Irish Soil Classification — Class 1 to Class 5+ and What Each One Costs You in Foundation
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Class 1–2 (rock, gravel): strip foundation, cheapest. Class 3–4 (clay, silt): raft, mid. Class 5+ (peat): piled raft, most expensive.
The Irish Soil Classification System runs from Class 1 (rock) to Class 5+ (peat, made ground, fill). The class drives foundation choice and cost. Class 1–2 (rock, gravel, dense sand): strip foundation, €120–€180/m² of footprint. Class 3–4 (firm clay, silt, sand with clay): raft foundation, €180–€260/m². Class 5+ (peat, soft clay, made ground, fill): piled raft or driven piles, €280–€450/m². The site investigation (€1,500–€3,000) is non-negotiable; the foundation choice follows. Get the soil test before you commission the foundation design — never assume. Geological Survey Ireland has free soil maps that can narrow down the class before you commission the test, but only a borehole on the actual footprint confirms it. Sloped sites add €40–€80/m² for stepped foundations or retaining; high water table adds €30–€60/m² for tanking; high-radon areas (Carlow, Wexford, parts of Wicklow) add €8–€15/m² for a radon barrier and sump.
TL;DR
Irish Soil Classification System runs Class 1 (rock) to Class 5+ (peat, made ground). The class drives foundation choice and cost.
Class 1–2 (rock, gravel, dense sand): strip foundation, €120–€180/m². Class 3–4 (clay, silt, sand with clay): raft, €180–€260/m². Class 5+ (peat, made ground): piled raft, €280–€450/m².
Get the soil test before the foundation design. €1,500–€3,000 for the test saves €30,000–€60,000 in over- or under-spec'd foundations.
When this matters most
You're choosing a site, you've gone sale-agreed, or your engineer is about to specify the foundation.
When this doesn't apply
The foundation is poured and cured.
Frequently asked questions
What are the Irish soil classes?
Class 1 (rock), Class 2 (gravel, dense sand), Class 3 (firm clay, silt), Class 4 (sand with clay, mixed), Class 5+ (peat, made ground, fill). Each class has a different foundation type and cost.
How much does a soil test cost in Ireland?
€1,500–€3,000 for a site investigation. Includes boreholes, soil samples, and a geotechnical report that the engineer uses to specify the foundation.
Can I build on peat in Ireland?
Yes, with a piled raft foundation (€280–€450/m² of footprint). The piles transfer the building load to a firm bearing stratum below the peat. Standard solution for Class 5+ ground.
Sources
Geological Survey Ireland, Irish Soil Classification System. https://www.gsi.ie/
Building Regulations 2023, Technical Guidance Document A (Structure), Department of Housing. https://www.gov.ie/
Frequently asked questions
What are the Irish soil classes?
Class 1 (rock), Class 2 (gravel, dense sand), Class 3 (firm clay, silt), Class 4 (sand with clay, mixed), Class 5+ (peat, made ground, fill). Each class has a different foundation type and cost.
How much does a soil test cost in Ireland?
€1,500–€3,000 for a site investigation. Includes boreholes, soil samples, and a geotechnical report that the engineer uses to specify the foundation.
Can I build on peat in Ireland?
Yes, with a piled raft foundation (€280–€450/m² of footprint). The piles transfer the building load to a firm bearing stratum below the peat. Standard solution for Class 5+ ground.
When this matters most
You're choosing a site, you've gone sale-agreed, or your engineer is about to specify the foundation.
When this doesn't apply
The foundation is poured and cured.
Where to go next
Next decision
The next step in your build sequence.