Raft Foundation in Ireland — The All-Rounder for Class 3–4 Ground and Subfloor Insulation
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Raft on Class 3–4: €180–€260/m². Spreads the load across the whole footprint. Cheapest path to a high-performing subfloor.
A raft foundation in Ireland is a reinforced concrete slab that spreads the building load across the entire footprint. It is the all-rounder for Class 3–4 ground (firm clay, silt, sand with clay), at €180–€260/m². A 150 m² house: €27,000–€39,000. Raft is the standard for BER A1 builds because the subfloor insulation layer is continuous — underfloor heating integrates easily. Building Regulations 2023 TGD-A (Structure) governs. The site investigation (€1,500–€3,000) decides whether a raft is viable. Raft is faster to build than strip+wall on variable ground. Add €30–€60/m² for tanking if the water table is high. A radon barrier is mandatory on every raft in Ireland.
When this matters most
Your site has Class 3–4 ground, or you want a continuous subfloor insulation layer.
When this doesn't apply
Your site has Class 1–2 ground (strip is cheaper) or Class 5+ ground (need piled raft).
Sources
Building Regulations 2023, TGD-A (Structure). https://www.gov.ie/
Frequently asked questions
How much does a raft foundation cost in Ireland?
€180–€260/m² of footprint. For a 150 m² house: €27,000–€39,000. Add €30–€60/m² for tanking on a high water table. Radon barrier mandatory.
When is a raft foundation the right choice?
Class 3–4 ground (firm clay, silt, sand with clay), variable ground conditions, or when you want continuous subfloor insulation for BER A1. Cheapest path to integrated underfloor heating.
What is the difference between a raft and a strip foundation?
Strip is a continuous concrete strip under each load-bearing wall. Raft is a single concrete slab across the entire footprint. Strip is cheaper on good ground; raft is more forgiving on variable ground and integrates subfloor insulation.
When this matters most
Your site has Class 3–4 ground, or you want a continuous subfloor insulation layer.
When this doesn't apply
Your site has Class 1–2 ground (strip is cheaper) or Class 5+ ground (need piled raft).
Where to go next
Next decision
The next step in your build sequence.
- Foundation Depth in Ireland — 900 mm, 1.2 m, or Below Frost Line →
- Foundation for a Timber Frame House in Ireland — Raft Is the Standard →
- Piled Raft Foundation in Ireland — For Peat, Made Ground, and Soft Clay Sites →
- Subfloor Insulation in Ireland — PIR, EPS, or Mineral Wool, and What Part L 2022 Demands →
Branch into
Adjacent decisions at the same stage.
Watch out for
Common mistakes, hidden costs, gotchas.
- Foundation Cost by Soil Class in Ireland — The €/m² by Class 1 to Class 5+ →
- Foundation Gotchas in Ireland — The Six Things That Catch Self-Builders Off Guard →
- Radon Barrier in Ireland — Where It's Needed, What It Costs, and the Carlow-Wexford High-Risk Map →
- Strip Foundation in Ireland — When It Works, What It Costs, and What the Engineer Will Check →
Compare with
Alternative approaches worth knowing.