Underlayment & Flashing
In most areas of the country, a roof can't withstand exposure to the elements with just a layer of shingles to protect it. An additional underlayment layer(s) is needed to deter water penetration.
Properly sealing out water before shingling requires these elements:
Laying A Waterproofing Membrane
Start by adding a waterproofing membrane along eaves and valleys. These are high-risk areas for leaks; valleys due to improper flashing installation and eaves because of ice dams. We used a thick, bituminous material with an adhesive backing.
The membrane protection provides extra protection against water penetration and is also required by code in "Snow Belt" states at the eaves.
Waterproofing membrane material can be considerably more expensive than just using felt. Most professional roofers recommend the extra protection, and for a do-it-yourselfer, it's good insurance against a leaky roof.
Rolling Out Builder's Felt
Builder's felt (tar paper) is the most widely used roofing underlayment. It comes in rolls and its thickness is gauged in pounds. Typically, a roll of 15 lb. felt may cover about 400 sq. ft. and a roll of 30lb. would cover half that area.
Laying felt is much easier and faster with two people; one rolling, one stapling/nailing. Position the felt roll flush with the gable end.
Working toward the far end or valley, unroll about 5' of felt, square it with the roof edges, press out any wrinkles, and staple/nail it in place as you go. Proceed at 5' intervals to the far end. When you run into a valley, angle cut the felt to lie down the middle of the valley. If you forego the membrane, lay down extra felt at the valleys.
Overlap the second row of felt on the first row by 2". Work up the roof this way to the ridge (peak). Leave the ridge exposed and continue underlayment on the other side. When you reach the ridge, fold the felt over both sides (overlapping 2") and fasten it into position.
Once the roof has underlayment, it can withstand exposure for a few days. However, if it gets wet, the felt may wrinkle up a bit and that increases its chances of tearing by wind.
In some cases, metal flashing can be saved and reused during a tear off. But any flashing that is bent, torn or looks bad should be replaced, instead of repaired.
One type of flashing we recommend is galvanized steel, W-shaped valley flashing. It fits in the valley like typical "V" flashing, but has a small ridge sticking up in the middle. This ridge prevents runoff rain from running down one side of the roof and splashing up under shingles on the opposing side.
Set the flashing flush with the drip edges at the bottom of the valley, trimming it with tin snips if necessary. Nail the flashing every 1' or so near its outside edges (never nail in or near the middle). For extra protection, seal seams between pieces and all nailheads with roofing cement.
Fastening Drip Edges
Drip edges are corner-shaped metal strips that nail along the edges of the roof. They allow water from the roof to run cleanly off the edge. Without a drip edge, water may run down the side of the fascia and siding -- causing stains and eventual damage. The drip edge also supports the part of the shingle that extends past the decking.
Along the eave, nail the drip edge under the builder's felt and over the fascia. As an extra precaution, you may want to cover the nailheads with roofing cement then stick the felt down.
At the rake (overhang), install the drip edge over the felt and fascia. This protects the felt from high winds and blowing rain.
NOTE: The length of eaves and rakes may require using more than one piece of drip edge. Like flashing, lap upper pieces over lower pieces.