Slab depth for an extension
For a ground-floor extension in Ireland, the concrete slab should be at least 150 mm thick, on at least 200 mm of compacted hardcore (Building Regulations TGD C). That's 350 mm before the insulation, the screed and the floor finish.
The total depth is worked back from the finished floor level of the house, so the new floor lines up with the old one. The insulation thickness comes from the U-value: 0.18 W/m²K, or 0.15 with underfloor heating (TGD L).
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The build-up, layer by layer
| Layer | Minimum or rule |
|---|---|
| Floor finish | Per the product |
| Screed | 65 mm floating sand and cement in a house; liquid screeds thinner |
| Insulation | Whatever reaches 0.18 W/m²K, or 0.15 with underfloor heating |
| Concrete slab | 150 mm, thicker if the design needs it |
| DPM and radon barrier | DPM continuous with the wall DPCs; radon barrier and sump in a High Radon Area |
| Blinding | Enough to fill the voids |
| Hardcore | 200 mm, gas permeable (T2 Perm) |
Matching the existing floor
The finished floor of the extension has to meet the house's floor, so the dig depth is set from the top down: the floor finish, the screed, the insulation, the slab and the hardcore. If the old floor has little insulation and the new one needs 0.18 W/m²K, the extension's floor zone can be deeper than the old one.
Where a step or a ramp isn't wanted, a thinner screed can help: liquid screeds go down thinner than sand and cement. See screed depth over underfloor heating on our sister site.
What else goes in before the pour
- The radon sump: TGD C expects a standby sump in every new floor, with a sealed barrier in High Radon Areas
- Drains and service ducts
- Edge insulation at the walls
- The hardcore docket, showing it meets S.R. 21 Annex E
Floors laid by the contractor who takes your call
Calls on this site ring JAM Floor Screeding: James Mulhall, based in Portlaoise, 25 years laying floors. Lines open Mon to Sat, 7am to 6pm. He pours concrete floors, slabs and foundations as well as screed. These are his jobs.
“JAM were punctual, professional and efficient and did an outstanding job on pouring our new floor as part of a renovation. Very highly recommend!”
“Excellent work, laser level finish, neat and tidy worker, highly recommend.”
“Top class work by james and his crew, highly recommend”
Quoted word for word from JAM Floor Screeding's Google reviews. Read them on Google
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Questions people ask
How thick should an extension floor slab be?
At least 150 mm of concrete under TGD C, thicker if the structural design needs it, on at least 200 mm of compacted hardcore.
How deep do I dig for an extension floor?
From the finished floor level down: the floor finish, the screed, the insulation, 150 mm of slab and 200 mm of hardcore. 350 mm of that is slab and hardcore.
What U-value does an extension floor need?
0.18 W/m²K, or 0.15 W/m²K where the heating is underfloor (TGD L 2022).
Does an extension need a radon barrier?
In a High Radon Area, a fully sealed barrier and a standby sump. Elsewhere, TGD C still expects a standby sump.
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