How to approach builder's problem?

[Dave W]

I'm confused now. I don't know anything "3 section". All I mean is on your photo where you've written "Water falls at speed down roof, hits top of gable and deflects pushing existing water under the tiles if sufficient flow". The flashing under your yellow arrow looks as if it sits on top of the slates of the main roof, and is under the slates over the window.

You could be right - maybe the flashing should be on top of both sets of slates, not underneath as I was proposing. Also perhaps I should not refer to it as "flashing" but as "lead valley" as someone else described it.

[AnthonyL]

Yes, the tiles are laid over the valley.

I wish this drizzle would develop into something stronger so I could have another look but the valley is upturned on the window side at almost a right angle, ie the valley looks like a __| The angle and perspective of the photo tends to make it look flatter than it is.

I've had a RICS surveyor on the phone for an hour. He's had a look at some photos and has a lot of concerns. He doesn't rule out water getting under the dormer roof tiles from the sheer speed of the flow as it comes down the main roof. We're not too sure how the felt from the main roof flows around the dormer, or how any felt under the dormer roof allows any leakage to drain away.

Unfortunately he's busy and then on holidays and unavailable for a month but he does agree that the builder shouldn't be let back anywhere near this job.