SIPs Extension

Lighter walls, same ground

A 142 mm SIP wall panel weighs roughly 20 to 25 kg per square metre before cladding. A brick outer leaf, a block inner leaf and the plaster come to nearer 400 kg. Add the roof and a single-storey SIPs wall puts a very small line load on its footing.

Approved Document A sets minimum strip widths by soil type and wall load, and a SIPs extension sits at the light end of that table. So the trench can often be narrower than a bricklayer would expect, and you pour less concrete.

Depth is a different question. The same document sets 450 mm as the minimum to stay below frost and asks for more in shrinkable clay, where 750 mm is the usual starting point. Near trees on shrinkable clay the NHBC's published tables, which most building control officers follow, can call for two metres or more. A light building is, if anything, more easily lifted by swelling clay than a heavy one. Do not let anyone tell you SIPs need "hardly any foundations".

Strip and trench fill

The standard answer. A trench 450 to 600 mm wide is dug to the agreed depth, inspected by building control, and filled with concrete either part way (strip, with blockwork up to ground level) or almost to the top (trench fill). A ground-bearing slab or a suspended floor then spans between the walls.

It suits most gardens, any groundworker can build it, and it is the cheapest route on firm soil. It produces a lot of spoil: a 4 m by 6 m extension dug a metre deep fills several skips.

Raft

A reinforced concrete slab, thickened at the edges, that carries the whole extension as one piece. Engineers choose it for made ground, soft clay and sites where strips would need to be impractically deep. An insulated raft, cast inside a tray of structural insulation, gives the floor its U-value at the same time and removes the cold edge where slab meets wall.

A raft is always engineer-designed, and the edge detail has to suit the sole plate, so the SIPs supplier and the engineer need to speak before the steel is ordered.

Screw piles

A compact rig screws steel shafts, each with a helical plate near the tip, down to the resistance the engineer has specified. A steel or timber ring beam sits on the pile heads and carries an insulated timber floor and the panels above.

This is where the low weight of SIPs pays off. Piles for a light timber structure are short and few, an extension's worth goes in within a day, there is no spoil and no wait for concrete to cure. They work in gardens a digger cannot reach and between tree roots you want to keep. They cost more than a simple strip, they must be designed and load-tested to the engineer's figures, and the floor above is suspended, with a ventilated void beneath it.

The sole plate, and why level matters

Whatever is in the ground, the panels start from a treated timber sole plate, fixed down over a damp-proof course. Structural timber guidance keeps the underside of that timber at least 150 mm above finished ground level, so the slab edge or a course of brick or block is brought up as a kerb. Agree the height early: it sets your door thresholds.

Panels are factory-cut to the millimetre and cannot be trimmed to suit a wavy slab. Suppliers typically want the base level to within about 5 mm over its length and square across the diagonals. Ask for the figure in writing and have the groundworker work to the supplier's setting-out drawing, not the architect's plan.

Under the floor, a damp-proof membrane linked to the wall DPC keeps ground moisture out, as Approved Document C requires, and enough insulation goes in to bring the floor to 0.18 W/m²K.

Neighbours and sewers

Digging within 3 m of a neighbour's building, to a level below the bottom of its foundations, triggers the Party Wall etc. Act 1996. You serve notice at least a month before starting. The Act covers England and Wales.

If a public sewer runs within 3 m of the footprint you will need a build-over agreement from the water company. Find the drain runs before the design is fixed; moving an extension 500 mm on paper is free.