How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is the difference between plywood and OSB?
Sunday, July 29, 2018
Here’s the big question we get asked regularly when we come out of an attic: “Is my roofing sheathing plywood or OSB?” Everyone seems to have a friend that has told them that one is good and the other is bad—or, at least, not as good. OSB (which is an acronym for Oriented Strand Board) is the new kid on the block, but more popular, with almost twice as much produced annually in the U.S. This is primarily because plywood and OSB have comparable, but not identical, performance properties, while OSB is cheaper. A contractor saves hundreds of dollars by choosing OSB instead of plywood to build an average home.
They are easy to tell apart at a glance, if you peek up into the attic at the underside of your roof sheathing. OSB (shown in the attic above) looks like wood chips laid at random angles and pressed together, while plywood has a recognizable overall wood-grain pattern.
They are also manufactured by different methods. Plywood is made from thin veneer layers of wood that are peeled from a spinning log and laminated together. There is always an odd number of layers and each layer is glued with the grain at right angles to the one underneath it, for strength and stability of the material. OSB starts with rectangular wood strand pieces that are arranged in cross-oriented layers which are pressed and heat-cured.
Here’s the pros and cons of each material:
- Both off-gas formaldehyde, but plywood produces less because not as much glue is used to manufacture it. While formaldehyde is primarily an irritant, that can cause eye and throat irritation, coughing, and allergic reactions, the EPA states that “long-term exposure to high levels has been associated with cancer in humans.” See our blog post How can formaldehyde gas in a house be a problem? for more information on this issue.
- Plywood is slightly stiffer, by about 10%, than comparable OSB.
- Better at holding nails and screws.
- Plywood tends to delaminate as it ages in exterior applications, especially at the base of siding in the high humidity of Florida.
- More expensive than OSB.
- OSB is considered a “greener” material, because it can be made from small, younger and farmed trees.
- OSB is more structurally uniform, without the soft spots at gaps in the veneer layers of plywood.
- Can be manufactured in larger sheets.
- Swells more when wet, especially at the edges of the panel. Stays somewhat swollen after drying.
- About 15% heavier than plywood.
Also, see our blog post What is the average life expectancy of plywood siding?
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To learn more about roofs and attics, see these other blog posts:
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