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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.
I'm not sure about that. Cost of fixing ours is much less than repairing / replacing the flat roof behind it (£600 vs £8k). Obvs. the scaffolding cost is spread out across three different jobs (pitched repair, flat roof replacement, chimney stack repairs) and the scope of the repair job to ours is fairly limited.
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.