There are three ways to install a metal roof over shingles. In this article, I will go over the three different ways and what their pros and cons are. The local building department should always be consulted with because each department might have their set of requirements for doing a replacement roof in their county or city.
1) Remove the Shingles before Installing the Metal Roof The first way to install a metal roof over shingles is by completely removing the old shingles before installing the new metal roof. A benefit to doing it this way is that you can see if there is any rotten plywood that needs to be replaced before installing the new roof. Most roofers can walk around the roof and feel for "soft" spots, which indicates rotten or deteriorating plywood. However, this technique is always "fool proof" either. If you have a bad leak in your roof for a long time, it might be best to get into the attic and look up at the roof to see if you notice any plywood that needs to be replaced. You will have a much stronger metal roof if it is being installed into good wood as opposed to rotten wood.
Here are the Pros and Cons to Removing the Shingles
2) Install Underlayment Right over the Shingles before Installing the Metal Roof The second way to install a metal roof over a shingle roof is by applying an approved underlayment right over the old shingle roof. The main benefit of doing it this way is that it is the fastest way to install a new metal roof over a shingle roof. This is the minimum that needs to be done. Some people think that they can just install the metal roofing right over the old shingles roof without any barrier in between, but this is highly not recommended. The problem with this is that as the metal heats up and expands or cools down and contracts, it moves a little on the roof. What ends up happening is that the shingles act much like sandpaper to the back of the metal roofing, gradually scraping away the back and exposing the bare steel.
Here are the Pros and Cons to Installing the Metal Roofing Right over the Shingles
3) Install 1x4 Purlins down over the Shingles before Installing the Metal Roof The third way to install a metal roof over shingles is by applying 1x4 purlins over the shingle roof. This is by far the most common way to install a metal roof. As long as the plywood under the shingle roof is in good condition, this way can provide many benefits.
Here are the Pros and Cons to Installing the Metal Roofing over 1x4's
Obviously, there are always different ways things can be done, and in the case of installing a metal roof over a shingle roof, there are three main ways. Depending on what the building department says in your area because that takes priority over everything else, the right way boils down to what you feel comfortable with. If you asked several roofing contractors what they felt was the best way, I wouldn't be surprised if there wasn't a majority winner, and therefore I don't want to say that there is a preferred way over another.
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