PART III: The Basics of Roof Underlayment Installation

Roof installation does not start and end in the nailing of shingles. It involves installing solid decking for adequate support and applying underlayment that will serve as the waterproofing membrane.

Basics of Roof Underlayment Installation

Roof underlayment installation procedures and intricacies vary per type. While your contractor is probably already adept in installing the material properly, it is still better for you to have an idea about it so you will know what to check for.

Felt Underlayment

•    In low-slope roofs, which include 2:12 up to 4:12, courses of felt underlayment should overlap at least 19 inches. This will make for a double layer of felt across the entire roof.

•    In steep-slope roofs, which include 4:12 and up, the upper courses should overlap the lower ones by at least 2 inches.

•    Felt is usually fastened using staples, but if you are in a high-wind area, plastic windstrips are recommended to be used along the edges to prevent tearing.

•    Plastic caps may also be used as fasteners for high-wind areas.

•    Felt underlayment should also overlap the edge metal at the eaves while being overlapped by edge metal on the rakes.

Synthetic Sheet Underlayment

•    Slope limitations in the installation of synthetic underlayment vary by manufacturer. Some require overlap for low-slope roofs while others don’t.

•    Many synthetic underlayment products are meant to be wrapped around the roof edge, protecting the edges of the roof sheathing. This is also done to avoid problems caused by moisture.

•    Plastic caps or roofing nails are generally used for fastening synthetic underlayment. The use of staples is not recommended, as this type of underlayment is not self-sealing.

Self-Adhering Membrane Underlayment

•    This is practically a peel-and-stick product so installation is generally easier.

•    Once the adhesive is set, self-adhering underlayment is very difficult to remove – so laying it properly on the roof deck in the beginning is of utmost importance.