Add an underlying screen. The HPV (Highly Permeable to Water Steam) underlayment screen helps to waterproof your roof with air and water while allowing water vapour to escape. It reduces pressure-depression phenomena of the roof, responsible for the flight of tiles during storms which enables the installation of pipes on lower slopes. It finally avoids the penetration of powdery snow, dust, pollen. How to choose the screen? Everything depends on the frame, the spacing of the rafters, the type of coverage, but also the presence or not of attics. For more accurate information call your Toronto Roofing Contractor experts today. Only a professional can judge the feasibility of the operation.