I'm pretty sure we're near the end of the existing roof - we're about 25 years out on what I'm guessing is 25 year shingles. I see numerous nails coming up even though I hammer them down and tar them as much as I can – I can’t keep up with them. The shingles seem to be dried out - cracking and losing their flexibility. We have some leaking, although I try to keep up with it. We have a little leaking from the skylights.

However – the biggest problem I notice when I’m on the roof is the waves. It looks like the sheathing is sagging between the rafters so that the ends of the sheathing sheets are pulling up, which is causing the nails to pull out. Or there were just not enough nails put in, so that as the sheathing warps, it’s pulling up the nails and creating voids. In any case, it’s leaving us with high and low spots, and definite voids.

In this town we are permitted to lay 2 courses of shingles over the existing shingles.

Seems to me that just putting new shingles over the existing shingles without doing something about the sheathing will cause the new shingles to fail early. My thought is to pull off the existing shingles, and put down another layer of sheathing over the existing sheathing. Then – as long as I’m doing that - cover with some sort of barrier material over the sheathing. Then cover with new shingles.

Or am I over-thinking this and should I just put up new shingles over the old?

I want to do this once – we have lived here 20 years and I want to stay here