Plastic roof channels/valleys - condensation issue

That's if the builders are still running. 3-4 days sounds like a big job maybe that includs a day of putting up scaffold. I am not sure they would retro fit a breathable membrane the tiles either side of the valley will need to be taken off with the cement bedding then the GRP valley then membrane added but it should go under the existing so any water that gets in will run off over it and not under it, so I am not sure how far they need to strip back tiles to get under. Then valley boards then new GRP valley as the old one will be destroyed getting it off then re set tiles.

Yeah, all of which combined is why I'm really stressed about it, and dread asking the builders to do that. If I were the builder I'd be really annoyed being asked to re-do all that. Seems like a huge upheaval to put something basic right, but if not done now then I'm looking at a self-made workaround of insulating under the valleys, which I frankly don't feel confident doing, and I would have a timebomb of moisture seeping into the wood over the next 10-20 years, by which time the builders probably won't be going any more, and I'll be a crusty old man.
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