Torch on built up flat roofing question

A good friend of mine is having leaking problems with his garage extension.

After looking at it, i quickly noticed the pitch was far to shallow to accomodate the tiles on it. It`s around 10 degrees.

After looking at various solutions, hes decided he wants to put a built up flat roof onto it.

I`ve done a bit of flat roofing in the past, but it was years ago (my 1st roofing firm if i remember) so i`m no expert, im just trying to get some advice for him. My question is, whats the correct way to use torch on felt onto a wooden base.

can you "tack" on an underlay as the 1st layer, then torch onto that with the top 2 layers? If so, can the torch on felt used for the top layers be used for the underlay?

would use 2mm torch on underlay spot stuck and nailed then 2mm fully bonded next finished with 4mm green mineral finish fully bonded

If you're not accustomed to a gasgun, nail a cheap torch on underlay bitumen face up, then gun whatever 2 layers you use on top.

Needs less flame to bond that way. You can torch an HT felt underlay straight on the ply if you think you won't incinerate it.