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Q.Two years ago, in the middle of the summer, I re-roofed a house here in Montana with three-tab asphalt shingles. The south-facing slope came out perfect, but the north-facing slope would not seal. I know the temperature was hot enough and the installation was done correctly. Why aren’t these shingles sealing?

A.Architect and roofing consultant Harrison McCampbell responds: There are several factors that may have affected the sealing of your shingles. One factor is climate: Many parts of Montana do not experience stifling heat during the summer months.

The second factor is the slope of the roof. The sun’s rays will strike the roof at different angles, depending on the roof slope. If the roof has a low slope — between 4/12 and 6/12 — then the effect of the sun on the north and south sides will be similar. But with the steeper roof slopes typical on newer houses — from 10/12 to 18/12 — the north roof might stay in the shade, no matter what time of day or year. If the north roof slope with the unsealed shingles is parallel to the sun’s rays, then the sun never had a chance to soften the asphalt. Conversely, the south side sealed well because the slope probably put the shingles nearly perpendicular to the sun’s rays during the hottest time of day, thereby sealing the shingles well to each other.

The slope of the roof can affect shingle sealing in another way as well: On steeper roofs, the weight of each shingle does not bear as heavily on the course below. The flatter a roof gets, the more the weight of the shingle can contribute to effecting a seal.

One solution to the problem of unsealed shingles is to install a dab of roofing cement or an asphalt-based caulk below each shingle. Otherwise, it is a moot point unless you have high wind conditions that cause a blowup of some shingles or blow off a section.

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Much of the year, the rays of the noon sun can be almost perpendicular to a south-facing roof slope, while the north-facing roof slope is mostly in the shade. This may prevent northfacing shingles from sealing.