The “pyrite problem” is due to the swelling of certain stones used as backfill under the concrete slab of houses. Under certain conditions of humidity and exposure to oxygen, pyrite oxidizes and swells. This chemical reaction can attack the concrete itself, lift the concrete slab in the basement and garage, and cause cracks in the floor and foundation walls.

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Swelling of foundation embankments can cause cracking of basement and garage floor slab. Sometimes the damage also affects the foundation walls, but only those surrounding the garage.

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Generally, the damage first appears on the garage slab, since the backfill is thicker there than in the basement.

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The backfill used in the basement can be the same as that used under the garage, or be of a different quality. It is therefore not possible to predict, even when there is damage to the garage, whether or not there will be damage to the basement.

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The swelling causes the slab to lift. Cracks, often in the shape of a cross or a star, then appear. The formation of gypsum during the chemical reaction can cause a white powder to emerge from the cracks. In some cases, there are unevenness in the floor and indentations in the cracks, that is to say an unevenness between one side of the crack and the other.

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It also happens, especially on the exterior side of the garage, that the pressure of the swelling is exerted laterally and thus causes the foundation wall to crack. In general however, the phenomenon only affects the walls of the garage or slightly those of the basement. In the majority of cases, this damage to the foundation walls is not severe.​