Shingles over good SBS on 1.5:12 shed roof addition

Reroofing main structure 5:12 with dimensional shingles and need to do something with the low slope to get another 20 years out of it.

Since the SBS is OK at this pitch, I can’t see a real downside on a shingle over.

NOTE: Long ago I’ve used “half lap” asphalt roll roofing and for more life it could be laid up “third lap.” Has anyone considered this method with shingles? - seems like it would be effective on low slopes with non-tab shingles.

I would shy away from putting shingles on a flat roof. Once you put the nails in you will compromise the sbs. What is wrong with the sbs now? Why is it not a good idea to leave the flat roof as it is now? Have any pictures of it?

The SBS is about 10 years old, and while still in good shape, there are a
couple of patches with barely detectable alligatoring. It is still very
flexible and will not crack or tear.

New SBS won’t last 20 year and I want to finish this roof once and for all.

As far as once you put a nail through the SBS, is it really any different
that ice guard that you put down under shingles for 2 to 4:12 pitches? A
properly seated roofing nail is supposed to create a seal.

I could just put coating on the SBS every 5 years and probably get the 20
year objective. They say you cannot glue SBS onto Capsheet, but coating
sticks which is a contradiction.

Basically trying to figure how to get 20 years of life and NOT tearing the
SBS off.

Whatever you do don’t use shingles on that slope under any circumstances. The reason is all the laps and seams without proper slope to drain. It doesn’t matter what tricks you use you are asking for trouble if you do. If it were me what I would do is use one of the self adheared SBS modifieds. They make a nailable base sheet which you can nail on top of your old SBS, then a self adheared base sheet on top of that. Then a self adheared cap sheet to finish it off. I believe that system will have about the same life as the rest of your shingle roof. If you want to be safe you could also coat it every 5 years or so.

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Nope. Never. Asking for trouble. Walk away.

APP modified bitumen cold process (glue). Any time you roof over another roof you loose about a third of the new materials life but climate definetely plays into that. If you’re in say Ohio or Michigan that’s not as much a factor but if you’re in say Georgia or Alabama it’s more. If you’re looking for 20 years you can glue down a smooth surface then cover that with granular surfaced. That should give you the 20 year life you’re looking for.
Never ever ever install shingles on that low of a slope. Never!