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Metal Roofing Underlayment

Underlayment

Metal is the fastest-growing segment of the American roofing market. It is durable, wind- and fire-resistant and practically maintenance-free, but it must be installed properly with an appropriate metal roof underlayment. Metal roof installation typically requires specialized tools and skills. However, the options and installation process for metal roofing underlayment are largely the same as shingles or other roofing materials, though you may need one extra layer.

Preventing Ice Dams

To prevent ice dams, start your metal roofing underlayment with a layer of ice guard, also known as ice and water shield. It forms part of the metal roof underlayment over the eaves. It should extend about two feet up the roof from the top edge of the overhang.

Start installing the ice guard by snapping a chalk line 35 ½ inches up from the edge of the roof, and laying the ice guard along the line. This will give the 36-inch ice guard a half-inch overhang from the eaves. If necessary, add a second strip of ice guard material.

Underlayment Options

The next part of a metal roof underlayment is typically a layer of felt paper, or building paper. It can help keep leaks out of the attic, and it provides a little bit of insulation. Local building codes may specify a certain weight of felt paper, but 30-pound paper is usually a good bet. Metal roofing underlayment is unique in requiring an extra slip sheet layer, which is typically red rosin paper. The slip sheet helps keep the metal from sticking to the felt paper while installing the roof.

There is also another option for metal roofing underlayment. New polymer underlayments sheets are thinner and easier to work with than layers of tar paper and rosin paper. The sheets are strong, waterproof and slick, so they can replace both of the other underlayment layers.

How to Install

Installation of either type of metal roofing underlayment is fairly straightforward. Add strips of felt paper or polymer underlayment horizontally across the roof, making sure to overlap the layers by at least four inches. Staple the ends of each strip, and once per square foot in the middle of the strip. Where two parts of the roof meet in a valley, overlap the metal roof underlayment generously.

Proper metal roof underlayment is important for keeping the attic dry. It is largely the same as the underlayment for other roofing materials, but may require an extra sheet to ease roofing installation.

Author Steve Graham is an expert on green building who writes for several home improvement publications. He’s full of great, practical home improvement answers, and incidentally, he’s pretty funny – so send him a message.

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