Comparison · 6 min read
Strip vs Raft Foundations
Strip footings carry loads on narrow trenches under walls and suit firm ground at shallow depth. Rafts spread the load across the whole footprint and suit weak, variable or filled ground.
How a strip foundation works
A strip foundation is a continuous band of concrete cast in a trench beneath every load-bearing wall. Loads travel down the wall into the strip and out into the bearing stratum below. It is the default in Ireland wherever competent ground is found within a reasonable dig depth.
Strips are efficient because you only excavate and pour where the load is. The trade-off is that they concentrate load on narrow lines, so they need genuinely uniform bearing.
How a raft foundation works
A raft is a reinforced concrete slab covering the entire building footprint, usually with thickened edge and internal beams. It spreads the load over a large area and behaves as a single stiff element, so localised soft spots cause far less differential settlement.
Rafts come into their own on made ground, soft clay, peat margins, sites over old fill, and where the bearing stratum is too deep to reach economically with strips.
Choosing between them
The engineer decides, but the logic is usually:
- —Firm, uniform subsoil within 1–1.5m — strip footings, lowest cost
- —Variable ground or shallow soft layers — raft, to bridge the variation
- —Made ground or old infill — raft, or piles with a ground beam if the fill is deep
- —Deep peat — usually piled, with a raft or beam layout over it
- —Shrinkable clay near mature trees — deeper strips or a raft with heave precautions
Cost comparison in practice
A raft uses more concrete and much more reinforcement than a strip, so the material cost is higher. But rafts often need far less excavation and cart-away, and a single pour can be quicker than trench digging, blocking up to DPC and a separate slab. On a poor site, the raft can be the cheaper route once the whole sequence is priced.
The honest answer is that neither is universally cheaper — it depends on how much digging the alternative would need.
Frequently Asked Questions
Can I choose a raft to avoid a site investigation?+
No. A raft still needs a design based on known bearing capacity. A trial hole or site investigation is what tells the engineer which foundation is safe and what reinforcement it needs.
Do rafts work on sloping sites?+
They can, but sloping sites usually need stepped construction or cut-and-fill with retaining elements, which affects both options. It is a design decision, not a preference.
What about piled foundations?+
Piles are used when competent ground is too deep for either strips or a raft — typically deep peat, deep made ground, or heavily loaded structures. A ground beam or raft usually spans between pile caps.