What would cause the granite to crack?

Question

We recently rented a cottage where the owner had a 3" granite slab mantle located at the bottom of the fireplace. Upon speaking with the owner after we had left he said the granite had cracked and showed me pictures as verification. Would the heat of the fireplace cause the the granite slab to crack? What else could cause it? Keep in mind that that there were no impact marks to suggest that something heavy had hit it.

Answer 1

Granite, along with other natural stones such as marble, appear to be very hard. But, in fact they are a soft stone. As well they have cracks ( faults ) running throughout the stone. For the most part the stone stays in tact. but over time some of these faults get weaker and eventually crack. It may take a slight knock, or the expansion and contraction of heat may also cause this to happen. The owner should have been told of this potential problem from the installer or dealer when purchased. Research the net and get the facts, then calmly explain to the owner your sorry for the loss, but it was not your fault.
Oh, you better look for a different cottage to rent next year....

good luck.

Answer 2

Cracked Granite

Answer 3

Well, the others have covered heat expansion and fault lines pretty well... there is another source for cracks tho... and that is improper support.

If a stone slab support, has a 'high spot' under the stone, the stone slab can crack over that high spot under its own weight.

For example, if the stone was simply laid directly on the sub floor.... I would expect it to crack at some point, because sooner or later, the sub floor will sag under the weight, between the floor joists. This leaves the floor joist as the 'high spot', and it acts as a fulcrum point, over which the slap will break, over time. There doesnt even have to be any extra weight on the slab itself for this to happen.

I would investigate this as the prime suspect. To check to see if this is the case, put a level, or straight edge across the top of the granit, and see if it does indeed, slope down, over a high spot, or between two high spots. I would also check to see if the granite had a good support base, and was not simply installed on the sub-floor. I would also put a straight edge on the floor itself, across the front of the hearth (thats the name of the slab that lays across the front of the fireplace, on the floor ;D )

Ive replaced a lot of fireplace hearths myself over the years because of just this problem. A homeone thinks, "a stone hearth would look good here"... but then they don't provide the proper support for one.. .and it eventually cracks across it at some point.

Now.... to repair a situation like this... and prevent it from happening again, I remove the stone, add extra timber underneath the floor, spanning between the joists (to help prevent sagging), and then I provide a stiffer underlayment, like cement board, to go between the sub-floor, and the stone. Takes a little more work than just setting the stone on the subfloor, lol, but it will save you in the long run.

Good Luck

Answer 4

I don't think the heat of a fireplace -- by itself -- could harm granite, which was formed at much higher temperatures than a fireplace. Was it cracked when you got there?

My guess is that there was a flaw in the stone before it was installed. Perhaps it was cracked partway through on installation?

Answer 5

Extereme changes in temperature can crack granite. If it got hot then was cooled down very quickly it may cause it to crack. This method has been used for thousands of years to quarry stone.