Why have our limestone floors started cracking?

Our limestone floors have started cracking for no apparent reason. We have underfloor heating but have had the floors for years, yet the cracking is a recent occurrence. Any ideas as to the cause and how we can stop it getting worse?

Andrew Ramsey replies: Limestone is an excellent tile, as the mineral composition makes it extremely durable and hardwearing. When cracking occurs to tiling, in my experience it is rarely because of a poor batch or inferior tile laid during construction.

Any form of cracking is indicative of movement, whether slight or severe. As part of a review by a chartered building surveyor there are several considerations that need to be examined when completing an inspection and assessment of cracking.

The location is important: is the cracking appearing within the middle of the floor or around the perimeter of the floor? Has the crack occurred in an area of high traffic (footfall) or within the location of a heavy appliance? The direction of the crack and level of the floor would also need to be assessed. Has the floor recently experienced a period of wetting? This is particularly relevant as limestone is porous.

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I believe monitoring of the crack to evaluate further deterioration is the best course of action

Sound testing could assist in determining the extent of “bedding” within the affected area. If a tap test reveals a slight sound difference, it could indicate that the mortar bed is incomplete. This would allow a differential in floor movement. Differential movement would create locations of stress for the flooring and result in cracking.

The underfloor heating could have contributed to the cracking. I assume that warm-water underfloor heating (UFH) is installed as opposed to an electrical mat heating system. Cracking associated with underfloor heating is a common cause of problems in rigid tiled flooring. The Building Research Establishment has investigated many cases where stone or ceramic tiles have been laid on a concrete floor only for cracks to appear in the tiles and along grouted joints;, however, these findings identified that the cracking normally occurred within weeks of the heating system being operated post installation. As you outline, the heating system has been operating for years and the cracking has only recently occurred. As a result , I think it’s more likely that the cause of the cracking is not linked to the UFH, but lies elsewhere.

Another consideration is early evidence relating to pyrite damage (assuming a concrete ground-bearing floor slab construction). Pyrite exists in the stone fill which forms part of a concrete floor construction. If the amount of pyrite in the stone is below a certain proportion, no problems occur; however, if the stone fill has a high proportion of pyrite this can result in ground heave.

Early indications that a problem is occurring are cracking to flooring and internal doors sticking or jammed in an open or shut position etc. If pyrite damage is thought likely after the initial visual inspection, the next step is to arrange to have a core sample taken from under the floor for testing of the fill to confirm the analysis.

I believe monitoring of the crack to evaluate further deterioration is the best course of action. This is recommended over a period of six months with a record of condition completed (photos and measurement). If the problem has not worsened over that period, it may have simply been an isolated incident and the tile could be replaced. If the tile continues to deteriorate, I would recommend inspection by a chartered building surveyor.

[ Society of Chartered Surveyors Ireland ]