11 March, 2015

Plywood vs. Oriented Strand Board

Several years ago, Sid and I were stripping the roofing material from a leaking flat porch roof when he launched into his ritual diatribe about how little I was paying him (he was being paid as much as I was). I was just about to reply in my customary string of profanities that he should get busy with the job, when I disappeared through the roof. All that Sid had seen was the column of dust rising where I had been just a moment before. I had landed unhurt but angry on the porch floor below. When I got back on the roof, I found that a previous contractor had used Oriented Strand Board (OSB) for a section of the roof deck. It had swollen and disintegrated due to the water damage. Thus began my hatred of the use of OSB for roofs.

On most old flat porch roofs you would find what is called “one-by” pine planking (1”X 8”, 1”X 10”, etc.) used for the decking. Planking is the costlier choice for a roof deck nowadays. Plywood first appeared around the turn of the 20th. century, but did not become a common decking material until the late 1940’s. Plywood is constructed of thick layers of veneer oriented at right angles to one another for strength and stiffness. OSB came onto the market in the late 1970’s and is made from 3- to 4-inch wood strands that are applied in layers and pressed together with adhesive. Like plywood the layers are oriented at right angles and weighs 10 percent to 15 percent more.

OSB costs about $5-6 less per panel than plywood, which is a significant savings when 16 or more panels may be used for a two-car garage roof, for example. Georgia-Pacific (a major producer of OSB) states in a 2003 research paper that CDX plywood holds up better under excessive wettings, has an all-wood surface that results in better glue adhesion. The ‘C’ and ‘D’ are the finish ratings of the panel surfaces and the ‘X’ means it’s rated for exterior humidity.

Plywood is 15 percent lighter and flexes less than OSB. They further state that their tests indicate that plywood holds nails better, too. Personally, I have no problem using OSB for wall sheathing, but I’d never use it for a walk-on roof deck and am loathed to use it on a gable roof--especially with the snow loads that we get in our area (the Northeast Ohio snowbelt). Several Ohio communities and several states including the state of Florida have banned it for roof decks due to swelling caused by high humidity along the edges.

Admittedly, OSB is better than it was ten years ago, I still will not allow it on any job that I'm involved in.  For a walk-on deck I specify 3/4” CDX plywood and 5/8” CDX for a roof that’s not walked on (though I prefer 3/4” CDX). On gabled roofs the building code specifies 1/2" four-ply CDX, I specify 5/8" for the snow loads we get. On shallow pitch roofs, I'd still would want 3/4" CDX. I would rather overbuild than have a roof deck collapse under snow

One last thing--if you ever do a roofing job with Sid, please smack him with a shovel for me.

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