In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention

Q: My granite worktop has slowly developed cracking but the support appears to be sound. What could have caused this?

If you are truly confident the support to the worktop is sufficient then there are very few valid reasons remaining that could explain why the stone has cracked. For a stone to crack under an applied load the support must give way to some degree to allow the stone to flex sufficiently to be able to crack.

One of the most common reasons put forward for a worktop cracking is that someone, normally a child, may have been climbing on it. But people may occasionally need to climb on to worktops to access something, so this must be considered a predictable scenario. Therefore I would argue that any natural stone worktop ought to be of sufficiently strong and rigid construction to resist such applied force – and most worktops built following the Stone Federation guidelines typically are.

Another common reason given is that a worktop has been heated up, using a blow torch or other means such as a hot plate. But, again, this is unlikely as most stones should be able to resist very high heat differentials. Evidence of scorch marks and localised shattering features would also be expected to be present. There have even been occasions when worktops have been hit with hammers, but this normally leaves evidence of an impact mark.

Although it has not been definitively proven, your occurrence of cracking that has slowly developed over time appears to be a consequence of delayed expansion of the stone trying to relieve locked-in stresses.

This seems to occur when large volumes of material are being quarried and stone is cut from a block that has not finished unloading in response to the removal of the former constraining stresses when it was in the ground.

It has been reported that some granites may expand by up to 7% once the constraining stresses have been removed. Such expansion is used to explain why granite (and sometimes other stones) form large dome structures when exposed at the Earth’s surface.

The number of incidences of cracked stone is relatively small when compared with the number of worktops that are installed, and such problems are very much an unfortunate exception.

However, there are enough incidences to suggest that there might be a particular problem with black igneous stones such as those from Rustenberg in South Africa. This has been most notable where the stones exhibit evidence of mineral alignments, which geologists describe as a foliation, that may be controlling the direction of expansion and related cracking.

The result is cracking occurring in directions completely independent of where cracking could have been predicted to have occurred had the stones cracked through flexure.

The only other possible cause of slowly developing cracking is an even rarer phenomenon that may be related to the cyclic expansion and contraction of the substrate.

In one situation, the plywood base suffered repeated wetting and drying that put low cyclic loading and unloading on the stone. That appeared to fatigue the stone so it cracked at a much lower stress than might normally be expected. There is increasing evidence from the study of marble cladding panels that stone can suffer fatigue. If the substrate were part of the problem, evidence of water damage and related warping should be quite obvious.

Reference:

Natural Stone Kitchen Worktops. Code of practice for the design, manufacture, installation and maintenance of natural stone kitchen worktops – including reception desks, counter tops and similar horizontal and associated vertical surfaces for use in domestic and commercial situations. Published by Stone Federation Great Britain, February 2005.