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Kulp Roofing

What is Underlayment?

Underlayment helps protect your roof decking (plywood, OSB, etc.) and serves as the critical waterproofing layer over the exposed deck.

Never use non-breathable felts made out of polyethylene as underlayments over the roof deck. These materials may trap moisture and prevent it from escaping which is detrimental to a roof and its surface.

Make sure your contractor follows all local codes when it comes to installing deck protection, especially if you live in a high-wind area. In the event of a shingle blow-off, deck protection can provide short-term water resistance. It can also protect the roofing material from resin and other materials in the wooden deck that is harmful to asphalt.

Ice and water shield is a must at least over the first three feet from the eave. If you have an extended overhang, you should, by local code, install six feet. Many local codes recommend extending the ice and water shield two feet past the exterior wall into your home. One layer of deck protector must be applied for decks with a 4:1 slope.

For low-slope applications, install GAF’s underlayment half-lapped up the entire roof slope to promote water-resistant protection. Many roofing contractors often overlap (us included) the ends by at least six inches if two or more pieces are needed to finish a course. This prevents water from seeping between the ends.

Moreover, we try and keep end laps at least six feet away from end laps in adjacent courses. At penetrations, such as rakes, sidewalls, hips and ridges, the deck protection must be installed before the leak barrier.

The lap deck protection should be lapped over hips and ridges by at least six inches securing the deck protection with enough fasteners to keep it in place.

With proper application, the material will stay in place, keep the surface safe, and prevent it from blowing off. This is why the underlayment is critical to a roofing project and why prices vary by the products chosen.


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