FAQ’s about pyrite in homes

With property prices continuing to fall, negative equity is a reality for a significant number of home-owners, especially those who bought in new developments.

The structural damage that pyrite can cause is an expensive problem to fix and stressful to the property owner. If if an individual home has no pyrite in its back-fill, the idea that pyrite may have been used in a number of homes in your estate can have a very negative affect on all property values in that development. The presence of pyrite in the backfill in almost all cases cannot be seen from a visual inspection. Pyrite in backfill can take upwards of 30-40 years to swell where no more expansion can be expected and become stable. Excavating the pyrite infested backfill is an expensive operation and on the structure can involve supporting the existing first floor structure and roof, removing the existing concrete floor slab, partition walls resting on the floor slab, removal and replacement of all plumbing, heating and electrical services in the affected area, removing all existing pyrite backfill.

Building Surveyors and insurance companies working in this area estimate that pyrite contaminated backfill has been used in house construction for possibly up to 20,000 homes in Ireland.

What is pyrite.

Pyrite (FeS2) is a very common mineral found in quarries in Ireland. Pyrite can be found in the sedimentary rock. This material is excavated from quarries and used to make the backfill used under concrete floor slabs and other structures.

Pyrite in its natural state within sedimentary rock is not normally a problem. However when it is quarried and mistakenly used as a backfill under floors or road structures it will combine with moisture and oxygen and expand, therefore becoming a problem.

To be somewhat technical, when pyrite is exposed to moisture and oxygen, the pyrite oxidizes and produces sulphuric acid. The sulphuric acid then reacts with the calcium carbonates (gypsum) which is found in the gravel. This results in the swelling of the backfill and its associated problems..

Swelling of the backfill under floor slabs.

If pyrite within the backfill never got wet, or was not exposed to oxygen, it would not swell. In reality there are methods used to slow down this process but in the long term they are not effective. In time the chemical reaction from the pyrite material leads to the formation of sulphate and gypsum. The gypsum continues to crystallize over time and causes the backfill to swell under the concrete floor slab or other structure. Over time this chemical reaction will result in the flaking and eventual crumbling of the concrete floor slab.

This swelling will lift the floor slab and interior structural supports resting on it. Swelling is usually worse if there is a higher concentration of pyrite in the backfill and the amount of swelling is also determined by the thickness of the backfill.

This is usually a slow process. On average, building surveyors experience suggests it can take about two to five years before any damage is noted.

Frequently Asked Questions about Pyrite in Homes.

I suspect pyrite in my home, what should I look for?

In most types of house construction, backfill is compacted over the soil and then the concrete floor slabs is poured over same. The presence of pyrite material in the backfill will cause the concrete floor slab to lift over the swelling backfill, resulting in cracks in the concrete floor slab. Interior partition wall will also lift including the sticking of interior and exterior doors which will catch on the rising floor.

Needle-like white crystals would also be visible in the cracked concrete floor.

Depending on the depth of the back fill, various types of cracks can be seen in the floor, including rising in the corners of rooms or the creation of a star type crack towards the centre of the floor.

Does my insurance policy cover for pyrite damage.

A home owners insurance policy would not normally cover for defects caused by pyrite. A building guarantee or insurance bond may cover. It is important to read these policies in full

Does Pyrite cause a health issue?

No, other then the stress it leads too. Medical opinion suggests that the quantities of sulfuric acid are too small to cause health problems. However it is possible that the radon barrier maybe compromised by the swelling backfill leading to ingress of radon gas into the dwelling.

Will my concrete floor slab eventually swell and crumble?

Yes, the pyrite will eventually result in the concrete floor slab swelling and crumbling. When sulfuric acid reacts with the underside of the concrete floor slab, the underside of the concrete slab swells from sulphation and starts crumble. When the backfill expands, the strip foundations and rising walls may prevent horizontal expansion; so the swelling usually occurs upwards from the centre or corners of the floor slab.

Will the pyrite damage my foundations?

In limited cases, the lateral forces of the swelling backfill will produce cracks in the foundation rising walls around the property. Most of the swelling is directed upwards.

Are older properties likely to be affected?

The majority of properties affected have been built during the celtic tiger years, However some older properties are also affected and the defect was not correctly diagnosed.

How quickly will it take to see the effect of pyrite.

The speed of reaction will depend on the depth of the backfill, the extent of the pyrite within the backfill and the amount of oxygen and moisture available to allow the reaction to take place.

What if I have a long fine crack in my concrete floor?

Most concrete floor slabs will develop drying cracks. This is normal when floors dry out too quickly. As concrete dries, it shrinks. These cracks are most likely not caused by swelling due to the presence of pyrite.

What is the main longterm solution to remedy a pyrite problem?

The only longterm solution is an expensive solution which involves substantial building works as follows.

Break and remove the concrete slab floor

Excavate and the backfill and any contaminated subsoil

Replace backfill according to Part D of the Building Regulations.

Pour a new slab.

Existing services, partition walls and first floors would also need to be replaced and supported during this re-instatement.

This work should be undertaken by competent building contractors.

Peter Sweeney FBEng, Building Surveyor.