Irish Strip Foundations: Widths, Depth & Concrete (TGD A)

The requirement

Ireland sets no fixed foundation depth. TGD Part A (par. 1.1.5.1–1.1.5.3) leaves the depth to your ground and your site investigation. Width does have numbers. Common soils land in a 400–650 mm band on TGD A’s strip foundation width table. The width can never be less than the wall width plus 75 mm on each side.

Depth: no frost rule, no fixed number. Unlike some countries, Ireland has no frost line to dig past. TGD A is the Technical Guidance Document for Structure. It is the deemed-to-satisfy route to Part A of the Building Regulations. Par. 1.1.5.1–1.1.5.3 sets the depth by the ground and the site investigation. There is no number on the page. What your report calls for is what you dig to. Any site that prints a single depth figure is guessing.

TGD A 2012 · strip foundations

1 footing
2 ground

Width: read it off the table. TGD A’s strip foundation width table works by subsoil type and condition, plus the total wall load (30–70 kN/m). Widths for common soils sit in that 400–650 mm band. Read the exact figure off the table for your ground and load. Outside the table’s scope, the design falls to Eurocode 7.

The concrete. A plain strip runs unreinforced. The mix is Class C12/15, which means 15 N/mm² characteristic strength. Cement content must be at least 200 kg/m³. The water/cement ratio can be no more than 0.85. On small jobs you can use a 1:7 cement:aggregate mix by volume instead (par. 1.1.5.2(c)).

Thickness must be at least 200 mm. If the projection P is greater, use that instead (par. 1.1.5.2(d)). P is how far the strip sticks out past the wall face.

Rebar cover: not printed here. If the strip is reinforced, TGD A gives no cover figure. That sits with Eurocode 2 (I.S. EN 1992-1-1). Check that standard, or ask the engineer who designed the steel. Never take a cover figure off a website.

Work it out on site

The depth comes from your site investigation. The width comes off the table. Once you know both, the concrete volume is plain maths. It feeds straight into the calculator. Say you have a strip to pour. These are sample numbers, not code figures. Your real width and depth come off your soils report and the width table:

Total trench length: 28 m.
Foundation width: 0.45 m. A sample figure. Read yours off the TGD A width table for your soil and wall load.
Foundation depth: 0.5 m. A sample figure. Your site investigation sets it.

Volume = length × width × depth.

28 × 0.45 × 0.5 = 6.3 m³.

So you would order about 6.3 cubic metres. Then add a bit for spillage and a rough trench. Most builders tack on 5 to 10 per cent. Swap in your own numbers and the calculator does the sum.

Do it in 5 seconds

Part of the Irish Building Regulations hub. One rule book for the whole country, with the TGDs and the calculators that solve it.
Setting out the stairs on the same build? See Irish stair & barrier rules.

Sources

TGD Part A (2012) — Structure, par. 1.1.5.1–1.1.5.3 (foundation depth set by ground/site investigation, not a fixed figure), the strip-foundation width table (subsoil type/condition × wall load) and par. 1.1.5.2(c)–(d) (plain concrete mix and minimum thickness). Read it at gov.ie.

Read this first. These figures are from the 2012 edition of TGD Part A, in force since 1 July 2013 and referencing Eurocode 7 for foundation design — an older edition referencing BS 8004 circulates online with a different width table, so confirm you’re reading the current one. TGD A is the deemed-to-satisfy route, not the only legal one — a designer can instead show compliance by other means. Like every TGD, it carries transitional arrangements, so the edition in force can vary by the commencement or permission date of your job. Always confirm the current edition on gov.ie — and get your ground checked by a site investigation — before you dig.

Frequently asked questions

Is there a minimum foundation depth in Ireland?

No. TGD Part A sets no fixed minimum depth and no frost-line figure (par. 1.1.5.1–1.1.5.3). Your ground and your site investigation set the depth, not a number in the document. Whatever depth the site investigation calls for, that is the depth you dig to.

How wide does a strip foundation need to be?

Read it off TGD A's strip foundation width table. The table works by subsoil type and condition, plus the wall load (30–70 kN/m). Common soils land in a 400–650 mm band. The width can never be less than the wall width plus 75 mm on each side. Don't guess it.

What concrete goes in a plain strip foundation?

Plain, unreinforced strip concrete is Class C12/15 (15 N/mm² characteristic strength). Cement content is at least 200 kg/m³, and the water/cement ratio is no more than 0.85. Small jobs can use a 1:7 cement:aggregate mix by volume instead (par. 1.1.5.2(c)). Thickness is at least 200 mm, or the projection P if greater (par. 1.1.5.2(d)).

What's the rebar cover on a reinforced foundation?

TGD A doesn't state a cover figure for reinforced foundations. Check Eurocode 2 (I.S. EN 1992-1-1) for that. Reinforced foundation design generally sits with an engineer, not with the deemed-to-satisfy route in TGD A.

Is this the current TGD A?

Yes. These figures come from the 2012 edition of TGD Part A, in force since 1 July 2013. An older edition points to BS 8004 instead of Eurocode 7 and has a different table. It still circulates online. Check you are reading the current edition before you rely on it.