New Roof Shingles
I bought a house 2 years ago, it is a 60's rancher. The pitch is low maybe 4x12. I believe the current shingles were put on in 2000 or 2002. They are 20 year 3 tab. I am on a bluff and the winds get high at times, 25-35 with gusts to 45 or so. Since I moved in every strong wind storm bends back and breaks tabs off. I have noticed that many tabs were not stuck to the next shingle, I have also noticed that they seem to get debris under the tabs witch prevents them from sticking. The ones that are stuck down seem ok. I have been putting roof cement under the tabs of the ones I find not stuck.
I am thinking about a new roof with 30 year single tab shingles which look like these. They are 140mph rated and about 2x as heavy as the old ones.
The roof sheathing feels like 1/2" plywood and is a little wavy in places.
Although the underside looks good. I know the best thing to do would be remove the old shingles and re sheath it with 3/4" plywood.
My question is: If I just re shingle over the existing shingles just how would it affect the life and wind resistance? If I get 25 years out of them I would be 90 so I don't think I would still be in this house.