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?
When re-roofing and doing a tear-off do I have replace the roof deck with plywood
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Replace it now. Like TODAY. That's BAD.
With that kind of bad shingle wear, your shingles are likely to create ice dams which will cause water to back up and leak through the roof, causing interior damage, compounding the cost.
In the summer, the roof won't hold the water because it can't freeze, so the curls will only grab a very little water before it drips free.
In the winter, however, a little retained water will freeze, expand, and capture more water, which will freeze, and so forth.
I wouldn't worry too much about it. The plywood is not typically supporting a load of more than the paper and asphalt shingles that were placed, even more so if you have a considerable pitch on your roof to where it would be difficult or impossible for a person to walk on. It basically serves so that you have something to nail the shingles down securely.
Most newer homes use plywood, I have an older home that used what looks like rough cut deck planks and those are mostly still original. The last time I had my roof replaced, they had to change out a few boards that were terribly dry rotted and some that were badly damaged from previous water damage by the oh-so responsible homeowners who had the home before me. Other older boards look noticeably damaged by the most recent nails but they continue to hold down the shingles and I would still feel safe to stand on my roof.
As long as the shingles are doing their job then the plywood will not be exposed to any water, and as long as your attic has good ventilation during the summer time then you should have nothing to worry about for the next 15-20 years at least.
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Best Answer
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.