- 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.
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