Garden Room Foundations: Ground Screws, Pads or a Concrete Slab?

Nobody admires a garden room for its foundations, but they decide whether the floor stays level, the timber stays dry and the doors keep closing sweetly for years. Here's how the main options compare, and how we choose between them.

What a good base has to do

• Carry the weight of the building and everything in it, including heavy gym equipment or a sauna.
• Stay level as the seasons change, without settling unevenly.
• Keep the timber floor clear of the ground, with air moving underneath.
• Deal with rainwater, so it drains away rather than collecting under the building.

Ground screws

Steel screws are driven into the ground at each support point, and the floor frame sits on top. There's very little digging, no concrete to cure and little waste to take away, which helps in gardens with difficult access. They're adjustable, so they suit gentle slopes well. Very stony ground or buried obstructions can make them harder to install, which is one reason we check the ground first.

Concrete pads

Individual concrete pads are cast at the support points, with the floor frame bearing on them. Pads need less concrete than a full slab and can be sized and spaced for heavier loads, such as a gym. They involve some excavation and time for the concrete to cure.

Concrete slab

A reinforced slab spreads the load over the whole footprint and gives a very solid base. It means more excavation, more concrete and longer curing, and it must be detailed so the timber structure sits above any splashing and damp. A slab can make sense where the ground is soft or variable, or where a design calls for it.

How we choose

We decide after surveying your garden, looking at:

• Soil and ground conditions, including any made ground or rubble.
• Slope. Sloping gardens may need taller supports, steps or retaining.
• Trees and roots, which affect where we can dig and how the ground may move.
• Drainage and where surface water goes.
• Access for machinery and materials.
• The load, from the building itself and from what will go inside it.

Questions to ask any builder

• Which base do you recommend for my garden, and why?
• Is the base included in the price, and what happens if ground conditions are worse than expected?
• How will the floor be ventilated and kept clear of the ground?
• Where will rainwater from the roof go?

Building near Exeter? Our Exeter and East Devon page covers local access and groundworks. Or tell us about your garden.