Need a new roof. Why can no one agree on whether shiplap sheathing is acceptable?
My house is from 1950. Zone 4C. Roof is 1 layer of 3 tab asphalt shingles that are in need of replacement (it will be a full year off). Sheathing is 1×8 shiplap boards (actual dimensions ¾x7¼, with â…œxâ…œ rabbets). The boards are not butted tightly; there is probably about a ¼” gap on average.
I’ve gotten 4 quotes to re-roof, and 2 of them either insist or highly recommend laying new plywood over the existing sheathing, while the other 2 have no problem roofing over the shiplap. I found this strange and did some internet research. It did not help very much — opinions seem to be about 50/50 on the issue.
The arguments for adding the plywood layer mainly center around the issue of gaps, knot holes, and possible splitting of the shiplap boards. That all makes sense and seems reasonable, but then why do so many contractors seem to not feel that that’s such a problem? And why does my roof have no leaks due to this problem? It’s probably already on its third roof. If the shiplap is such a problem, then you’d think it would have manifested itself by now.
So I’m basically left feeling that it’s a coin toss… except that it costs $2000-3000 extra, which obviously sways me toward the opinion to leave it alone and omit the extra plywood layer.
I’m not really concerned about shingle warranty issues because I don’t expect to ever collect on that anyway, as they seem loaded with exceptions, prorating, etc.
Is there any more compelling information that could settle the issue?