Slate roofing looks great, and it’s extremely durable. People in the roofing trade call it the 100-year roof, but slate can last up to 200 years with proper maintenance. Occasionally, one of the slates might crack, but it’s not difficult to repair and replace. Here are some tips for replacing a piece of roofing slate.
When a slate cracks, the best option in the long run is replacing it quickly. Slates are usually fixed to the roof using two nails made from copper or aluminium alloy for durability, and these will need replacing along with the slate itself. Slide the slate out and pull out the nails with a shingle ripper or a claw hammer.
The replacement slate will need holes punched in it, just like the holes in the slate that’s being replaced. These days slates are often pre-holed at the quarry prior to shipping, so this should not be an issue. If you do need to make holes, slate rippers have a spike for doing this, but a nail punch should work. Slide the slate in, and slot the hole over the replacement nail.
Source: http://www.doityourself.com/stry/how-to-clean-a-slate-roof#b