On the discussion on sized materials and what to use, as opposed to the original question of what is the most common size uzed, then there are some good points, and some that I feel are misleading.
For walls, the requirement for sheeting is to prevent racking and creat shear strength. Stud walls will just lay over if there isn't any diagnal bracing. For centuries, this was done by cutting boards into the corner studs at an angle to provide this bracing. It's worked for a very long time at keeping a house stiff and standing. Then when plywood became available, it was used in just the corners of homes. Half inch was the standard. In some homes, that was all that was used, but other homes, foam or the black, crappy, paper like stuff was used. I forget what it's called, but you can still buy it. I just don't see it used anymore.
Now it's pretty much common building practice to wrap the entire exterior walls with OSB. Oriented Stand Board. It is nothing like particle board or MDF. It is very strong. Is it stronger then plywood? I honestly don't know. CDX plywood is what we used to use on roofs in CA all the time, but then switched to OSB. With the way code is there, and how ever change is dont to make the houses stronger and better to withstand earthquakes, I figured OSB was stronger then plywood. I never looked into it, and don't have any facts to say either way. I use it, and I'm very pleased with the results.
Half inch plywood is actually 15/32 thick and 7/16 OSB is 14/32 thick. The difference is minimal in comparing thickness. The plywood is made of three layers of wood, glued together in oposing directions. Three very weak peices of material glued together to make one strong materil. The direction of the grain and the glue are what make plywood strong. 4/5 will have a fourth layer of wood, and 3/4 will have five layers. The layers are where you get all your strength.
The glue and strands of wood in OSB is what makes it so strong. I've seen allot of half inch plywood fail for a variety of reason. Mostly a layer will come off, but it will also snap when too much weight is on it. I've seen OSB fail after its' been wet for awhile. Mostly, the ends expand and the nails don't hold in those locations anymore.
If kept dry, both are pretty much equal in strength.
For roofing, one of the important things to consider is how well the material holds a nail. For this, I also don't think it matters too much what you use. What is VERY important is that you use nails long enough for the shaft to penetrate the material. The point must be out of hte wood with some of the shaft showing. This is the only way that the nail are able to stay in the wood and have any holding power. I've seen too many roof overs and even new roofs where short, one inch nails were used, and they didn't penetrate the wood all the way. They just pop out real easy. If you are losing shingles, that's a good thing to look for.
For those who claim that OSB leave ripples in a roof, I disagree. If the rafters or trusses are perfectly flat and aligned, then you'll never know what was used on the roof for sheething. With H clips, OSB will be impossible to distinguish between half in plywood. While I agree that thicker wood is stronger, and will support a heavier load, you will still have waves or riples in the roof if the wood that you attach it to isn't perfectly true.
Since it's actually fairly common for their to be some minor variations in the wood used and trusses that are bought, the best solusiton is to use premium, architectual shingles. This will hid any imperfections in the lumber. Three tab shingles are the least expensive to buy,but they show every dimple or high spot on a roof.
For example, I'm building a deck right now, and I've seeing a consistant 1/8 variation in the width of my lumber that's bigger then 2x6's. Lengths are all over the place too, some are dead on, others are up to 3/4 of an inch longer then the others. I've had some 2x10's that were 3/8 wider then the thinnest ones. It got so bad that I had to rip a 2/8 down to size so I could use it.
This happens in roofs,but usualy it's the angle or curve of the board that I see more then anything. If you take care, and check every board, then it's not an issue. If you have a crew of 20 somethings trying to get it done so they can do whatever it is that's more important to them, sometimes a board ends up on the roof that you wouldn't really want up there. You don't even see it unless you're looking for it, or after it's been decked. There are some very big, very expensive homes that I've seen shims used between the decking and the rafters. If you're not looking for it, or trying to figure out what's wrong with the roofing system, you'll never know it's there.
What's really sad is that how a house is built, and what is used to build that house doesn't have anything to do with selling that house. It's all about the finish and what materials are used, depending on the latest trends. I worked on a house a few months ago that has allot of issues. They painted the cabinets, put in new tile floors and a granite counter top. At first, it looks nice, but the granite used wasn't of a very good quality and every time I was in that house, I noticed more things about it that I didn't like. That house sold in two months time for the full asking price. It was very nice looking at first glance, but the house itself was a money pit in my opinion.
There's a dozen ways to do the same thing, but most of the time, only one that is cost effective. The question will always be what is the best way to build for the least amount of money.
Eddie
