House subsidence in Ireland

Definitions of settlement and subsidence from the Aviva Homeshield policy booklet; pyrite heave from the C&AG. Drawings are schematic.

What causes subsidence in Irish houses

Leaking drains, most often

Civil engineer Kieran McCarthy says that in most cases, though not all, subsidence is caused by drain leakage. Older sewer drains were short clay pipes pushed together with unsealed joints. A slight leak at one joint weakens the ground under it, the pipe slips, the next joint opens, and the process continues until the ground next to the foundations has lost much of its strength (Irish Examiner, 2021). A chartered building surveyor told The Irish Times the same: most subsidence comes from damaged underground drains, particularly in older period properties where clay pipes have collapsed or their joints have opened, washing away the soil (The Irish Times, 2020).

Trees drying the soil

Soil shrinks as it dries. Trees close to a house draw water from it (The Irish Times, 2020). A surveyor answering an Irish Times reader also asked whether a large tree had been cut down near the house recently, because changes in the moisture in the soil make it shrink or swell (The Irish Times, 2019).

Weak, made or soft ground

Some houses were built on weak or filled ground (Irish Examiner, 2022). Geological Survey Ireland describes made ground as anything from engineered fill to material containing voids or material that can compress, decompose, dissolve or expand, and maps soft, compressible river and estuary deposits under parts of Dublin and Cork (PanGeo Dublin, PanGeo Cork). Limestone can dissolve: GSI says karstified bedrock can cause subsidence (GSI).

Work nearby

Tunnelling under a property can disturb the ground (The Irish Times, 2020), and heavy excavation on a neighbouring site can send vibration through the ground (The Irish Times, 2019).

Things that look like subsidence but aren't

Pyrite in the hardcore under a floor can swell and crack floors and walls from below (C&AG), and defective concrete blocks crack and crumble in the walls themselves (RTÉ). See pyrite and defective blocks and when underpinning isn't needed.

The signs

• Cracks running at about 45 degrees from the top corners of windows, inside and out (Irish Examiner).
• Cracks in the brickwork, windows that no longer fit properly, and floors that slope (The Irish Times).
• Skirting boards that don't line up, and fireplace hearths out of level (The Irish Times).
• Doors that jam, or have been shaved at the top. Check whether the top and bottom of a door are still at right angles to its edge (The Irish Times).
• Blocked drains alongside the cracking (Irish Examiner).
• In a house from the early 1970s or before: manhole covers and gully heads that aren't plastic. Lift one and look at the pipe below (Irish Examiner).

Which houses

Kieran McCarthy says most suburbs in Irish towns and cities built in the early 1970s or before will likely have clay drains, and that while they may not all be leaking now, they will eventually leak (Irish Examiner, 2021). In Census 2022, 523,039 of Ireland's 1,836,728 households (28.5%) lived in homes built before 1971 (CSO). See the figures for each county on underpinning by county.

What to do first

1. Photograph and date every crack.Inside and out, with a ruler in the shot. Do it again every few weeks.
2. Don't fill them yet.Filling the cracks is the last thing you do (Irish Examiner).
3. Get a chartered structural engineer.Engineers Ireland awards the Chartered Engineer title, CEng MIEI (Engineers Ireland). The engineer reads the pattern, the ground and the drains.
4. Have the drains surveyed.A camera survey of the drains shows whether they're leaking. In 2020 one company charged €385 including VAT for a pre-purchase survey with drain maps (The Irish Times).
5. Tell your insurer if you plan to claim.As soon as reasonably possible, with full details within 30 days (Aviva policy conditions).
6. Monitor if the engineer asks.A crack monitor across a crack for six to 12 months shows whether it's still moving. If it's getting bigger, the movement is recent (The Irish Times).

Is there a subsidence map?

There is no public record of where pockets of subsidence exist, an engineer told The Irish Times, but Geological Survey Ireland holds geological information from past site investigations that can help (The Irish Times, 2020). GSI's PanGeo project maps areas of potential or observed ground instability in Co. Dublin and Cork City, and GSI also keeps a karst features database and a national landslide susceptibility map (GSI PanGeo, GSI geohazards). GSI says the PanGeo maps are broadly indicative and should never be used for site-specific purposes.

Questions people ask