Should I tear off my old roof or just roof over it?
Complete removal of the existing roof system is mandatory if it contains
moisture, or if the additional roof layer exceeds safe load design, or if
local codes require that the existing roof be removed. Most local codes
will only allow two layers of 3-tab shingles.
A second layer of shingles cannot be installed over Dimensional /
Architectural style shingles, because the weight would exceed most local
code requirements and the irregular surface of the existing shingles would
not allow the new shingles to lie flat and seal properly.
Installing shingles over an existing roof system requires proper
preparation of the surface. Blisters, splits and other roof membrane or
flashing defects should be repaired in accordance with good roofing
practices to attain a surface which is smooth, dry, clean and free of
sharp projections and depressions. If the roof is not smooth and flat, the
flaws will project and be seen through the new roof.
Finally, the service life of the new shingles should be considered. The
U.S. Department of Housing and Urban Development performed a study which
concluded that as much as 25% of the service life of the new shingles will
be lost when installed over an existing layer of shingles due to heat
How long should my roof last?
Just as the human body ages and changes appearance over the years, so too
will your roof. Due to the severity of the roof environment, even a
one-year-old roof may look different from a roof that was just installed.
Research indicates that aging begins soon after the shingles are installed
and progresses rapidly during the "Initial Curing" phase of its life
cycle. During this stage, minor granule loss may occur, a few small
blisters may develop or the shingles may curl slightly at their edges. The
good news, however, is that after this "Initial Curing" stage the shingles
enter a long period of slow aging, which lasts for the major portion of
the shingles’ natural life.
Is ventilation really important?
Yes. Attic ventilation might seem like a minor consideration, but when
done properly, it can extend the life of your attic and roof structure;
and save you hundreds of dollars in repair and energy costs.
In the summer, hot, moist air in the attic can warp roof sheathing and
cause shingles to deteriorate prematurely. A hot attic also makes a home
more difficult to cool and can result in added energy costs.
The Roofing Institute Tile Installation Manual