if you have a sizeable open fire which can give off a lot of heat, there is a maximum size for marble slabs for the hearth. this is to allow adequate expansion without splitting. My house has a marble hearth (on the floor I mean), and has two slabs about 1 metre square each, separated by a very thin line of cement. I have re-cemented these myself after the heat took it's toll (crumbling cement), without problem. As was mentioned, make sure you lay them without unnecessary strain induced by uneven foundation. It just takes a coal bucket dropped or a child jumping on it to spoil all your hard work! My hearth is only about 5cm thick and has stood up to all sorts of abuse. Hope this is of use. I don't come here often, so email SandyBuchan@hotmail.com, thanks.