Hello, First time post here. I need to repair my detached garage roof, as the asphalt shingles are crumbling. underneath the one layer of shingle is a layer of underlayment(like a tar paper), and underneath that are wood shingles. I checked inside the garage, up in the rafters, and all wood hingles look in great condition, and there is no sigs of leakage. My question is, can i just take off the layer of asphalt shingle, and underlayment, and put on a new layer of heavy weight underlayment and a new course of asphalt shingles? The other option is to completely strip the roof of both asphalt and wood shingles, and purchase and apply 1/2 inch plywood decking to the roof rafters, and then tar paper and shingle over that. These are 2 very different options, with different costs, and time associated. I am doing this myself, with the help of my dad, brother, and buddy, all of us who have at best intermediate skills. Thanks for your insight.

Last summer I had my roof done - everything except for a flat section approx 12x9ft, (we were debating on getting the chimney removed) anyway...I now want to replace the roofing - it has roll roofing and it seems not to be leaking but is old and patched with tar in a few spots, I bought the new stuff and I just need opinions on should I rip up the old roofing first or should or can I even install it right over the old stuff... I am kinda pressed for time so the quicker the better, however, I don't want to be doing it over again next year. if I can just go over it do I just use roofing adhesive or ? Thanks for any help...