Few things can be as annoying to a home owner as roof leaks. Sometimes they are mysterious, as well. Different types of roofing materials are subject to different types of leaks. For example, flat roofs have seams that can lift-up and leak. Flashings against the chimney or parapet walls (fire walls) can pull away. Edge seams can split open. While these defects are easily remedied, they are impossible to identify unless you are on the roof. The most common flat roof leaks are caused by the roof coating wearing-out. Bituminous coatings (tar) need to be recoated every 3 years or so, depending on the coating thickness or the exposure to the sun. Reflective silver coatings, usually on rubber roofs, can last longer. Naturally, poorly installed roofs will not last as long as better installed roofs. Common installation errors include neglecting to replace rotten roof sheathing or rafters, neglecting to build-up low spots so standing water attacks the coating or roofing material, re-roofing over moist sheathing so trapped water vapor causes the roofing material to lift-up or bubble. Many times, roof penetrations for roof deck supports, plumbing vents, air conditioner lines, bathroom vents, skylights, etc. are incompletely or too thinly sealed.
Leaks in shingled roofs can be caused by loose or poorly installed chimney flashings. These metal flashings should be applied in overlapping steps, and the top edges tucked into the brick mortar and caulked, or tucked under the chimney siding in the case of non-masonry chimneys. The roof penetrations need flashings that are tucked well under the shingles and extend vertically above the roof. Plumbing vents usually have rubber collars (seals) around them in addition to flashings. After about 10 years of sun light, these seals start splitting, and leaks commonly occur. Obviously, worn shingles or shingles scratched by tree branches can cause leaks. Shingles are installed in overlapping layers, so if the top layer fails, the shingle underneath still offers protection. Underneath the bottom layer is a layer of thin roofing felt. Usually, however, once the top layer of shingles show patches of worn-off granules, leaks will start to appear.
Slate and clay or cement tile roofs can last decades, but they are brittle. One or more may crack and slip down each year. These must be replaced or repaired as soon as possible. These types of roofs also have flashings and penetrations that require sealing and resealing with roofing cement every 2-4 years. While tile roofs have a row of tiles on the ridge (peak), slate roofs have a large bead of roofing cement that can split open and leak. Slate and tile roofs should be carefully inspected annually.
Another common cause of leaks is not due to the roof, but to clogged gutters and downspouts. In the winter, clogged gutters can freeze with water to form an ice dam that can cause melting snow to be trapped on the low edge of the roof. Water can then back-up under the shingles or slates, get through the felt and leak into the house. The same kind of ice dam can occur if the attic is not insulated well. Heat from the house melts the snow. Water runs down the warm roof until it hits the area over the soffit or eave that is outside the house and is still cold. At this point the water freezes again to form an ice dam.
All the above issues are usually associated with older homes. Newer homes have well insulated attics, trees shorter than the gutters, and younger roofing materials. Flashings and seals, however, are shorter-life materials that must be periodically inspected, no matter when the house was built.
by John R. Berry, PE
© Copyright 2003
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