3 ply vs. 4 ply 1/2" CDX plywood


#1

My roofer replaced the entire plywood deck on my house this past summer. Our contract specifically stated the use of 1/2" 4 ply CDX plywood. While in my attic the other day, I noticed he used 1/2" 3 ply CDX instead of 4 ply.

I also recently noticed some plywood buckling on the new pieces. I cannot help but think the 4 ply would have been more unlikely to buckle or bow. I am correct to believe this even though both are still 1/2"?

Is 4 ply stronger than 3 ply when the thickness is still 1/2"?


#2

Absolutely,4 ply is better and stronger. Costs more too. Hard to tell why you got buckling,but my guess is that the plywood is laid too closely or was wet when installed. If you have a contract that spells out the plywood thickness and you didn’t get it, I would ask for a refund of the difference. It probably isn’t worth tearing off the entire roof over the plywood. Now, buckling should be investigated.


#3

I think more important than plys is
whether it is Southern Yellow Pine or Fir.

You can tell one from the other with a blindfold on…just try hand nailing into SYP…and it bows
real bad when left in the sun.


#4

So fir is better than the yellow pine, right?


#5

I have not addressed the 4 ply vs. 3 ply issue with my roofer yet, but he knows about the buckled plywood sections. He said the plywood probably buckled from too much moisture in the attic coming from the living space below. The roof was installed in May and it starting buckling in November when the cold weather arrived. Even if attic moisture was too high, would the 4 ply have resisted the buckling better than the 3 ply? That’s what I’m trying to determine.


#6

OSB…


#7

Mono,

Sorry for your problems and I wish that I did not have to Post-Critique someone else’s work that I looked at, so please let me know if my opinion comes off as biased and out of character from previous critiques.

The 4-ply is Extremely more resilient. That additional ply counts as an actual additional layer of adhesive and another veneer of sheathing applied to it.

The structural and dimensional stability of the 4-ply is akin to comparing the 3-ply to 3/8" plywood.

For the reasons you stated, that is specifically why I only use 4-ply CDX or better now-a-days, although it does cost several dollars per sheet more than the 3-ply.

One time back around 1991, the lumber supply shipped out the wrong sheathing on a job we did and when I got there and noticed it, I made my crew remove the entire one side of the house that they installed and replace it with the proper product. This was when I was still experimenting with different decking products and we did not use 4-ply exclusively. If that were to happen currently, my crew would know immediately.

OSB, well…, you will get multiple opinions on that cr*p.

I usually find that if a contractor had to pay the exact same amount for the 2 different products, then they would purchase the CDX, but money talks so loudly when it affect the pocket book, that they will still proclaim it to be better. That goes against the actual manufacturers reports defining which product performs better and with less adverse affects. (I reference a report and study with conclusions between the 2 different products, done by Georgia Pacific, who manufactured both styles of products. They have since then, shifted their manufacturing of OSB to a Canadian division, to limit liability issues. The report was done while they still manufactured both products though, which I feel to be a very unbiased source, considering)

At least the guy and his crew that screwed up is local to your area, so you should have some leverage.

Check with your building code for deck sheathing “Minimum” requirements and see if 3-ply is even allowed. That will give you the best leverage on getting the job done the “Right Way”, like it should have been done in the first place.

If they specified 4-ply in their contract and stated that they only use that version, there should have been no reason that the wrong product got supplied.

Ed


#8

Ofcourse 4 ply is stronger than 3 ply.

But three ply is strong enough to do the job right.

1/2 inch 3ply would only be insufficient and against code if the house was being specced with some heavy roofing(tile). then 5ply required.

Almost No new construction builder buys 4ply unless you ask for it.

Usually No new construction builder installs 3ply unless you ask for it.

They almost exclusively use OSB.

ALL plywood can warp regardless of plys.

OSB never warps…


#9

Old grunt…yes I prefer fir…seems lighter, lays flatter, and is deftinitely more hand nail friendly.

David


#10

Dont introduce moisture to your wood. (cdx or osb)

Try not to butt up your 4x8 planks next to each other.
Leave a small space.

Edit. Donl beat me to it.
“but my guess is that the plywood is laid too closely or was wet when installed.”


#11

If it were OSB bet you wouldn’t have buckling problems.

I cringe every time I start tearing off a roof with plywood from the 80’s. That must have been when they used 2 ply? Complete garbage.

My brother got in a little trouble for this a while back. His contract indicated plywood and the roof was re-decked with OSB.


#12

Dougger,

His new roof and plywood decking were just installed less than 6 months ago.

Ed


#13

[quote=“ed the roofer”]Dougger,

His new roof and plywood decking were just installed less than 6 months ago.

Ed[/quote]

Not good!

Not very often do you see a roofer tearing off the entire decking and replacing it, was it 1970-1980’s plywood or something? Only heard of one roofer having to do this as all the plwyood delaminated while tearing off the shingles.

Are the shingles thin three tab fiberglass?


#14

He posted here before, around last spring time and identified that the plywood decking on his upper house roof section had become inundated with mold.

Also, from various advice from all of us posters, he realized not only that he had ventilation problems, but was also shown how th improve the intake ventilation properly for a hip style home with 100 % intake ventilation being added to the existing set-up.

Ed


#15

[quote=“ed the roofer”]He posted here before, around last spring time and identified that the plywood decking on his upper house roof section had become inundated with mold.

Also, from various advice from all of us posters, he realized not only that he had ventilation problems, but was also shown how th improve the intake ventilation properly for a hip style home with 100 % intake ventilation being added to the existing set-up.

Ed[/quote]

We improved the attic ventilation by doubling the size of the soffit intake openings. The roofer removed the all the 16â€Â


#16

A few years ago we had an issue with buckling plywood on a new build.
This was 4 ply and there was a deflection of over 4" between trusses in 5 separate areas.
The house was only a year old when the plywood started buckling.


#17

Mono,

You will have to refresh my memory regarding your ventilation square footages and total NFVA produced for the house section.

Remember, the 1/300 rule only applies if you either have a properly calculated and balanced intake and exhaust ventilation and have a vapor barrier under the warm side of the insulation.

If you do not have those requirements met, then the calculation changes to double the requirements, which is the 1/150 rule. This would mean that you need one square foot of total ventilation calculated for every 150 square feet of attic floor space.

Generally, when the slope of the roof is 8/12 up to a 10/12, you should add an additional 20% ventilation and if the pitch is greater than that, then 30% more should be added, due to the increased cubic footage of air in the attic area.

I don’t recall you mentioning the vapor barrier aspect before, but that was quite a while ago, so maybe my memory isn’t as clear about it as it should be.

Probably the only way to have increased the total ventilation on your home would have been to add the shingle over intake ventilation products on at least 50% of the eave edges, if not more, using something similar to the Smart Vent, since you had a limited amount of ridge line to increase that capacity.

Then again, this entire problem may just be due to a poorly fit installation of the weaker than specified plywood deck sheathing, by being butted too closely together, without the proper gap spacing.

It is very good though, that the contractor has addressed the situation with you and is further wiling to work out a solution, even if he is pushing the causation factor off on the vapor barrier.

Ed


#18

[quote=“ed the roofer”]He posted here before, around last spring time and identified that the plywood decking on his upper house roof section had become inundated with mold.

Also, from various advice from all of us posters, he realized not only that he had ventilation problems, but was also shown how th improve the intake ventilation properly for a hip style home with 100 % intake ventilation being added to the existing set-up.

Ed[/quote]

Makes sense now thanks! Gotta be nice to have a memory like that!!!


#19

I prefer the 4-ply CDX myself. Don’t care for osb, but have used it.


#20

I started my insulation project this weekend. What a pain! I’m laying JM ComfortTherm R-19 plastic wrapped batts in on the ceiling. It wouldn’t be so bad if I didn’t have to pull the loose fill fiberglass out of the bays before laying the batts. I have no idea why any builder would not install a vapor retarder in a home based in the upper Midwest. After I lay the batts (with vapor retarder towards the heated living space), I’m pushing the loose fill fiberglass back on top. This is providing about 16â€Â