Pat is 100% right about a three piece slate if installed correctly not coming apart at the seams. But I've gone way beyond that with steel plating the slate seams together. I'll post some pictures on the whole process of converting a 3 piece slate to a 1 piece slate in a thread of its own shortly.
Do you come across the shims coming out often? I've noticed lately when I've covered tables in basements that the owner is saying that they have a bump going across there table and it's always a shim that needs tapped back into place. I attribute it to bing in a basement and the wood shims expanding and contracting with the humidity.