When do you replace the whole roof?

[newroofadvice]

If you have a leak from the roof, does the whole roof need to be replaced, or just the part the leak is coming from? i.e. a patch work?

How does a building surveyor know whether the whole roof needs replacing or not?

When should you replace a whole roof?

Thank you.

We had to do major work on our roof many years ago because you could see daylight through the ridge. We decided to replace the purlins, trusses and the main beams because the roof was early Victorian (although, they weren't bad considering their age). Replaced slates and ridge: job done. Georgian town house, scaffolding, materials and all the work cost £4k -- in 1996. The work was done in January and (believe it or not) there was no rain (in Cornwall!) until they finished.