Pełna deska i papa zamiast membrany. Dlaczego?

We're moving on to forming the trusses with 15mm thick OSB tongue-and-groove boards. Icopal roofing felt. Why didn't the builder opt for a membrane? Music used in the video: Fashion Saxophone Hip-Hop by Infraction [No Copyright Music] / Do It    • Fashion Saxophone Hip-Hop by Infraction [N...