You have to tear the roof off first and if you're not up to that, go home. Determine the thickness and if it's on an open cornish, be sure and buy a BC grade, which is good on one side.
Whether you use plywood or OSB, it doesn't matter. The following rules apply to both.
The new decking must span across a rafter, joining three, or it will sag. When you cut out the decking, make sure you don't leave a piece next to the repair that doesn't meet this requirement or it will sag.
Sometimes, when repairing a roof and not replacing it, you might want to replace as little as possible. So if you only cut out the decking between two rafters, you need to install 2x4 blocks to support the new wood.
Don't forget to make sure the decking has gaps on all sides to allow for foundation movement, and humidity and temperature changes. When new decks are applied we use "h" clips not only to strengthen the decking between the sheets but to space it.
Don't forget to nail it down with coated nails that won't back out. The nail head heats up first and then along the shaft. The reverse happens when the day is over. The head cools last. This makes for an alternating wedge, growing and shrinking daily. That is one reason nails back out. The other is the movement and heaving of the home. Have you noticed how fence nails back out so easily.
Have you ever seen a redecked home where the roofer didn't space the sheets? He thinks the bows forming in the roof are either defective plywood or foundation movement. He's an idiot.
Bet you thought I wasn't going to do that.
If the decking you want to replace is bowed, don't try to machine gun it down with a lot of nails. Won't win in the long run. The memorized torque in the wood can lift a bus. Cut it out right now.
The rough side of the deck goes up and the waxy side goes down so you won't slip and break your neck. Or my neck for that matter.
OSB is superior in my humble opinion. It is not wafer board. It is Oriented Strand Board and has a superior glue and move of it. It does gas a little longer but if you vent, like I do from time to time, it will be okay. That's formaldehyde I'm referring to.
It is heavier, harder to drive and pull nails out of and burns hotter. The last trait I have not verified yet.
If you need to repair the deck on an open cornish, you'll need to match the wood. It won't be as wide as what's up there now. It just won't be. So use your eyes to make it look as good as possible.
In the Dallas/Fort Worth Metroplex, most older homes have plank decks while in the suburbs like Richardson the homes have plywood. Newer residences as in those of Colleyville and Southlake have OSB. In Grand Prairie I've seen black sheet rock that was installed after WWII.
One pet peeve: bend over the nails on the wood you've pulled. Whether it's on the roof looking for my foot or has fallen down and can't get up, bend the nails over. The homeowner is an NRA member and his barefoot granddaughter can't quit screaming because her foot is impaled on your board.
Time to run a away. Hear that cocking sound? It's not an attorney.
