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I have seen where people say that using an underlayment on a roof over is a must, and I have heard some say that it isn't. I have roofed new roofs and also done tear offs, but I have never done a roof over as of yet. During some tear offs, they had multiple layers of shingles and I never saw one that had an any underlayment between the layers. Just curious as to anyone's take on this.
 

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40 yrs roofing here. Much to my surprise, Atlas calls for 30# over existing shingles before overlaying. My experience, and the conclusion of local building inspectors, is that felt between the layers traps moisture and causes the deck to rot. I've seen it on several occasions. I use Certainteed almost exclusively--they do not require felt between the layers.
 

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Seen a 500+ square apartment complex being roofed with no underlayment. Just decking and 30 year directly over it. We always use it cause its cheap, why not. Builders call it the "dry in" stage. From my understanding it is the felt stage when builders building a ton of houses at one time and have the decking crews felt them which gives them a temporary dry roof system to have other tradesmen come inside and continue working for production purposes until they can get the roofers to roof the houses. Once shingles are nailed down, the underlayment, whether its regular felt or the new synthetic stuff, gets punctured thousands of times by roofing nails rendering it useless. When I saw those multi million luxury apartments being put up with no underlayment, I began to question the real need for it. We use it on every home, but more for the homeowner who is told it comes with any new roof. Is it "necessary" , who knows. Like I said, its cheap, why not
 

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It’s code here in SC and certain manufacturer’s warranties call for specific underlayment i.e. GAF Feltbuster for System Plus/Silver/Golden Pledge for example.

We also have a hurricane preparedness SafeHome grant that requires underlayment and then Ice and water on all the seams along with a lot more addtl work.


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40 yrs roofing here. Much to my surprise, Atlas calls for 30# over existing shingles before overlaying. My experience, and the conclusion of local building inspectors, is that felt between the layers traps moisture and causes the deck to rot. I've seen it on several occasions. I use Certainteed almost exclusively--they do not require felt between the layers.
My 30 years agrees with your 40. I do the butt and run method. The top of the new shingles butt up against the bottom of the old. Little trick getting started but as you suggested there is minimal space between layers. I have seen a few disasters with roof-overs, which are taboo in Miami-Dade now and putting roof coatings on shingles is even worse.
 

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When you do a roof over, you don't have to put an underlayment between the layers because the previous roof acts as the underlayment.
 
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