How Roof Underlayment Protects a Home

Learn about moisture management systems and their role in keeping water out of your home.

The roof is your home's first line of defense against bad weather. While shingles stand as the first layer to keep water out of your home, they rely on a crucial ally below— the roofing underlayment —to effectively seal out moisture and reduce the potential for water damage if your shingles are blown off. That's why what a builder uses for roofing underlayment matters.

Building science has come a long way regarding these moisture management systems, especially in the last decade. Traditional roofing felt (No 15 and 30) used to be the standard for roofing underlayment. But, thanks to innovations, a lighter, more durable alternative, synthetic underlayment, is growing in popularity. Not only are synthetic underlayments better at repelling water, but they also are a safer alternative to roofing felt, providing enhanced walkability for the roofing contractor.

When selecting a moisture management system for your home, here are three best practice installation methods to best protect your home from moisture- approved and recommended by the Insurance Institute for Business Home & Safety- FORTIFIED ROOF™

Method #1: Tape the seams between the roof sheathing that forms the roof deck. Apply the underlayment over the self-adhering tape. Horizontal laps must be a minimum of 4 in. and end laps must be a minimum of 6 in. Space fasteners at 6 in. O.C. along all laps and at 12 in. O.C. vertically and horizontally in the field- all fasteners should have a minimum 1-in diameter cap.

Method #2: Install a double layer of underlayment, also known as a half-lap. Apply the underlayment in a half-lap fashion along the horizontal seams and 6 in on the vertical seams. The spacing should be 6 in. O.C. along all laps and at no more than 12 in. O.C. in the field of the sheet between the side laps- all fasteners should have a minimum 1-in diameter cap.

Method #3: Cover the entire roof deck with a full layer of self-adhering underlayment that meets ASTM D1970. This method is commonly referred to as “fully adhered” or “peel & stick” underlayment. While this method offers the most protection from moisture, it is important to ensure your roof deck has adequate ventilation before installing.

Note: Always verify that the installation method follows the manufacturer's installation requirements and all the applicable building codes.

For Method #1 & #2, the type of fastener used to secure the underlayment is critical. Selecting a cap fastener with a minimum of 1- in diameter will improve the holding power in strong winds and help reduce moisture penetration. Additionally, cap fasteners are often required by most shingle and underlayment manufacturers' warranties and building codes. Why? There's no comparison when you stack them up to a typical A-11 staple. Sure, a staple is fast to install, but cap nails provide superior holding power compared to a staple alone, and they can help reduce moisture penetration at the fastener hole.

When it comes to cap fastening, you can hand drive caps or you can use an innovative cap fastening system, like the CN100B Cap Nailer paired with STINGER® NailPac®. Hand-drive caps are prone to overdriving and can deliver an inconsistent finish. They also crack under extended UV exposure and don't do as well directing water away from the fastener hole. Not only is a collated cap fastening system quicker, it helps reduce waste and clean up time for the crew, another time saver!

The NailPac includes 1-inch plastic collated caps and 1" x .083 stainless steel or electro-galvanized ring shank nails that seal out moisture and provide superior holding power in up to 150 mph hurricane-force winds with gusts up to 180 mph. They stand up to the elements better than other fasteners on the market, help keep water out of fastener holes, and are the only fastener of its kind approved by the Insurance Institute for Business & Home Safety FORTIFIED Home™ program. Plus, they boast dynamic speed on install-over-hand drive caps with the STINGER CN100B Cap Nailer. The pneumatic cap nailer is less labor intensive, making for an easier job that reduces human error, delivering a consistent finish every time.

When protecting your house from future moisture problems, what's on top of your roof deck is important. Rely on the brand that's made a name for itself with its moisture defense systems: STINGER.