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My house is in CA (USA, we don't have many stroms / heavy winds) and i am undergoing a re-roofing project. House is about ~65 year old the current "plank decking" looks in decent condition.

Is it a good idea to put a OSB on top of it to make it a "SOLID SHEATHING" before putting the singles ?

My roofing contractor told that very few people chooses that option (1/100) or so.

Cost of adding OSB (solid sheathing) is ~20% higher (2K $). Is it worth the investment?

I already have adequate attic insulation and my question is primary related to longevity of the new roof.

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    Are you asking if it is worth the investment as it pertains to insulation value? If so, can you edit that into your question along with info. on your current attic insulation status? – Jimmy Fix-it Nov 3 '20 at 1:48
  • How many roof vents do you have? Do you have good soffit intake? Have you run the numbers for adequate ventilation? Are you doing a ridge vent? – Fresh Codemonger Nov 3 '20 at 3:18
  • Yes ridge vent will be there. There are lot of soffit vents. – David Nov 3 '20 at 6:21
  • @JimmyFix-it "Is it worth the investment" pushes this into "opinion based" territory and risks closure. "By what percentage would it increase (insulation value/structural integrity/something else)" is quite objective and can be answered. – FreeMan Nov 3 '20 at 13:41
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What do you expect to gain by this?

Your roof deck is in good shape, so if you put a roof on it that keeps it dry, I'd expect it to stay that way.

Redoing the deck happens when you let the roof go to long, it leaks and rots the deck, and you need to remediate that.

If you are worried about meteors, they will not care about another layer of wood. I doubt you are getting hailstones of a size to break your roof deck, or it would be broken, and lacking some reason like that, I don't see much advantage to adding a layer of OSB to support your roof while it sheds water and ordinary size hailstones, when your current roof deck is doing that job just fine.

  • But it is 2020. Hail stones the size of Volkswagens is not out of the question. – Kris Nov 3 '20 at 3:51
  • A layer of OSB won't stop those, either. Might help with grapefruit sized... – Ecnerwal Nov 3 '20 at 3:52
  • I think the OSB in tandem with the original decking is rated up to cantaloupe sized hail but you will need check with the local AHJ – Kris Nov 3 '20 at 3:59
  • @Ecnerwal thanks for input. Does OSB will help in achieving better nailing pattern (and holding of singles) . In the current plank decking there are gaps (although < 2mm but they are there all over the roof) – David Nov 3 '20 at 6:19
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    OSB sheathing on top of the existing plank sheathing would add significant weight. Could this cause shifting in the structure leading to cracks in drywall or plaster? What types of rooing are the existing and new? – Jim Stewart Nov 3 '20 at 10:44

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