March 5, 2025

Pyrite in houses: causes, testing and solutions

In Quebec, the presence of pyrite and pyrrhotite can cause significant damage to buildings, especially those built between 40 and 50 years ago. Read on to find out how to tell if a property is at risk, what tests can detect the presence of pyrite in a house, and what measures should be taken. Whether you’re a buyer, owner or seller, this article will give you useful tips on pyrite in the house to protect your health and your real estate investment.

What is pyrite and how is it formed?

According to the Régie du bâtiment du Québec , pyrite is a mineral present in the backfill used when building houses. When exposed to humidity and oxygen, this sedimentary rock undergoes a chemical reaction, forming sulphuric acid and sometimes gypsum. When gypsum crystallizes, it can cause the crushed stone in the backfill to swell.

Over time, the pressure causes concrete to crack and crumble, especially under the slabs of garages and basements. Pyrite damages foundations and can cause internal damage to load-bearing elements and floor coverings. This phenomenon occurs slowly and becomes visible after about a decade. That’s why houses built between 40 and 50 years ago are the most affected.

Pyrrhotite is an iron sulphide mineral varying in colour from yellow-bronze to black. If it crystallizes into blades, pyrrhotite can cause cracks in concrete foundations.

	Tips and tricks During construction of a home, the ground may be uneven. To remedy this, backfill is used, i.e., a layer of material is added to level the ground. Unless you opt for DB-certified (“dalle de béton”) backfill, it may contain harmful minerals such as pyrite or pyrrhotite.

What are the consequences of pyrite in backfill?

The presence of pyrite or pyrrhotite can cause significant damage, such as cracks, concrete heaving and, in the long term, structural damage. These problems can lead to depreciation of the property’s market value and may make it more difficult to access mortgage financing, thereby discouraging potential buyers. Financial institutions may also be reluctant to provide a loan for an affected building, and insurance companies may refuse to cover pyrite-related damage.

It is essential to be well informed about pyrite testing options and understand related costs prior to any real estate transaction. If the presence of pyrite is confirmed, it is recommended that foundation problems be resolved as soon as possible to avoid difficult negotiations. Moreover, you should be aware that any information about pyrite detected in the house must be included in the Declarations by the seller.

Which areas and cities are at risk?

Numerous studies have identified some common factors of properties affected by pyrite. Two of them stand out specifically:

1. Year of construction

Properties built in the 1980s are the most affected , but cases have been observed before 1970 and after 1998.

2. Origin of materials

Some at-risk areas are more affected than others. On the South Shore of Montreal, the affected areas span from Châteauguay to Varennes, from the St. Lawrence River to the Richelieu River, as well as the Eastern part of the island, including Rivière-des-Prairies, Pointe-aux-Trembles, Repentigny, and the West Island, including Lachine, Dorval, Dollard-des-Ormeaux, Pointe-Claire, Kirkland, Pierrefonds-Roxboro, and Beaconsfield. However, the problem is most widespread in Montérégie because of the quarries that provided backfill containing pyrite.

Classifications have been established to assess the level of risk for the most at-risk Quebec cities and neighbourhoods according to the swelling potential petrographic index (SPPI).

Swelling potential petrographic index (SPPI)
Result	Level of risk
0 to 20	Low
21 to 40	Low to medium
41 to 60	Medium to high
61 to 80	High
81 to 100	Extremely high

If you are considering buying a house that meets these criteria, ask for a pyrite test during the pre-purchase inspection . A visual inspection is not sufficient to detect the presence of pyrite. An assessment of the backfill materials, concrete slab, and soil is recommended. The average cost is around $350 , and the seller is generally responsible for covering this cost.

What are the general indicators of pyrite in a house?

The following issues may indicate the presence of pyrite in the house:

Sulfation: a gradual process that degrades the concrete.
High humidity levels in the basement: potentially caused by cracks in the concrete.
White dust around cracks: a sign of sulphate crystallization.
Jamming doors: potentially caused by a slight deformation of the structure.
Cracks in foundation walls: an indication of pressure on the concrete.
Uneven floors: a symptom of swelling backfill or a collapsing foundation.

Buyers: why test for pyrite before buying?

Before signing a promise to purchase, ask questions during the showing to identify signs of potential pyrite-related problems. A thorough backfill analysis can help you make an informed decision.

During a real estate transaction, the Organisme d’autoréglementation du courtage immobilier du Québec (OACIQ) recommends that brokers work with laboratories that use the CTQ M-200 methodology . Testing for pyrite is not mandatory, but it is the only recognized way to inspect affected elements and analyze the backfill.

	Tips and tricks Several costs are to be expected when purchasing a home before, during and after the transaction. Once you have the pyrite testing report in hand, you will be able to determine with certainty whether the property you’re interested in is affected and what measures you’ll need to take.

Sellers: how to detect pyrite in your home

The damage caused by pyrite appears gradually, causing between 1 mm and 5 mm of concrete lifting per year. After a few years, the reaction stops. However, it can be difficult to identify the problem in its early stages and assess the impact on quality of life.

If you suspect the presence of pyrite in your home:

Make an inventory of all visible cracks.
Regularly measure and photograph these cracks.
Add markers to better observe changes.
Record the date, indoor and outdoor temperature for each observation.

If the cracks grow rapidly, getting a pyrite test done through a specialized laboratory is recommended. The experts will first survey the apparent damage before taking samples of the slab and backfill. These samples will then be tested to detect the presence and concentration of pyrite.

	Tips and tricks According to the Association des consommateurs pour la qualité dans la construction (ACQC) , the CTQ-M100 standard aims to certify the non-swelling quality of backfill used in construction, but it is not mandatory. If the house is more than 10 years old, it may be wise to have the backfill analyzed, including under the concrete slab in the garage, to assess swelling potential (SPPI).

How can pyrite be removed?

There is no way to stop the oxidation of this mineral. To remove the pyrite, the concrete slab must be removed, the contaminated backfill must be dug out down to the foundation base, and a new slab with DB-certified backfill must be poured.

Assess the extent of the damage before beginning any work. Backfill swelling is a chemical process that stops after a while, so excavating the backfill is not always necessary. This solution is long-lasting, but it does represent a significant investment.

How much does it cost to remove pyrite?

The price for removing pyrite varies depending on the amount of backfill to be removed and the extent of related tasks. On average, the work lasts from one to four weeks . An entire basement is more expensive to fix than a garage due to the necessary excavation and backfill.

If you are considering this type of work, it is essential to choose your contractor carefully . Whether you’re setting your budget or planning the different stages of your project, check out our Complete Home Renovation Guide to better plan your work.

Assistance programs, grants and guarantees

If you have a pyrite or pyrrhotite problem, check if you are covered by the Guarantee Plan for New Residential Buildings . Be careful! This plan expires five years after you purchase your house or condominium.

The RénoRégion or RénoClimat financial assistance programs may also be helpful. Check with your city to find out if it offers financial assistance for affected owners.

With regard to pyrrhotite, the Quebec government supports owners of residential buildings through the Program for Residences Damaged by Pyrrhotite . This program covers 75% of recognized eligible costs, up to a maximum of:

$75,000 if the building is not covered by the Guarantee Plan for New Residential Buildings.
$15,000 if the building is covered by the Guarantee Plan for New Residential Buildings but refurbishment work on basement areas is not covered by this Plan.

	Tips and tricks In Quebec, there is no time limit for taking legal action for hidden defects . You can sue someone for an event that occurred 30, 40 or 50 years after a house was built. All former owners can sue the previous owner. These are called cascade lawsuits . However, there is a deadline to notify the seller once the hidden defect has been discovered.

Working with a broker for added protection

Using a real estate broker to buy or sell a house in a pyrite risk area offers additional protection. The OACIQ requires brokers to inform their clients of pyrite risks and potential consequences , in addition to recommending the right actions to take. They must also disclose any known information about the presence of pyrite, even if no problems are visible.

If any doubt remains, the broker may recommend an analysis to confirm the presence of pyrite. If pyrite is found, corrections must be made or the selling price adjusted. The broker will also ensure that the buyer obtains the report, regardless of the results.

Finally, the broker may suggest including a clause requiring an expert assessment in the promise to purchase. However, they cannot add a clause specifying a threshold that would allow the buyer to withdraw their offer , because brokers do not have the authority to do so and this data is insufficient to assess the risks.

Selling or buying a house with pyrite: there are solutions!

Most commonly found in some houses built between 40 and 50 years ago, pyrite in houses can damage foundations and reduce the value of a property. Fortunately, there are solutions to prevent and remedy this problem. Whether you are an owner, buyer or seller, it is crucial to get informed, do testing and act quickly if pyrite is detected.

Looking for a new property ? Team up with a real estate broker who can guide you and help you avoid the issues that come with pyrite and pyrrhotite. They will be able to identify the signs and help you make an informed decision.

Frequently asked questions (FAQ)

1. Is pyrite dangerous to your health?

According to the Département de santé publique de la Montérégie, pyrite produces only a small amount of sulphuric acid. Although it is harmless to human health, it can cause cracks to appear in some basement floors, which increases humidity. Mould then develops, affecting air quality. It is therefore important to remain vigilant.

2. Is pyrite a hidden defect?

For pyrite to be considered a hidden defect, it must be unknown to the buyer at the time of the transaction. In addition, pyrite must have been present prior to the purchase and have caused damage to the property. If pyrite is detected but no damage—such as cracks—has occurred, the seller may not be required to reimburse the buyer.

3. Does a home inspection include testing for pyrite?

The building inspector does not take samples or perform laboratory analysis; this must be done by specialists.