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Community and Q&A

Need a new roof. Why can no one agree on whether shiplap sheathing is acceptable?

Nick Welch | Posted in General Questions on

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?

Replies

  1. KEVIN ZORSKI | | #1

    Nick - Wood boards have been used successfully for sheathing for hundreds of years.In rural areas with local sawmills they are still used frequently. Plywood is a great material-it goes down quickly and is really strong,especially against racking loads.That's why it's so popular in new construction.( Though it has taken a back seat to a much more inferior product - OSB.) Your own roof has no leaks. The boards may split? Yeah, they might, but probably not badly. Wood board sheathing has the least embodied energy of all sheathings. Especially when it's already on the roof! Stay with the boards unless you hear a compelling reason not to. The roofers who want to put plywood over are either younger and less experienced or just want to feel safer with "new" sheathing down. Sorry, but you may get other opinions! Good luck with your project.

  2. Malcolm Taylor | | #2

    Kevin has pretty well covered the bases. We haven't seen your roof though, and there is a fair amount of variation between the quality and condition of old shiplap. Perhaps some of the roofers don't think it's in good enough shape? If the only issue is the presence, not the quality of the shiplap, then the sole potential downside I can think of is that the roofers might be more likely to air-nail between boards without knowing it than they would be with plywood.

  3. Joe Suhrada | | #3

    What is the pitch, may I ask?

  4. Aaron Gatzke | | #4

    When I re-roofed, it was the shingle manufacturer that required I install plywood over my shiplap sheathing to maintain the shingle warranty. I had my shingles replaced a few times previously. Both times by companies doing work in the neighbourhood. They said the shingles had a "so many years" warranty but I never got anything in writing. I now feel that maybe they were not telling me the whole story. The last company that installed the plywood before the shingles, gave me a written warranty. This is in North Vancouver, BC.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Nick,
    If the boards are sound (without rot), you'll be fine. I think this is mostly an old roofer / young roofer problem.

  6. David Meiland | | #6

    I agree you'll be fine with the board sheathing. When they strip the roof, you might want to check the condition of the nails that hold the boards to the rafters. On my house, which is older than yours and had cedar shake roofing, I re-nailed the board sheathing because the nails were very rusty and somewhat loose in places. A couple of hours and a case of ring-shank gun nails improved that quite a bit. You may not have any issues but I would at least check it out.

  7. Bob Irving | | #7

    There is another issue - whether the shiplap is aiding your household air infiltration. In most cases, if it is a "cold" roof, shiplap will be fine; if it is a "hot roof" and the design assumes airtight sheathing, it would be easy at this point to install plywood/zip sheathing and tape it.

  8. D Dorsett | | #8

    On a hot roof a self healing peel'n'stick membrane underlayment (Grace Ice & Water Shield or similar) installed per the manufacturers' instructions can air-seal as-well as (or better than) an OSB layer, and would be cheaper. But if there are sections where the shiplap is getting punky or there is excessive splitting,etc, it's probably worth the extra expense of an overlay of taped ZIP.

  9. Nick Welch | | #9

    It's a cold roof. Attic floor is air sealed and insulated with cellulose. Pitch is 7:12. Overall the shiplap seems to be in fine condition as far as I can tell. The roofers that recommended plywood didn't even look in the attic. There is one area that has a leak and may need some wood replacement, but it's just a small area next to the chimney -- I'm pretty sure the step flashing is what's leaking. Thank you all for the advice!

  10. D Dorsett | | #10

    With a vented attic sealed at the attic floor, if the shiplap isn't moldy rotting or splitting, there's no need to put down a layer of OSB.

  11. Joe Suhrada | | #11

    I think you are fine repairing the small damaged area and forgetting new sheet goods. The shingle warranty isn't worth a damn. Buy the thick shingles with the longer warranty if you are going to live there a long time. Maybe you get twenty five trouble free years. None last "a lifetime." Good luck collecting in any event. All the old houses in all the old cities have this kind of roof, and I own two. One is a 5/12 ranch circa 1957 and another is a 7/12 Bungalow circa 1925. Every board is still there on both buildings under newer architectural hurricane-rated shingles.

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