Plywood roof sheathing deflection 3

[jfmann]

Seeking experience with sagging of plywood roof sheathing for cases where the sheathing has been properly sized per the building code.......I have encountered such a situation and it does not appear to be a case of overload.......seems like the code provision is allowing plywood that is too thin.........

 

[MikeVV]

Have you checked the plywood itself - it may be chip board and will not have the same stiffness as plywood.

 

[4n4s4n1s]

Could it be that the supporting joists that are<br>inadequately designed and that sagging took place<br>across the joists? <p>azmi ibrahim<br><a href=mailto:azmi@civil.engr.itm.edu.my>azmi@civil.engr.itm.edu.my</a><br><a href= > </a><br>I'm always keen to learn more to advance<br>
myself in areas that I'm interested in.

 

[neilb]

codes usually require H-clips or solid bridging under abutted sheathing joints. are these in place? is it possible sheathing has warped due to condensation? was sheathing installed with short side between truss rafters instead of lengthwise across rafters? if it's OSB, consider if it's Louisiana Pacific because LP has had a slew of problems with their OSB board.

 

[PDHal3]

Need engineer/expert to analyse the edge delamination / bulging of SYP roof sheathing that causes unsightly telegraphing of plywood joints through roofing material. Any help out there?

 

[JAE]

jfmann and PDHal3

Could be that both your cases involve a similar situation that I encountered on a project where there was a multitude of warping across a pitched roof with plywood sheathing. In this case, the attic was not properly ventilated and in colder weather, moisture in the warm, interior air condensed on the underside of the plywood, which was cold.

This moisture was absorbed by the plywood and initiated warping. The attic was insulated along the slope, rather than across the flat ceiling. Thus, the interior attic was kept warm instead of vented.

Don't know if this applies to either of your situations but you might consider it. Delamination of plywood edges (by swelling) definitely sounds like a moisture source - either leaks or condensation.

 

[PDHal3]

Thanks JAE .. however, the more I hear about ventilation of attics, the more suspicious I am becoming ... what relative humidity can sheathing plywood tolerate? Seems that the plywood industry is hanging their hat on vague science. When selling the product, they say it can be immersed in water for weeks .. but it can't tolerate a little high humidity or even a bit of condensation when nailed in place? hmmmm ... thanks again.

 

[Aton]

Just joined the Forum. Sorry I'm late. In response to Jfmann and PDHal3, the problem may well be differential moisture, as JAE suggested. If there is moisture built up under the ply roof sheathing, due to inadequate attic ventilation, coupled with poor vapour diffusion retarder membrane, then the moister side of the ply will swell more than the dryer side, causing convexity (cupped up). Add to this the intense heat on the upper surface of the ply due to (usually dark-coloured) roof membrane (which causes drying, and thus shrinkage, of that surface), and you can often have quite a sag - especially if the rafters or trusses are 24&quot; oc.

 

[Trussdoc]

The plywood manufacturers recommend providing a 1/8 inch gap between sheets of any structural panel to absorb any movement due to thermal or moisture caused expansion. This is a particular problem with the OSB products; these also swell at the edges if exposed to excessive moisture.

I believe its the cyclic effects of moisture/drying, rather than a single event, that cause plywood problems.

Also, check if the product involved is Southern Yellow Pine based...I've seen several dimensional problem jobs that used it. SYP just seems to like to twist, whether its plywood or dimensional lumber.

 

[boo1]

The common thickness of OSB for roofs is 7/16&quot;, cdx plywood is 15/32&quot; or 1/2&quot;. Most plywood panels are also overbuilt to cover the statistical range that guarantees each sheet of plywood meets the minimum standard. OSB, on average, is 7% less stiff because it stays closer to its target spec. The problem could be a combination of elastic creep, lower section modulus, differential moisture intusion and installation errors.

Osb and plywood share the same exposure durability classifications: Interior, Exposure 1 (95% of all structural panels), Exposure 2 and Exterior. They share the same set of performance standards and span ratings. Both materials are installed on roofs, walls and floors using one set of recommendations. Installation requirements prescribing the use of H-clips on roofs are identical.

We normally specify 7/16&quot; OSB for shear wall installation and 1/2&quot; four ply for roof decking. Insure all roof panels are installed perpendicular to the truss.

Cheers

 

[PDHal3]

Any roof sheathing that could not tolerate at least as much relative humidity in the attic as exists outside of a ventilated attic space seems inappropriate for use. Plywoods solve cost, structural and conveniece issues but perhaps that is where benefits stop. Gapping requirements beg the question ... why is the sheathing expected to move and where are these movements absorbed in the roofing layers since they are generally fixed accross panels?

 

[Trussdoc]

With regard to gapping requirements...my contact at APA tells me that the movement is expected only while the sheathing is exposed during construction; once the membrane is on and protecting the sheathing, there should be no further movement.

Plywood and OSB do tolerate humidity as well as any material, and without the expansion or corrosion problems inherent in metal. The problems occur when unventilated spaces, or improper insulation, create an environment of extreme temperature and humidity.

 

[Miper]

There is considerable warpage in certain plywoods that would look like sagging. I believe it is fir plywoods that are stiffer than pine but more prone to warpage. In high humidity applications, roof panel warping can occur with fir plywood.

However, if the sagging extends across trusses, then it is more likely that you have a truss problem.

 

[Trussdoc]

My experience with plywood sheathing has been the opposite...Southern Pine is more likely to warp than any others (Sorry, Y'all)

 

[Miper]

You are probably right, trussdoc, especially if you have recent experience with it. It has been a while since I worked with the two types of plywood. What I remember most clearly is that one type of wood has a definite tendency to warp. Sometimes even the sheets at the lumber yard look like potato chips, but that could be a different problem.

 

[neilb]

been in the boonies, so haven't kept up with my own thread....(sorry)..the purpose of my original question was not directed at causation so much as remediation..let me rephrase..suppose after a coastal hurricane i inspect plywood deck panels from inside an attic space to check for wind-driven rain damage.. now, if i was on the roof with covering removed, i could bang with a hammer and listen for delamination .. but if i'm under the deck (trying to decide whether to pull the covering)....what rules of thumb do i use to decide if the panel is crapped out? (other than moisture content) is there a dimensional warpage standard? if panel is dry but water-stained, can i presume anything simply from the fact that it's water stained? APA research shows loss of strength after a panel is wet, but if i get there and it's dry -- how do i presume to know?

 

[jerrom]

OSB and Plywood have a standard deviation of 50/1000 of an inch from the nominal thickness average. These materials also won't shrink back to original size so it should be easy to tell if it got really wet.

 

[tingwong]

This is only what I've heard from an experienced custom home builder.

When there is sagging of plywood roof sheathing, 7/16&quot; or 1/2&quot;, 90% of the time, the plywood were rained on before shingles were installed during construction of the home.

you be the judge.

 

[boo1]

APA technical note titeled &quot;Buckling of Wood Structral Panel Sheathing D481&quot; lists recommendatios to avoid this problem. The recommend using metal clips to create 1/8&quot; gaps beween panels. The problem is edges of the seathing swell due to high moisture environment or when the seathing is installed without leaving expansion gaps between panels.

http://WWW.apawood.org