Roof underlayment is essential to insuring against a leaky roof and is made up of several layers of different materials. In general construction, roofs cannot withstand constant beating by the weather simply with a single layer of shingles, thus the underlayment materials must be of high quality and installed in a specific manner. Many do-it-yourselfers often forego some or all of these underlayments, however, if not installed, there is a greater chance of roof damage.
Install a waterproofing membrane on top of the plywood roof first. The membrane comes in large rolls and simply is spread along the length of the roof in strips. Each strip overlaps the one below it, offering a waterproof seal against moisture and leakage. Extra waterproofing can be laid down in the eaves and valleys of the roof to prevent leaking when ice dams form and cause excess water runoff.
The next layer of underlayment is the tar paper, also known as building felt. This helps to provide another moisture barrier on the roof and adds a small amount of insulation. The tar paper is also rolled and stapled on, just as the waterproof membrane was. Each strip overlaps the strip below it and is installed right overtop of the waterproof membrane. Extra felt can be installed at the eaves and in the valleys as well.
The final layer of roof underlayment is metal flashing. Flashing provides the extra strength and barrier needed to reinforce the valleys and each edge of the roof. The edges are the most susceptible to water damage and thus need protection. The valleys of the roof are where the water will lay the most, thus additional water sealant is needed. Flashing also provides increased stability in the roof, as it ties the different parts of the roofing together.
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