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Pressure treated plywood for roof deck?

diditmyselfer's picture

Can pressure treated plywood be used as roof decking without being destroyed by the dry heat a roof is exposed to?


I have a low sloped roof to replace where the 1/2" OSB has rotted out at the edges and water has been penetrating behind the gutter and I am afraid will soon be into the interior walls, possibly even rotting the ends of the trusses sitting on the top plate. So far the place hasn't fallen in and I can rebuild and reinforce what damage I find once I tear the roof off. What I am aiming at is redoing the mess and solving the problem, once and for all. I will use Storm Guard, 30 lb felt, with 30 or 35 year shingles. I always use the best quality materials and like doing things the right way with the best workmanship, only once. I am not sure this will completely solve the problems associated with ice damming or heavy downpours. But I thought if pressure treated ply were used at the most vulnerable places on the roof deck, even if water did seep in, the deck would not rot out again so readily. The other alternative is to strip the roof and completely rebuild a new, steeper roof. Even  then there is a limit to the slope I could apply and if it's a good idea, I'd use the PT plywood on that as well.


Has anyone ever tried this before? 

(post #82815, reply #1 of 57)

not me!

(post #82815, reply #2 of 57)

Welcome to BT.

I think you're a WHOLE lot better off working at preventing the leaks in the first place. Treated plywood won't keep water out or keep other things from rotting or getting moldy.

Ice and water shield come to mind as a first defense. Checking out attic ventilation would be second.

In other words - Treat the problem, not the symptoms.

Carelessly planned projects take three times longer to complete than expected.
Carefully planned projects take four times longer to complete than expected, mostly because the planners expect their planning to reduce the time it takes.

(post #82815, reply #3 of 57)

What do you mean by low pitch ?


That could be your biggest problem in you are using shingles on  to low a pitched roof.


We used 3/4" pt plywood for sheating on a flat  roof in 1982. I recovered the pt plywood in 2002. It has been leaning agianst a tree behind my shop or used as walk boards since then.


Good stuff, that old CCA treated plywood.


Don't know about the the new ACQ treated.


Treated plywood isn't going to solve your problem if the roof pitch is to low for shingles. It is just going to hide the problem untill your rafters have rotted  out.


Need more information.

(post #82815, reply #4 of 57)

I can't imagine any need for PT ply for roof sheathing ever.

Anytime PT is needed, the roof is not doing it's job and that is the place to deal with the issue. Let's plan to do that.

You mentioned low slope roof. That first had me thinking you had a BUR or steel roofing material but you are talking shingles.

What pitch is this? And what is your climate?

 

 


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Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
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Oh Well,

We did the best we could...

(post #82815, reply #5 of 57)

That is what got me wondering too..


BTW we had to use the pt per the drawings. It was two layers of sheating with a steel roof over our CNG compressor station for our alternate fuel fleet.


(Bet the company wishes they hadn't given that thing away now.)

(post #82815, reply #6 of 57)

A properly constructed deck should not require use of PT plywood.  You may need more of an overhang with functioning drip edges.  Covering the whole deck with bituthane is an option, but this still doesn't address any overhang issues.  If you decide to use PT sheathing, you'll probably want to use stainless or copper nails, but I really don't think that's an appropriate solution.

(post #82815, reply #7 of 57)

didit...

Is it possible to share a couple photos of this roof situation with us?

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #82815, reply #8 of 57)

Thanks for all the input.


I did contact some plywood manufacturers and university architectural engineering departments today as well. What I found was, that all chemical pressure treatements weather-out. In an enclosed system, they will remain embedded almost indefinitely, hence they are used in pressure-treated foundations. Which I have been told are built in northern Canada! Imagine that, a wooden foundation wall! So, PT is an excellent usage on low or flat roof decks. If any slight seepage or moisture does occur, it does absorb more than other deckings and will not rot, nor decompose during drying out, and remain structurally sound throught many many years no matter what the conditions. I plan to use it along the bottom edge which should be plenty sufficient to resolve ice damming and any capillary action. Adding a wider 12" overhang and foam insulation a couple feet into the attic space should help as well. I also plan to blow in cellulose insulation when I open the deck up and add a ridge vent instead of those ugly vent caps.


Thanks again for the input. Always good to hear what others have tried.

(post #82815, reply #9 of 57)

I thought I was the only one who put PT plywood on a roof deck.  We did an entryway for a 11 story building last year.  The architect spec'd 3/4" PT ply with PT rafters.  I could't get an answer why...now I have it. 


 


Just use regular plywood and  ice and water shield. 

(post #82815, reply #15 of 57)

Is your archy planning on a leak too?

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #82815, reply #22 of 57)

I guess...this spec must be in some college textbook.

(post #82815, reply #10 of 57)

The normal preference is to build a roof that will not leak through to the sheathing, nor wick water in at the edge.  I don't think anyone here will disagree that PT is the way to go if you are anticipating that the roof will fail.  If that is your expectation, then PT is the way to go.  Good luck with it! 

(post #82815, reply #11 of 57)

That's all well and good about PT weathering better and if it gets wet it'll dry and all that, but just remember then to do as Piffin said and use stainless steel or copper nails. Your roof won't last too long if the nails have all rusted away and the shingles (or the decking itself) are blowing off! (New ACQ-treated wood is corrosive, especially when damp/wet and will eat regular nails in short order!)


Edited 6/16/2008 8:54 pm ET by Danno

(post #82815, reply #12 of 57)

I just can't agree. 


Using PT for residential roof decking is like using greenboard through the interior just 'cuz the walls might someday get wet.


It's adressing a symptom, not the problem.


Hell, why not treated rafters AND joists AND plates AND studs AND floor decking . . .


See where this is going?  Where do you stop anticipating?  Plus the cost of SS fasteners. . .


Forrest -


Edited 6/16/2008 9:07 pm ET by McDesign

(post #82815, reply #13 of 57)

I think didithimselfer got someone from somewhere else to agree with him and is not listening to the builders. There's nothing really wrong with using PT decking, just as there's nothing wrong with using greenboard throughout a house. Preparing for a leak rather than preventing it.


You can lead a horse to water, but you can't make him drink it.



--------------------------------------------------------


Cheap Tools at MyToolbox.net
See some of my work at TedsCarpentry.com


Edited 6/16/2008 9:45 pm by Ted W.


Edited 6/16/2008 9:45 pm by Ted W.

~ Ted W ~

(post #82815, reply #14 of 57)

But you are missing the point. There is NO REASON in the world to plan for the contingency of this sheathing getting wet!

I'm trying to help you build a roof that doesn't leak, but I need feedback to do that.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #82815, reply #16 of 57)

Well hey, who are we to argue with a plywood mfr. and the campus arky? I hope it rains like he11 in never-never land.


--------------------------------------------------------


Cheap Tools at MyToolbox.net
See some of my work at TedsCarpentry.com

~ Ted W ~

(post #82815, reply #50 of 57)

build the roof not to leak and take the money saved by buying regular sheathing and have a beer

(post #82815, reply #53 of 57)

yes, Wood foundations with PT lumber have been an accepted standard in the home building industry for a generation now. PT sheathing for decks is not. It is an anachronism.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #82815, reply #28 of 57)

 In an enclosed system, they will remain embedded almost indefinitely, hence they are used in pressure-treated foundations. Which I have been told are built in northern Canada! Imagine that, a wooden foundation wall! So, PT is an excellent usage on low or flat roof decks.


Application - Retention (lbs/CuFt) - Uses:



  • Above Ground - 0.25 - Decking, fence boards, hand rails, deck supports

  • Ground Contact Fresh Water - 0.40 - Fence posts, landscaping, piers, docks, etc.

  • Permanent Wood Foundations - 0.60 - Wood foundations, crawl spaces

  • Poles - 0.60 - Building, transmission and distribution poles

http://www.treatedwood.com/products/preserve/


There are three ratings for PT lumber - above ground, ground contact, and direct burial. The PT plywood that's available in my neck of the woods is above ground only. So your comparison may not be valid.


http://grantlogan.net




Who got Bo Diddley's money?

(post #82815, reply #49 of 57)

I am in northen canada.Prince Edward Island. I have been putting PT foundations in for the last 15 years.alot of them are only frost walls.e.i garages and small additions where basement space is not an issue.full height basements are constructed with 2x10,12"centers ,concrete footings. Once the basement is properly insulated etc. it is an extremely warm area. you dont have 10" of cold concrete to insulate against. exterior waterproofing is crucial but not difficult if done properly

(post #82815, reply #17 of 57)

I'd use the PT plywood on that as well.


With that mindset, you should probably use PT rafters, joists and studs. "Cause if the PT plywood was necessary, everything below it would also need to be treated. Better use that aforementioned greenboard as well.


http://grantlogan.net




Who got Bo Diddley's money?

(post #82815, reply #18 of 57)

And pressure treated blown-in insulation too! :)

--------------------------------------------------------


Cheap Tools at MyToolbox.net
See some of my work at TedsCarpentry.com

~ Ted W ~

(post #82815, reply #20 of 57)

Thanks for the advice. I see you used that on all your constrcution projects too. Must work real good.

(post #82815, reply #19 of 57)

>>>Can pressure treated plywood be used as roof decking without being destroyed by the dry heat a roof is exposed to?

Yes.

But why? As others have pointed out, it sounds like you're planning for it to leak. If so, adjust your design so it doesn't. Otherwise the building is doomed from the start.

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

(post #82815, reply #21 of 57)

Scott,


You at least seem to be one of the few intelligent and mature people on here with a opinion.


Have you ever seen what 13" of rain in 3 hours does to a roof? Any roof? 12/12 slopes even? Or 70+ mph winds, with driving rain? how about 6 months of winter? Ever walk down the street and see the ice hanging from second story roofs all the way to the ground like frozen waterfalls? 2' to 3' thick? Where the leaves didn't even get a chance to turn colors in the fall but are covered with snow all winter? Or 3 or 4 feet of heavy wet snow that crushes the roofs of commercial buildings? I have. And I've repair all such roofs just mentioned. This house in question happens to be in an area that has experienced all these weather conditions in the past 20 years. If I'm going to do a roof, it better last for more than 20 years without any problems.


Opinions are most often worth what you pay for them.


In closing, in all the 38 years I've remodeled and built homes I have retained 1 out of every 10 guys that came along with a hammer and often without a toolbelt, that tried to call themselves a carpenter. I have also kept 1 out of every 3 electricians, plumbers, masons and concrete finishers. The others were sent packing real quick.


I can always find a guy with a pickup truck and a hammer. But good people are hard to find, because they are always busy with work. Funny thing is that on a Monday afternoon I posted a question here thinking I'd get some reply from someone who knew something intelligent. Within an hour, I get all sorts of wise-guy ignorant responses and opinions from know-it-alls. But not one of them had any precise or accurate information nor directly answered the question. What were they all doing behind a keyboard on a Monday afternoon? Were these the same guys I fired, sitting back in the bar or at an internet cafe with a beer in hand on a Monday afternoon?

(post #82815, reply #23 of 57)

Usually I just sit around and read the messages and keep my mouth shut, granted there are certainly some sarcastic responses that are not warranted.  


You asked for opinions and you got opinions...they are different than your own...so everyone else must be idiots.  My opinion as well as many others is PT is not needed and a better solution is a better roof system. 

(post #82815, reply #24 of 57)

>>>Were these the same guys I fired, sitting back in the bar or at an internet cafe with a beer in hand on a Monday afternoon?

Well, if you're referring to Boss, Piff, Dave, McDesign, or Me, definitely not. Nonetheless, I wish you well with this project.

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

(post #82815, reply #25 of 57)

I thought pressure treated insulation was a good one. But then, I'm easliy humored.


Good luck with your project. If it were my house, you'd be the one getting fired.


--------------------------------------------------------


Cheap Tools at MyToolbox.net
See some of my work at TedsCarpentry.com

~ Ted W ~

(post #82815, reply #29 of 57)

I have asked twice now for information about your roof and climate so I can help you prevent the leaks in the first place.

Are you ignoring that, can't read, or don't know what pitch roof and climate you have?

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...