Self-adhering underlayment provides critical protection in vulnerable locations.
Synopsis: When you’re making sure your roof is safe from water penetration, it’s important to use the right materials in the right locations. In this article, FHB editorial adviser Mike Guertin describes his process for making sure a roof is protected from water infiltration by using self-adhering underlayment in vulnerable locations such as roof valleys, eave edges (to help deal with ice dams), rake edges, roofs to vertical walls (such as dormers), sidewall protection, the bottom of dormer valleys, and joints in sheathing. When working with self-adhering membranes, Guertin cautions that it’s important to work carefully and to adapt based on outdoor temperature, to buy the appropriately sized membrane for the job, and to get a strong bond so that the membrane adheres firmly to the sheathing.
I’ve done enough roof repairs to know that storm water that gets past the shingles and underlayment doesn’t penetrate through the roof sheathing. Instead, it leaks through the joints between panels and the places where the panels tie into other building elements and roof planes.
The first and most obvious areas to focus on when preparing a roof for shingles are roof penetrations such as chimneys, plumbing vents, and skylights. A more thorough approach, though, involves protecting leak-prone areas such as rakes, eaves, valleys, and dormers. Some of these storm proofing details are installed before the underlayment and shingles. Others are layered into the underlayment or roofing so that water is redirected to the roof surface.
Because I’m going to all the trouble with these details, some people ask me why I don’t just cover the whole roof with self-adhering waterproof underlayment. Call me cheap, but I think I’m getting 98% of the benefit of covering the whole roof with 10% of the material.
Working with sticky stuff
Plastic-surfaced self-adhering membrane is best for the detailed work involved in storm proofing a roof. The plastic folds tight into and around corners, and the adhesive is more aggressive than that on granular-surfaced self-adhering membranes. Still, there’s a definite learning curve to working with this material. Here are a few tips.
Change with the seasons
Your approach to working with these membranes should vary depending on the temperature. In general, the adhesive backing is less sticky and, therefore, more forgiving when it’s cold out. Long sheets can be lifted up and repositioned if necessary. Misplace a sheet or get it stuck to itself in hot weather, and you might as well kiss it goodbye. Rather than risk it, I work with 4-ft. to 6-ft. lengths in hot weather, and lap end joints by 3 in. to 4 in.
Size the membrane for the job
I use various widths of membrane, sized depending on the application. For example, W.R. Grace makes Roof Detail Membrane in 9-in. and 18-in. rolls, and York Manufacturing makes Home Seal in 4-in., 6-in., 9-in., and 12-in. rolls. I often just cut the strips to the width I want from regular 36-in.-wide rolls.
Divide and conquer
Several of the details require that a portion of the backing sheet be left on so that the membrane can be integrated later with the roof underlayment or roofing. Removing the backing sheet in stages also simplifies the installation process at wall-to-roof intersections and valleys. Grace embeds a thin wire, which it calls Ripcord, into its Roof Detail Membrane at strategic positions, making it easy to split the backing sheet into convenient sections. When I’m working with another brand of underlayment that doesn’t have embedded wires or when I want the backing sheet cut at a position where there is no wire, I score the backing sheet with a light pass from a sharp utility knife.
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