How to do a reroof over an existing roof?

October 17, 2008 7:41 AM

Reroofing: Should 15# felt used when doing a roofover on an existing asphalt/mineral 3-tab roof? The existing roof is is fair condition, so a tearoff isn't absolutely required.

I don't find much on the internet that tells much about roofover details. One site made it sound like felt was not required and that the new courses should line-up with the old course below. I can see how that would simplify the process and help to keep horizontal lines nice and even. But is that correct?

Any advice appreciated. TIA.

posted by toucano to home & garden (5 comments total)

It sounds like you're planning on leaving the existing shingles in place. Before doing so, make sure the asphalt base of these shingles is in good shape (meaning that the granules are off, but the tar-paper component is sound.)

While leaving existing shingles in place is not the best practice, if there are no leaks or potential leaks then installation of rolls of felt paper is not necessary over old shingles.

Have your roofer confirm that the substrate is in good condition - you cannot tell the condition of the plywood if you don't take off the old shingles.

I'd take off the old shingles, if at all possible, and start anew. Get a good guarantee from the installer whichever way you go, and, don't pay for the full job in advance of completion.
posted by mightshould at 8:22 AM on October 17, 2008

If the old roof is in good condition, with just one layer of shingles: You don't need felt and you would be wasting money to do a tearoff.
posted by bricoleur at 10:02 AM on October 17, 2008

Re-roofing over existing shingles will save you a little money as long as the existing shingles are flat and not cupped. You don't need to use felt when re-roofing over existing shingles. In fact new felt will prevent you from using the technique below that reduces lumps.

You should trim the width of the first two layers so that they butt up against the bottom edge of the older singles as shown here. After trimming the first two rows, all the rest should butt up against the next course without trimming. This prevents a lumpy roof in which you have multiple layers of shingles. This means your first row will have a reveal of only 3 inches but all the others will have the normal 5 inches.

Don't worry about trying to line up the slots in the tabs with the old shingles. Those will be covered by two layers of new singles. Besides, it is unlikely that you will be able to accurately duplicate the old spacing anyway.

Make sure you trim the first 5" row as shown in the picture above so that the sticky self-seal strip will seal down the second course.

Remove the old ridge cap singles to prevent a big lump there as well.
posted by JackFlash at 10:28 AM on October 17, 2008

Most of the above sounds good, but I would also ask these guys.
posted by Huplescat at 10:46 AM on October 17, 2008

From "those guys" book, one of the things to be worried about in roofing over old shingles is, how well can your structure handle the weight of an additional roof's worth of shingles. Once you carry one bundle of them up a ladder you may wonder how your roof can handle one roof's worth of shingles.
posted by Kid Charlemagne at 4:25 PM on October 17, 2008