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While watching the builders next door do their thing, I became curious with their technique to install plywood on edges neighboring the next townhouse, and OSB for the rest (which for these narrow interior units is a single panel in the middle, but they use OSB all the way to the non-attached side on the end units). What's the reason to pick one over another?

photo of the roof

Follow up: I'm guessing this is related to OSB's tendency to expand on the edges when wet, possibly for the flashing and the plywood side of the firewall that would rest on top of this edge of the roof. Here are some links that I came across when doing a bit of research:

Comparison from home inspectors

Comparison from UMass

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Twofold theory:

Perhaps OSB is cheaper at the moment for this builder than ply and wants to use OSB as much as they can. OSB and Ply are apparently seen as the same in terms of performance and code see: http://bct.eco.umass.edu/publications/by-title/choosing-between-oriented-strandboard-and-plywood/

However, OSB, when cut, is more susceptible to water damage on the cut (the cut exposed raw wood not covered in the bonding agent/glue). Once OSB edges get wet, it expands like a sponge...which would ruin the roof.

So, it looks like they might be using as much OSB as they can, but have to switch to ply whenever cutting.

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  • That makes a lot of sense, though seems to me they could get a lot more OSB usage if they laid it vertically instead of horizontally. Then again, that would create much longer vertical seams which would be more susceptible to leaking... and the point of the roof is not to leak in the first place...
    – The Evil Greebo
    Commented Oct 4, 2011 at 19:11
  • I'm seeing a lot of cut OSB in their install (every other board is cut, roughly in half). But I'm suspecting it has to do with moisture from the edges, where they have to install flashing along the firewall.
    – BMitch
    Commented Oct 4, 2011 at 19:13
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    @TheEvilGreebo, roofing should always be installed horizontally, joining together the most rafters for strength.
    – BMitch
    Commented Oct 4, 2011 at 19:15
  • Stronger in what sense? For static/live load or some other sense?
    – The Evil Greebo
    Commented Oct 4, 2011 at 19:19
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    You are right on both counts DAO1, OSB is cheaper, but not as moisture stable as plywood. Looks like they are using maybe 30 to 40% OSB, strictly as a cost savings. If they were building top quality, they would be using Advent-Tec T&G roof sheathing, which is about twice the cost of OSB and 25% more than good CDX. Using OSB on a sidewall, even with good step flashing and Grace is risky. If it gets wet, the nails will not hold and the panels will warp, wave and lift.
    – shirlock homes
    Commented Oct 4, 2011 at 19:36
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It could be that they are trying to slow down a fire from spreading. The material at the edges might be more fire resistant, so the fire will spread to adjacent buildings more slowly, allowing the occupants more time to escape. You might be able to verify this by checking the local fire codes for conjoined (not sure this is the right term) buildings.

See how if there was an MS Paint fire, the flames would stay away from the next building (assuming the material in the middle burns faster). enter image description here

The man would then have more time to get to safety, or be saved by Captain Construction.

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  • Many states and localities have code requirements to firewall between contiguous units of multi-unit dwellings.
    – dmckee --- ex-moderator kitten
    Commented Oct 4, 2011 at 20:48
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They're required by 2012 IRC to use a fire retardant treated plywood on the roof within 4' of the UL assembly (the firewall that's between each unit). They can use OSB in the middle as it's much more cost effective than plywood.

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  • Can you point to the actual section in the code that requires this?
    – Tester101
    Commented Jan 20, 2015 at 18:04
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Personally, I hate OSB for anything but temporary uses. It's not as strong as ply, it's more suceptible to moisture than ply, more easily damaged than ply. They use it for things like roofs and such cause it's cheap. They are counting on the roof being covered before any extended exposure to moisture, and they certainly don't expect constant foot traffic on it. When I had my own roof replaced, I specified ply, not OSB. But then, I'm biased.

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  • Interesting...you missed the one, main disadvantage of OSB: its weight. Given the significant increase of lighter-weight plywood cost, most people eventually get beyond their hatred of OSB.
    – Lex
    Commented Oct 27, 2012 at 0:12
  • From my experienced perspective OSB is stronger, more uniform, and more dimensionally consistent than plywood.
    – isherwood
    Commented Jul 26, 2016 at 15:38
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The outer edges are fire treated plywood and have a reddish tint. Code requires a minimum of 4 feet from the fire wall. From seam to seam must be a minimum of 2 feet.

So to reduce waste the stagger from 4 feet to 8 feet (full sheet). Anything in between fire treated wood can be plywood or OSB.

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Most likely the boards along the edge are a treated or coated sheathing product. Some common ones are "Blazeguard" or "Fireblock". In many areas earlier city codes called for gypsum board to be installed along the edges and these products provide the same fire blocking qualitites.

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The OSB these days has way better glues/resins than ten years ago. OSB, especially subfloor, holds up to moisture better than CDX. Advantech is one of the best and it is an OSB product! I wish people would get over their unbased biases and chill out!

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To reduce overall roofing cost, plywood panels are often used around the edges of a roof (near the eves) because this is where there is higher wind pressure pulling the nails out. OSB is then used in the interior or field panels of a roof where wind is not as much of an issue.

Plywood grasps the nails holding flashing, the finished roofing like asphalt shingles, or the like, better, especially long-term, than OSB which tends to soften more quickly with moisture cycles over the years.

Although OSB was specified by the engineer in a cabin I built for a customer, because the location was very windy, I convinced the customer to upgrade the whole roof to plywood so that it would hold up better. ..Especially because it was a 2 to 3 story roof in places and steep, and thus hard to fix if something went wrong.

I've had my own shed roof which was sheathed with particle board by the last guy fall apart as wind driven rain pushed moisture between the flashing and the felt. The moisture softened up the wood. The nails pulled out in a wind, and the whole thing needed to be replaced. I replaced the particle board decking with plywood. And I double flashed the edges so that wind driven rain can't push moisture in, even if the edge seal fails.

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  • This is a bit short to be a really effective answer. Are there any other pros/cons that would be helpful to note in your experiences? Also, just a note, looks like this question was asked several years ago.
    – BrownRedHawk
    Commented Sep 14, 2015 at 20:21
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I'm not sure of any difference at all - but I could swear I remember Mike Holmes saying that he liked OSB for roofs because it's stronger and a bit more resistant to water (more glue, I guess).

It kind of looks to me like they had a mix of OSB and plywood and they just were randomly using whatever they grabbed...

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  • It's new construction on a large project, so they have lots of everything. They're being very systematic about using it next to common walls which has me puzzled.
    – BMitch
    Commented Oct 4, 2011 at 17:36
  • @BMitch maybe they have different fire ratings, so the more fire resistant one has to be near adjacent buildings.
    – Tester101
    Commented Oct 4, 2011 at 17:41
  • Well its hard to tell but it looks like the entire sheeting is OSB... if the fire rating was the concern you'd think it would apply to the walls too...
    – The Evil Greebo
    Commented Oct 4, 2011 at 17:46
  • @TheEvilGreebo the walls would not be as much of a concern for fire spreading (aside from interior walls, which I'm assuming are protected in some other way).
    – Tester101
    Commented Oct 4, 2011 at 18:00
  • OSB is more water resistant? That doesn't seem to be correct. Most any OSB I've used turns into a sponge once wet. (I take that back. See my answer...I guess OSB has come a long way...)
    – DA01
    Commented Oct 4, 2011 at 19:00
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Respecting posts/comments referring to OSB water wicking and degradation (too little space allowance for reply by comment)--

If it were a matter of edge wicking of water and consequent swelling or crumbling, the photo would show only full, uncut OSB panels, but does not.

Could it be, simply, that the roofing subcontractor was using new materials along with materials he already had on hand and wanted to get rid of? That would seem to indicate that it was leftover OSB that did not have an indoor space for storage, so needed to be used up or be lost to the weather.

That theory is consistent with the OP pictorial, in that sheathing a roof would always start at a rake edge, or at interior, attached-unit-demarcating "wall" projections, and continue to an opposite edge, or inward, with interior sheets or partial sheets in between.

Note: it is clear that structure comprises individually owned units--each prospective owner to be responsible for upkeep of his/her own roof (not the "common" roof). Each section is a roof unto itself, needing to be build only to residential (and therefore minimally specified) construction standards.

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  • This is a major project with fresh materials being delivered for each stage of the project, so it doesn't appear to be a reuse of leftovers. My best guess is that the flashing around the firewall is a common location for leaks, or they may need a higher fire rating at that junction. If it makes any difference, those firewalls get cut about 1' above the roof, framed with plywood, and covered in flashing.
    – BMitch
    Commented Oct 27, 2012 at 0:19
  • @BMitch So you are saying that the plywood is solely/primarily for nailing (and re-nailing) effectiveness; the OSB for cost cutting; the per-se cutting of the OSB a mere irrelevancy--a red herring. Seems most plausible so I agree. The trimming of the projecting panels goes without saying. Thanks for the comment.
    – Lex
    Commented Oct 27, 2012 at 0:45
  • I figure the OSB is more susceptible to water damage whether or not it's been cut, so it's more about keeping it away from high risk areas.
    – BMitch
    Commented Oct 27, 2012 at 1:27

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