Foundations for a Timber Frame House in Ireland

The foundation is the part of the house your frame supplier will not build and your budget will not forgive. On an Irish rural site it is also the item most likely to move.

Start with the site investigation

Before any foundation decision, you need to know what is under the topsoil: soil type, bearing capacity, water table, and whether there is rock or peat. A site investigation is a small cost against the risk it removes, and your engineer cannot design without one. On a rural greenfield site it is not optional.

The three options

Insulated raft

A reinforced slab cast on a continuous layer of insulation. It is the natural partner for timber frame: no perimeter cold bridge, the floor and the foundation in one operation, and a flat, accurate surface to set the frame out on. Underfloor heating pipes go in before the pour.

• Good for: level or gently sloping sites with reasonable bearing
• Watch: every service penetration has to be set out before the pour — there is no second chance
• Best for: Part L performance, because the floor perimeter is where heat quietly leaves

Strip foundation with a suspended or ground-bearing floor

The traditional Irish approach. Works on slopes, tolerates variable ground, gives you a rising wall to level off. More operations, more cold bridge detail to resolve at the wall-floor junction.

Piles

Where bearing is poor — peat, made ground, deep soft strata — piles with a ground beam or a piled raft are the answer. Expensive, definitive, and decided by the engineer rather than by preference.

Tolerance: the part people miss

A factory-made panel is manufactured to the millimetre. It will not stretch to meet a foundation that is 30 mm out of square, and packing it out defeats the airtightness detail you paid for. Frame suppliers issue a setting-out drawing with a tolerance — typically far tighter than a blockwork build assumes — and the groundworks contractor has to be told about it before he prices, not after he pours.

Sequence

1. Site investigation and engineer’s foundation design
2. Frame supplier issues setting-out, loads and tolerances
3. Groundworks, drainage and radon barrier
4. Insulation, reinforcement, service ducts, UFH pipes
5. Pour and cure
6. Survey the finished slab and confirm it against the tolerance before the frame is delivered

That last step takes an hour and is skipped constantly. Do it.

Frequently asked questions