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Agreed, but if the rest of the roof is under 20 years old, I'd not be keen to replace it all.
The areas that need to be replaced are all the central spaces. A full job will go to the edges and chimney.
I imagine that during a full re-tiling even newer tiles will be removed and laid again and incur some wastage because of that.
Velocio-
You're thinking of doing a loft conversion in the future?
All or most of the roof will probably come off anyway during a conversion so it's a good time to replace it. If you're likely to do it soonish, I probably wouldn't replace the whole roof now only to take it off again then.
Obviously this depends on the correct value of "soonish" and whether the battens etc. are likely to survive that long with just patch repairs on top.
useless
Agreed, but if the rest of the roof is under 20 years old, I'd not be keen to replace it all.