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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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If it makes a difference, the yellow sound/fire block is being covered in plywood, too. –  BMitch Oct 4 '11 at 17:38
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Here's the product that Shirlock recommends (AdvanTech sheathing): advantechperforms.com/product-lineup/advantech-sheathing.aspx –  BMitch Oct 5 '11 at 2:26
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6 Answers 6

up vote 10 down vote accepted

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 Oct 4 '11 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 Oct 4 '11 at 19:13
    
@TheEvilGreebo, roofing should always be installed horizontally, joining together the most rafters for strength. –  BMitch Oct 4 '11 at 19:15
    
Stronger in what sense? For static/live load or some other sense? –  The Evil Greebo Oct 4 '11 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 Oct 4 '11 at 19:36
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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 Oct 4 '11 at 17:36
    
@BMitch maybe they have different fire ratings, so the more fire resistant one has to be near adjacent buildings. –  Tester101 Oct 4 '11 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 Oct 4 '11 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 Oct 4 '11 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 Oct 4 '11 at 19:00
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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 Oct 27 '12 at 0:12
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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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Call the MSPaint Fire Department! –  The Evil Greebo Oct 4 '11 at 18:16
    
+1000000 for the Captain –  Steve Jackson Oct 4 '11 at 19:01
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This just proves that answers should be allowed to be upvoted multiple times! –  The Evil Greebo Oct 4 '11 at 19:11
    
Many states and localities have code requirements to firewall between contiguous units of multi-unit dwellings. –  dmckee Oct 4 '11 at 20:48
    
That's the first time I've seen Comic Sans MS used properly! –  Chris Cudmore Dec 10 '11 at 14:19
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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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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 Oct 27 '12 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 Oct 27 '12 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 Oct 27 '12 at 1:27
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