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My home was build in 1920 and uses planks for the roof deck. I was planning on just removing the old asphalt shingles and installing new asphalt shingles. Makes sense to me; until a contractor friend told me it was code to install OSB or plywood over the old 8 inch planks. I don't see why this would be necessary if the planks are still in good shape?

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    In many places, code that applies to new construction doesn't apply to simple like-for-like replacement. So I'm sympathetic. It may be beneficial, it may be required in code for new build but it might not be legally required in your case.
    – RedGrittyBrick
    Commented Apr 7, 2016 at 15:31
  • I don't know the answer, but I'd question the wisdom of doubling the weight of the roof deck if the new OSB/plywood is required. I'd strip the old planks off rather an leave the extra weight, unless the new stuff added is allowed to be thinner than if it were standalone. Hopefully someone with more knowledge can comment on my two-cents opinion :)
    – pbristow
    Commented Apr 7, 2016 at 18:02
  • I think that your question triggers that age old adage, it depends on your local codes. The Internation Code Council says: R905.2.1 Sheathing requirements. Asphalt shingles shall be fastened to solidly sheathed decks. So, look online, or tell us what area you are from.
    – Ben Welborn
    Commented Apr 7, 2016 at 20:33

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Are your 8-inch planks laid tightly together, or is there significant space in between? Old roofs that were originally topped with rigid shingles (wood or tile) were sometimes installed with spaced decking planks, and that kind of decking is not suitable for use under modern asphalt shingles, which are flexible.

However, if the planks are relatively tight together this is not a concern. The planks should be fine as long as they aren't damaged, and you'd only bother replacing damaged sections. Strip old shingles, add new underlayments, add new shingles.

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  • 3/4" OSB works well as a patch material if you have occasional gaps, cracked boards, or large knot holes. Cut out sections down the center of the rafters and drop in the OSB pieces.
    – isherwood
    Commented Apr 7, 2016 at 21:11

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