Installing a second layer of shingles to a residential home is a significant home maintenance project. In the case of a simple, ranch style home, the project can be completed without extraordinary skills if a homeowner has the right tools, and a willingness to engage what can be a physically demanding task. Check the tips at the end of this article for a list of factors to consider before beginning this project.
Determining the Soundness of the Existing Roofing System
Examine the edge of the roof to verify the number of layers of shingles currently installed. In most areas, building codes allow for up to two layers of shingles installed on a roof. If two layers already exist, the existing layers must be removed before installing a new layer of shingles. Select a length of roofing nails that penetrates completely through the new layer of shingles, existing layer and the wood roof deck.
Physically inspect the entire roof to determine the soundness of the wood roof decking, which is under the shingles.
Identify any areas in which the roof deck is damaged or deteriorating. Roof deck which is sagging or excessively spongy can be evidence of rotting wood under the shingles.
Remove shingles from any area which is suspected of containing deteriorated or damaged roof decking. Replace the damaged roof deck with 3/4 in. OSB board.
Apply a layer of shingles over the newly repaired area to bring the underlying surface back to a level height, consistent with the rest of the roof plane.
Cover the landscaping, flowers and shrubs under the work area with the blue poly tarp. Shingles which fall from the roof can damage landscaping, or become embedded in the soil.
Installing 2nd Layer of Shingles
Cut the tabs off a number of shingles. The resulting shingle will be 1/2 the height and the same length as an uncut shingle. This is the starter row shingle. Cut enough shingles to cover the length of the lower edge of all roof planes.
Apply the starter shingle so that the top edge butts up against the bottom edge of the 2nd row of shingles, and the bottom edge is flush with the edge of the roof. Apply this starter shingle along the entire length of the lowest edge of all roof planes. The goal is to create a level surface for the first course of new shingles.
Apply the new singles according to manufacturer's instructions.
Begin at the lowest edge of the roof plane, and apply the shingles with galvanized or zinc coated nails. Apply the shingles to all roof planes, ending a row of shingles at the top of the plane (ridge or peak) and cut to follow the profile of any outside corner.
When shingles laying on two roof planes approach each other in a valley, weave the layers of shingles across the valley to create an integrated, unbroken water barrier across the valley.
Apply a liberal amount of plastic roof cement at the base of any fixtures, plumbing pipes or roof vents which break the plane of the roof surface.
Applying Roof Cap Shingle
Cut a number of shingles into thirds, separating each shingle tab into an individual roof cap shingle. Cut the upper portion of the shingle at approximately a 15 degree angle. These roof caps will lay atop each other, and the angled area will be completely covered by the finished portion of the shingle.
Apply the roof cap to any outside corner (ridge) beginning at the bottom of the ridge, and overlapping them toward the top edge of the ridge. Apply the final cap so that it completely overlaps the upper corner of the roof planes.
Apply roof cap shingles along the peak of the roof, overlapping the shingles on each side of the peak. Apply one nail in each side of each roof cap shingle.
Cut the final roof cap shingle in half, trimming off the unfinished side of the shingle.
Apply the final shingle with two nails through the finished surface. Apply a small amount of plastic roof cement over the nail head to seal the surface.