Roof window installation advice needed

[Dewi]

I am looking to install a roof window into my loft (Velux type window). The roof is tiled with a felt membrane underneath. The installation of the window seems quite straightforward but I am a bit concerned that after I cut through the felt, how do I ensure that the seal between the window frame and the felt is waterproof?. I know there are sealer units that do this job and I understand that the seal is overlaps the assembly at the top so this should not let any water in. What I cannot get my head around how do you seal the sides of the window frame with the felt. My thinking is that if any rainwater gets under the tiles higher up the roof and runs down on the felt along the edge of the window then there is potential for it to enter at this point, or am I missing something?

 

[Roofer]

No you haven't missed anything, the potential is there for leaks but remember that the purpose of the underlay is not to keep water out, the tiles do that, the underlay acts as a secondary protection only (its primary purpose is to prevent wind damage to roof)

Having said that it is obviously better if there is a seal between the rooflight and the underlay and Velux make an underfelt collar which does the job

 

[masona]

Not sure if you know this, the velux opening should have double rafter and trimmer.