Every Arden & Oak building sits on a steel-anchored foundation, not a concrete raft. It's one of the most important decisions we've made about how we build — and once you understand how it works, the reasoning is straightforward.
What a steel-anchored actually is
A steel-anchored is a large, tapered steel pile — typically 1.2 – 2.4 m long, 76 mm outer diameter, galvanised. It has a helical flange at the tip that lets it drill itself into the ground under torque from a hydraulic head. A single trained installer can drive a screw to full depth in under 15 minutes.
How the foundation gets designed
A structural engineer works out the load path for the specific building — the point loads at the corners, the intermediate loads along the walls, the increased load at any twin-volume or double-glazing corner — and specifies the number, position and depth of screws required. Every screw is installed to a torque reading that verifies it has bitten into competent ground.
Why it beats concrete for a garden room
1. Speed and dryness
Ground screws are installed in a single day. Once they're in, the building can start immediately — no waiting 7–14 days for concrete to cure. A traditional concrete raft installed in wet British weather adds a further 3–5 days of curing under a tarp before you can build on it.
2. No excavation
A concrete raft requires digging out topsoil to a depth of 300–500 mm across the entire footprint, then removing that spoil from your garden. Ground screws leave your garden intact — the only disturbance is a 76 mm hole at each screw position.
3. Sloping gardens
This is the biggest single advantage. A sloping site that would require expensive retaining walls or a stepped concrete raft can be handled by simply driving each corner screw to a different depth. The final finished floor is dead level whether the garden slopes 100 mm or 800 mm across the footprint.
4. Reversibility
Ground screws can be unscrewed and removed if you ever sell the property and the new owners don't want the building. A concrete raft is permanent.
Where a concrete base might make sense
Concrete is still the right answer in a handful of edge cases — very poor bearing ground where screws can't torque properly, buildings with unusually high point loads, or sites where the customer specifically prefers a solid slab. In practice, on the domestic garden rooms we build, this comes up on well under 5% of sites.