Thread: 3/8" plywood roof sheathing

3/8 plywood roof sheathing with clips. I called excessive deflection but didn't call the thickness as a defect because other than the deflection, I couldn't see any problems. Also deferred to a Structural Engineer for futher evaluation if further evaluation was wanted because the buyer was adamant that it needed to be replaced.

I wasn't very happy with the roof, but felt like I had to see probelms to call it a defect. Home built in '67, so it's been there a while. Any suggestions on this?

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As you probably know, the span rating for 3/8th inch plywood with edge support when used for roof decking is 24 inches. That means the thickness would only be considered a "problem" if the rafter spans were greater than 24 inches on center.

"Excessive deflection" is a bit of a nebulous term and you need to define just what "excessive deflection" actually is for a roof decking product.

When push comes to shove, I think you will find yourself getting into arguements with other "experts" what really constitutes "excessive deflection" and they would then beat you over the head with the span tables from the code (assuming the installation in question meets that minimum standard).

Originally Posted by Phillip Stojanik

As you probably know, the span rating for 3/8th inch plywood with edge support when used for roof decking is 24 inches. That means the thickness would only be considered a "problem" if the rafter spans were greater than 24 inches on center.

"Excessive deflection" is a bit of a nebulous term and you need to define just what "excessive deflection" actually is for a roof decking product.

When push comes to shove, I think you will find yourself getting into arguements with other "experts" what really constitutes "excessive deflection" and they would then beat you over the head with the span tables from the code (assuming the installation in question meets that minimum standard).

Nope, I didn't know the allowable span for 3/8", just that I never installed it as a carpenter.
"Bouncy", "squishy", "practically a trampoline"... got to call it something and nebulous beats not mentioning it. I hear "excessive deflection used a lot and I think generally, most people know what it means. Everyone knows what the first two adjectives mean, but I wouldn't use them ia report. I'm open to suggestions, it never hurts to improve although code arguments pretty much bounce off my back unless I see them as safety/damage issues. Egress, handrail/guardrail, pretty much everything electrical, some structural, etc., but I don't quote code, just call it a defect and recommend correction.

kenton,
if the ply has a 24/0 rating stamped on it and it's exterior,exposure 1 or exposure 2 it can span 24" with a 30# max live load! a 10# dead load is assumed.

Originally Posted by Phillip Stojanik

As you probably know, the span rating for 3/8th inch plywood with edge support when used for roof decking is 24 inches. That means the thickness would only be considered a "problem" if the rafter spans were greater than 24 inches on center.

What Phillip did not state, because he qualified it with "with edge support", is that 3/8" plywood WITHOUT edge support has a maximum allowable span of 20 inches.

Failed, bent, deformed, improperly installed, ineffective, etc., 'H' clips do not provide "edge support", thus 24" span (spacing of the trusses) exceeds the allowable span of the 3/8" roof sheathing.

I come across this quite a bit but without the H clips. My recommendation is to consult with a qualified contractor to determine the proper course of action. In my experience the deflection is not bad enough to recommend a SE. My main concern is possible further damage from heavy snow.

Originally Posted by Kenton Shepard

3/8 plywood roof sheathing with clips. I called excessive deflection but didn't call the thickness as a defect because other than the deflection, I couldn't see any problems. Also deferred to a Structural Engineer for futher evaluation if further evaluation was wanted because the buyer was adamant that it needed to be replaced.

I wasn't very happy with the roof, but felt like I had to see probelms to call it a defect. Home built in '67, so it's been there a while. Any suggestions on this?

Originally Posted by Kenton Shepard

Nope, I didn't know the allowable span for 3/8", just that I never installed it as a carpenter.
"Bouncy", "squishy", "practically a trampoline"... got to call it something and nebulous beats not mentioning it. I hear "excessive deflection used a lot and I think generally, most people know what it means. Everyone knows what the first two adjectives mean, but I wouldn't use them ia report. I'm open to suggestions, it never hurts to improve although code arguments pretty much bounce off my back unless I see them as safety/damage issues. Egress, handrail/guardrail, pretty much everything electrical, some structural, etc., but I don't quote code, just call it a defect and recommend correction.

On what basis to you call "excessive deflection" as a defect if you don't use the span tables in your current code as your point of reference? It was "bouncy, squishy, practically a trampoline" when you walked on it? If the construction meets your current code, and if someone calls your call, what do you tell them? What are the implications of the "excessive deflection"?

I agree with Vince that deferral to a SE seems like overkill. If it is a problem that needs correction, a competent contractor should be sufficient to determine an appropriate method and cost of repair.

Originally Posted by Jerry Peck

What Phillip did not state, because he qualified it with "with edge support", is that 3/8" plywood WITHOUT edge support has a maximum allowable span of 20 inches.

Failed, bent, deformed, improperly installed, ineffective, etc., 'H' clips do not provide "edge support", thus 24" span (spacing of the trusses) exceeds the allowable span of the 3/8" roof sheathing.

Jerry, you're so knowledgeable I don't get a chance to tweak your comments very often but this is my opportunity!

One finds out what constitutes "with edge support" from the footnotes. In APA publications the footnote reads: "Tongue-and-groove edges, panel edge clips (one midway between each support, except two equally spaced between supports 48 inches on center or greater) lumber blocking, or other."

In the 2006 IRC see Table R503.2.1(1) footnote d, (page 107) which reads almost the same as the APA footnote.

In short, H-clips or "edge clips" are considered edge support for purposes of Span Rating--they are not edge support for purposes of shear nailing. So if edge clips are installed one uses the "with edge support" column for the Span Rating.

APA has never actually tested H-clips to see how much they improve (reduce) deflection. In testing panels are checked for deflection under a 200 pound load but then the deflection gear is removed and the panels are loaded to failure. The H-clip always pops off before the panels fail (due to the amount of deflection).

All that said, 3/8" panels with a 24/0 Span Rating installed on supports 24" on center is not particularly robust--especially if you're walking on it. It is however capable of carrying the uniform loads for which it is rated 30 # LL, 10# DL at max. span. Also, in wind tests we find that fiberglass shingles tear out around the staples or nails before the staples or nails withdraw from the 3/8" panel. I guess the bottom line is that for those who need a roof and can't afford something better --this is a minimum answer.

Mike,

Thank you for the information - I'm always learning (at least trying to).

Code aside (I don't do code inspections), if visible, typical failure appears to be due to undersized sheathing, that's what I call. If no failure is apparent, 3/8" plywood roof sheathing not desireable but not defective. "Excessive deflection" is a personal judgement call, an opinion, my opinion. My concern is safety, system or component failure, or liklihood of early failure. Code is a tool I use but my inspections are not based on it.

Originally Posted by Kenton Shepard

Code aside (I don't do code inspections), if visible, typical failure appears to be due to undersized sheathing, that's what I call. If no failure is apparent, 3/8" plywood roof sheathing not desireable but not defective. "Excessive deflection" is a personal judgement call, an opinion, my opinion. My concern is safety, system or component failure, or liklihood of early failure. Code is a tool I use but my inspections are not based on it.

Agreed!

The minimum building code is a wonderful thing but it is ultimately just one tool in your box of many.