Pyrite and blocks · checked 29 October 2026
Pyrite, mica and underpinning
Two problems that look like subsidence but aren't, the State schemes for each, and where they apply.
Facts on this page checked against each source on 29 October 2026. Each one links to its source. How we check
Pyrite heave and defective concrete blocks both crack Irish homes, and both get mistaken for subsidence. Neither is a foundation sinking, so underpinning doesn't fix them. Each has its own State scheme.
Pyrite heave
Pyrite is a common mineral in some rock. Where it is in the aggregate used as hardcore under floors, exposure to oxygen and moisture can make the hardcore beneath ground-floor slabs swell, cracking walls and floors. The problem became apparent in mid-2007 in a number of private housing developments. An expert panel estimated that up to 10,300 dwellings in 74 estates may have reactive pyrite in the hardcore (Comptroller and Auditor General, 2019 report).
The Pyrite Remediation Scheme
The scheme is a last resort for homeowners with significant pyrite damage and no other practical way to get it fixed. The Comptroller and Auditor General set out its conditions in 2020 (C&AG):
- The home is in Dún Laoghaire-Rathdown, Fingal, Kildare, Meath, Offaly, South Dublin or Dublin City
- It was built and completed between 1 January 1997 and 12 December 2013
- It has been assessed, tested and certified with a damage condition rating of 2, attributable to pyrite heave
- It was bought before 12 December 2013, and one application per home
- The owner has no practicable option other than the scheme or their own resources
On 25 September 2020 the Minister for Housing extended the scheme to Limerick City and County Council's area. At that point up to 2,800 applications had been received and 2,000 homes remediated, at an average cost of about €70,000 each (gov.ie). By the end of 2019, homes in Fingal and Meath made up over 90% of applications (C&AG). These conditions are as published in 2020; check the current rules with the Pyrite Resolution Board.
Defective concrete blocks ("mica")
In Donegal and Mayo, muscovite mica in concrete blocks absorbs and stores water. Drawing on the 2017 expert panel report, TheJournal.ie explains that 1% muscovite mica is estimated to cut concrete strength by about 5%, and that wet blocks lose integrity in freeze-thaw. A 1% limit applies to mica and other harmful impurities in blocks. Cracks and fissures opened in more than 5,000 homes, most in Donegal, and many families face demolishing and rebuilding (TheJournal.ie, June 2021). In April 2022 a civil engineer told RTÉ he believed defective blocks affect homes in more counties, with pyrite in the blocks the main problem outside Donegal (RTÉ News, 24 April 2022).
The Defective Concrete Blocks Grant Scheme
The enhanced scheme covers homeowners in Donegal, Mayo, Limerick and Clare whose homes were damaged by defective blocks, with grants of up to €420,000 depending on the works needed. The Remediation of Dwellings Damaged by the Use of Defective Concrete Blocks (Amendment) Act 2025 and 2026 regulations allow an increased grant in some cases and extend the time allowed for remediation from 65 to 130 weeks. Applications go through your county council (Mayo County Council).
Why underpinning isn't the fix
| Subsidence | Pyrite heave | Defective blocks | |
|---|---|---|---|
| Where the problem is | The ground under the foundations | The hardcore under the ground-floor slab | The blocks in the walls |
| Which way it moves | Down | Up, from under the floor | The blocks themselves crack and crumble |
| Typical fix | Fix the cause, then support the foundations | Under the Pyrite Remediation Scheme | Under the Defective Concrete Blocks scheme |
| State scheme | None; insurance may cover it | Pyrite Remediation Scheme | Defective Concrete Blocks Grant Scheme |
Sources: C&AG; RTÉ; Mayo County Council; Aviva policy booklet.
A house can have more than one problem, and the symptoms overlap: cracked walls and floors. The pyrite scheme itself depends on testing and certification by an engineer (C&AG). If you're in a scheme county and your home was built in the scheme years, raise it with your engineer before anyone prices underpinning.
Start with a chartered structural engineer. Nobody can tell from a photo or a phone call whether a house needs underpinning. An engineer looks at the cracks, the ground and the drains, then decides. This site never tells anyone their house is safe or unsafe.
Questions people ask
Does underpinning fix pyrite?
No. Pyrite heave swells the hardcore under ground-floor slabs and pushes up. Underpinning supports foundations that have lost support below. The pyrite scheme deals with pyrite damage.
Which counties does the pyrite scheme cover?
As published in 2020: Dún Laoghaire-Rathdown, Fingal, Kildare, Meath, Offaly, South Dublin, Dublin City, and Limerick City and County, added on 25 September 2020. Check the current list with the Pyrite Resolution Board.
What is mica in blocks?
Muscovite mica, a mineral, in concrete blocks. Too much of it makes the blocks absorb water, lose strength and crumble, especially through freeze-thaw. A 1% limit applies.
Which counties does the defective blocks scheme cover?
The enhanced Defective Concrete Blocks Grant Scheme covers Donegal, Mayo, Limerick and Clare, with grants of up to €420,000 depending on the works.
Can a house have subsidence and pyrite?
It can have more than one problem. The symptoms overlap, so testing by an engineer is how they're told apart.
Sources (1)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- Aviva policy booklet: arachas.ie
Get it looked at properly
Five quick questions and one experienced groundworks and underpinning contractor who covers your county rings you. Or call 087 162 1846 between 8am and 8pm.