Welsh slates will last a long time, mostly the nails rot and the hole that was made for the nail gets bigger.Poor quality slates can have shorter lives.

I have seen Welsh slates that are over 200 years old still going strong. Difficulty will be establishing that they really are Welsh.

Posted By: ArtiglioMy mum has a welsh barn , built in 1802 one face of the roof is in the original slates ( locally called “welsh cows”, as they are variable in size , so batten spacing reduces as you move up the roof, takes ages to lay them) reclaimed when the barn was renovated, they’re as hard and solid as day they were first split. Probably last forever.

Usually called random diminishing.

When we did our extension/refurb we took off the front elevation of random diminishing slates numbering them in rows and position with hope of reusing them but we would not have enough and trying to match would have been costly. We resorted to new welsh slate. They must have been old they had been re-fixed with nails but original wood pegs were still evident in some. Property we think was from about 1750. I still have them in storage the biggest slate is about 4' x 3' we have outbuildings we may be able to use them on, So basically you should not be concerned about Welsh slate they last several lifetimes. An experienced slater will be able to tell you if they are welsh and also from what quarry they came from. Make sure what you get are thick around 8mm which is about medium thickness in grade they will taper slightly along their length and good slaters will grade into 3 or so thickness stacks the thicker slates on the first rows working up the roof. When handling the slate they will balance it in their hand so as to put the thinnest edge at the top so that when all laid the roof looks very even.

i was always told , give them a tap with your slaters hammer , if they ring theyre good , if they sound a bit dull then theyre not

Thanks for giving me confidents in reclaimed slates,ill test them with a hammer!.Im thinking the thicker they are the longer lasting they will be.

Exactly what I was thinking, Tony, although I then didn't post because in the 800+ slates I recently 'inherited' from a friend to do my extension the thicker ones actually did have a much lower failure rate, but then some of the thin ones were *really* thin and rang like a bell. I think it's about general quality of the mountain they came out of rather than thickness. (And yes, as far as degradation goes, I had a fair few which had gone 'fluffy' and some which resembled cardboard!)