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How much roofing material can a roof hold? There are codes that determine the limits and there are structural criteria that determine the limits. Many codes permit the application of two courses of
asphalt shingles assuming the roof is structurally capable of supporting the weight and that’s why it is important to have your home inspection conducted by a home inspector who is licensed to practice engineering.
When a roof needs re-shingling, many factors go into determining if it is advisable to overlay the existing course of asphalt roofing shingles or whether it is recommended to strip the roof down to the wooden roof deck prior to
reshingling. Factors such as the strength of the roof rafters, the weight of the existing and proposed roofing shingles (asphalt roofing shingles are manufactured in various weights per square), environmental loading factors,
and other factors must be taken into consideration by the home inspection engineer, as well as the condition of the existing shingles.
It is not advisable to install new asphalt shingles over badly cupped roofing
shingles because asphalt shingles provide the best service when installed on a flat surface. Other factors, such as wind blow off, will be taken into account by the home inspector; roofs with too many courses of shingles are
more subject to wind blow off because the roofing nails must penetrate a lot of spongy material that does not provide a good base to hold the roofing nails.
Be careful of the straw that broke the camel’s back, your
home inspector can advise you regarding how to approach a roof re-shingling project; look for a home inspector who is licensed to practice engineering, P.E.’s are trained to evaluate structural projects (don’t be
confused, many home inspectors are not engineers).
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