Obviously it depends on the size of the roof but once you factor in scaffolding, repairs and materials the difference gets negligible.
Those old roofs are hyper inefficient too so if your bills / the environment bothers you then it should be changed IMO.
If it's the original roof, I'm guessing there's not been any loft conversion - do the roof when that gets built instead.
@NotThamesWater started
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Obviously it depends on the size of the roof but once you factor in scaffolding, repairs and materials the difference gets negligible.
Those old roofs are hyper inefficient too so if your bills / the environment bothers you then it should be changed IMO.