you need a slate ripper dale, to cut through the nails holding it in place, when you have got the old one out, cut a strip of lead about 8 inches long and nail it to the joist at low level bottom of the slate, this is going to act as a strap and hold the slate in place, as you wont be able to nail it through the fixing holes, now slide the new slate up into position, this may be a bit of a struggle as sizes dont always match up, if too tight nibble a bit off the side of the slate with some snips, once in place fold the lead strap up to hold the slate, give the strap a double fold for additional strenght. slate roofs can be very dangerous, i hope that this is a low level repair.
ghyllsidebuilders@hotmail.com
November 2008