Guide · 7 min read
Types of Foundations Explained
The five foundation types used on Irish sites are strip, trench fill, raft, pad and piled. Each spreads building load into the ground differently, and the choice is made by the project engineer based on ground conditions, loading and depth to suitable bearing material.
The short answer
A foundation transfers the weight of a building into ground capable of carrying it. Which type is used depends on how strong the ground is, how deep suitable bearing material sits and how the loads arrive — spread along walls, concentrated at columns, or across a whole footprint.
The engineer makes that decision from ground information and the structural design. A contractor builds to it; substituting a different foundation type on site is not a site decision.
Strip and trench fill foundations
A strip foundation is a continuous concrete strip cast in the bottom of a trench under load-bearing walls, with blockwork or masonry built up off it. It is the traditional and generally cheapest option where ground is firm at a reasonable depth.
Trench fill is the same trench filled with concrete almost to ground level. It uses more concrete but removes most of the below-ground blockwork and the labour in the trench, which often makes it faster and safer on deeper digs.
- —Strip — less concrete, more blockwork and time in the trench
- —Trench fill — more concrete, fewer trades working below ground
- —Both need clean, dry, undisturbed formation before pouring
Raft foundations
A raft is a single reinforced concrete slab, usually thickened at the edges and under load-bearing lines, that spreads the whole building load over a wide area. It is used where ground is soft, made up or variable, and where a strip foundation would need to go uneconomically deep.
Rafts are reinforcement-heavy and level-critical. The fill beneath them has to be placed and compacted in layers, because settlement under a raft shows up across the whole building.
Pad foundations
A pad is an isolated reinforced base carrying a single concentrated load — a steel column, a portal frame stanchion or an internal support. Pads are standard on commercial, industrial and agricultural frame buildings.
Setting out is the critical part: pad centres and holding-down bolt positions come from the frame supplier's drawing and have very little tolerance, because the frame cannot be adjusted to suit a base in the wrong place.
Piled foundations
Where suitable bearing material is deep, piles carry load down to it — either by end bearing or by friction along the pile shaft. Piles are installed by specialist piling contractors, and the civils package then builds the pile caps and ground beams that connect them to the structure.
Piling is normally the response to a site investigation, not a preference. If your engineer specifies piles, expect the ground information to explain why.
What decides the choice on your site
Three things dominate: the bearing capacity of the ground, the depth at which that capacity is found, and the loads coming down from the structure. Water table, made ground, tree roots, radon requirements and proximity to existing buildings all influence the detail.
A site investigation or a set of trial holes is what turns this from guesswork into design, and it is nearly always cheaper than the change in scope that discovering poor ground mid-dig causes.
Frequently Asked Questions
Which foundation type is cheapest?+
A strip foundation is usually the lowest-cost option where ground is firm at shallow depth. Once the dig gets deeper, trench fill often works out comparable because it removes blockwork and time working in the trench.
Can I choose a raft to avoid a deep dig?+
Only if the engineer designs one. A raft is a structural solution to weak or variable ground, not a shortcut, and it brings its own requirements for compacted fill and reinforcement.
Who inspects foundations before the pour?+
Normally the project engineer, who checks the formation and the reinforcement before concrete is placed. The pour is held until that inspection has happened.
Do foundations need a radon barrier?+
Where the design requires it. Radon barriers and damp-proof membranes are installed as part of the foundation and floor slab works according to the specification for the site.