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.

1. 1 footing
2. 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 out the concrete volume

Use the concrete sizes on the approved plan. Embedment is the depth from ground level to the base of the footing. It is not the thickness of concrete to pour. A strip, wall and trench fill each have their own shape. Work out each part on its own. Where parts meet, count that concrete once.

This example uses a straight concrete strip with the same size all along:

• Total concrete length: 28 m.
• Concrete width: 0.45 m.
• Concrete thickness, measured from its top to its underside: 0.5 m.

These are example sizes, not code limits or a footing design. Check the shape and size on the plans. Then check how deep the base must sit. It must meet the soil and frost rules for the site.

Volume = length × concrete width × concrete thickness.

28 × 0.45 × 0.5 = 6.3 m³.

That is the volume before any extra for waste or uneven ground. Measure the trench or forms. Agree the extra amount and final order with the supplier. The tool does the sum. It does not choose safe footing sizes.

Do it in 5 seconds

Related

• 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.