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While this list isn't exhaustive, it will highlight some common mistakes that roofers make when they are installing a metal roof. These mistakes don't always point to intentional negligence, but they are things that should and likely would be considered by an experienced metal roof installer.
#1. Installing a metal roof over shingles without an underlayment
The question of installing a metal roof over an existing shingle roof is perhaps one of the most commonly asked questions from consumers. A metal roof can absolutely be installed over an existing asphalt shingle roof but an underlayment or other barrier (i.e., lathing boards) should always be used.
Why?
The underside of traditional metal roofing panels is protected by a primer, very similar in makeup to the coating that is on the visible side of the panel. If you were to install a metal roof directly over shingles, that primer would eventually wear down as the metal expands and contracts against the rough grit of asphalt shingles. As the primer wears down rust may introduced (not good). This event is likely not covered by the paint manufacturer's warranty.
#2. Improper screw installation
This isn't as crucial for concealed fastened systems as it is for through fastened systems like Tuff-Rib. Incorrect fastener installation on a through fastened metal roofing system is a fast track to a leaky roof. An under-tightened screw will not allow the washer to properly seal against the metal roofing panel and moisture could easily be introduced around the shaft of the screw and into the roof deck.
An over-tightened screw will "blow out" the washer resulting in premature degradation due to the washer splitting as well as from increased UV exposure. When the washer breaks down you'll again have an issue with moisture being introduced to the roof deck around the shaft of the screw.
A screw that is installed on too steep of an angle will also prevent the washer from achieving a proper seal against the panel. Care should be taken to install the screw as straight as possible to ensure the washer has a 360° seal.
Metal roofing fastener technology has greatly improved over the last several years to mitigate some of these issues. Learn about new metal roofing fastener technology here.
#3. Failing to use proper sealants
A properly installed metal roof utilizes sealants to prevent leaks. These sealants are easy to install and come at a minimal added cost, yet some installers still do not use them. An example of some sealants that you would find on a properly installed metal roof would include butyl tape. Butyl tape is a roll of butyl rubber that serves as a gasket for metal roofing trim and other select parts.
As a general rule, anytime metal is touching metal on your roof, a gasket or sealant should be used. For example when your rake trim is installed along the linear edge of your roof, one side of the trim touches the roofing panel. Butyl tape should be used along the entire run to prevent water from running under the rake trim and possibly onto the fascia. You may also find butyl tape or similar sealants on panel overlaps for a low slope roof, on top of the eave trim near your gutters, and under pipe boots and flashings to name a few examples.
Again, this is not an exhaustive list but does contain a few common mistakes that you should look out for when installing a metal roof or when you are having a new metal roof installed.