Before you underpin · checked 29 October 2026
When underpinning isn't needed
Settlement, drains caught early, uneven floors, roof spread, pyrite and defective blocks. The common cases where something else is the answer.
Facts on this page checked against each source on 29 October 2026. Each one links to its source. How we check
Most cracks in houses don't mean underpinning. BRE's damage scale treats cracks up to 5 mm as aesthetic, needing only redecoration (BRE). A chartered building surveyor told The Irish Times that most internal and external cracking is minor, caused by normal, uniform settlement (The Irish Times).
Underpinning is for foundations that have lost support and are still moving, or can't be relied on. These are the common situations where something else is the answer.
Definitions of settlement and subsidence from the Aviva Homeshield policy booklet; pyrite heave from the C&AG. Drawings are schematic.
Settlement in a newer house
An engineer quoted in The Irish Times says there may be evidence of settlement in the first six to 12 months of a building's life, and then it stops. What is worrying is evidence of recent movement (The Irish Times). Monitoring tells the two apart: a crack monitor across the crack for six to 12 months shows whether it's getting bigger.
Drains caught early
Kieran McCarthy, writing about older Cork houses, says the fix may be as simple as removing the old clay drains before they leak much further, or it may be more invasive, with underpinning, and that the longer it's left, the more expensive the repair (Irish Examiner, 2022). A CCTV drain survey is how you find out.
Uneven floors
If timber floors are only marginally uneven, around 5 mm or less, an engineer told The Irish Times you may be able to lift the boards and wedge the joists to get a level plane, and that concrete floors may take a simple screed (The Irish Times). Sloping floors that are getting worse are a different matter.
Roof spread, alterations and works next door
A chartered building surveyor answering an Irish Times reader about cracks in a 1987 dormer bungalow's gable pointed to other causes before the ground: roof members cut during an attic alteration pushing the walls out at wall-plate level, vibration from heavy excavation on a neighbouring site, trees close to the wall, and blocked drains (The Irish Times, 2019).
Pyrite heave and defective blocks
Pyrite in the hardcore under ground floor slabs can swell and crack floors and walls (C&AG). Defective concrete blocks crack and crumble in the walls themselves (RTÉ). Neither is a foundation dropping, and both have State schemes. See pyrite and defective blocks.
Repairing the cracks themselves
Once the cause is dealt with and movement has stopped, cracks get repaired. The Irish engineering consultancy ORS describes injecting external cracks with low-viscosity epoxy resin and re-rendering with expanded metal mesh, and, for severe cracks, stitching with stainless steel helical bars set into the mortar joints (ORS).
Questions people ask
Do all cracks mean subsidence?
No. BRE's scale puts cracks up to 5 mm in its aesthetic categories, and a chartered building surveyor told The Irish Times most cracking is minor and caused by normal settlement.
Is settlement the same as subsidence?
No. Insurers define settlement as movement caused by the weight of the building itself, and subsidence as downward movement within the ground independent of the building's load.
Can I just fill the cracks?
Not until the cause is known and movement has stopped. Irish engineers fill cracks last, six months to a year after the repair.
Is a new house settling normal?
An engineer quoted in The Irish Times says there may be settlement in the first six to 12 months and then it stops. Movement that continues is the worry.
Sources (2)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- Aviva Homeshield policy booklet: arachas.ie
- ORS: ors.ie
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