Is nailing OC durations over 3-tabs feasible, or is tear off mandatory?
It’s not “mandatory” but definitely much better to do so. With that said, I usually do one or two roof overs a year. My requirements to do so 6/12 or greater, minimal flashing details, old roof must be laying very flat.
Any problems with nails tearing through on the voids?
Not personally. I always try my best to hit the double thickness area with every nail.
With 3-tab layovers, always made sure the nailing pattern put the nails in the center of the old tabs. Plus, the 5 5/8 exposure going over a 5" on the old roof might not lay too flat.
I second everything that has beed said so far. I don’t do many layovers, but when I do, I butt the top of the new shingle up with the bottom of an old course. It makes it so the old layer does not cause extra or enlarge voids below. Downside is you have to cut the bottom off of the first course and you are at the mercy of the exposure and straightness of the original. I think it is/was printed on the bag, but I rarely see it done this way.
In theory there should be less voids than a single layer
I took the 1st 12" off, minus the overhang, flipped the 1st shingle for a starter, then ran with that. The first course exposure would be 3". They’d lay smooth and the edge wouldn’t look like a sub sandwich with 4 layers showing.
That works great if you can get old sized (12x36in) archs. Otherwise you will need extra bundles if using metric archs.
I wouldn’t advise. Can’t warranty it. Can’t fix pre-existing issues.
Yup. Told him tearoff or go fish…
Sorry guys. Even though manufacturer specs allow it, we won’t do it. There is true value by putting shingles down on a flat solidly sheathed surface. I’ve seen architectural layovers using a decent shingle and after 7 years, they looked old and beaten.