Unpainted PT plywood as Rainscreen cladding?
junglepete
| Posted in General Questions on
Does anyone have opinions about using pressure treated plywood as a siding over a 5/8 inch rain screen gap and 15lb tar paper over plywood sheathing?
I’m in the Pacific North west (zone 4 marine) about 1/2 mile from the ocean.Â
Thinking of using Hardie Panel on the lower story and PT plywood on the upper story. I have 24″ eave overhang consisting of exposed PT 4×6 lookouts and 2x boards (see picture)
Main concern that I can think of is the unpainted plywood absorbing significant amounts of moisture and solar drive issues. maybe I should paint the back of the plywood (and hardie), but I would rather not.
Any input would help
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Replies
junglepete,
PT plywood is treated against insects and moisture related decay, but not UV, so without a sealant or paint it will check, splinter and look weathered.
It will take on moisture and act as a reservoir cladding, but with a rain-screen that shouldn't be a problem.
Narrow rain-screen cavities and building paper don't work together very well. Unless you use 30#, it inevitably wrinkles and either blocks the gap, or bridges it defeating the capillary break. You are better off with a synthetic WRB.
I appreciate the feedback. Before I give up on the #15 building paper, I wanted to confirm whether you consider 5/8" or 3/4" a "Narrow rain-screen cavity" that would be inappropriate for #15 paper. Maybe I could do 12" oc furring (although the framing is 24" oc). I have already purchased the paper and like its properties better than synthetic WRB (at least theoretically).
My experience is that PT plywood is significantly more likely to delaminate and buckle than non-treated plywood.
Even painted it doesn't hold up that well.
Thank you both for the quick replies (did not show up by reloading the page). Both your responses make it sound like PT plywood can fail aesthetically. I have used the east coast green colored stuff in Florida on shed walls and was satisfied with how it looked, although perhaps it did buckle and split a little. Maybe the Brown pressure treated boards on the west coast (with west coast wood) behaves differently? (not sure where you are from DCcontrarian... maybe Washington DC?)
I still think I would prefer a worn look over worrying about rot or powder post beetle. And I like the idea of not painting, especially on an upper story. Stuff I'll continue to think about.
I also appreciate the information on the 15# (nominal) paper blocking a narrow gap. I'm assuming that 5/8" is considered narrow? I just like asphalt papers proven history and its "smart" permeability and hygric redistribution potential. I have 24" oc studs, but I am considering spacing the furring on 12" oc. Perhaps that would reduce blocks from wrinkling?
I kind of wish a coat of paint was considered adequate for the WRB under the rainscreen (I guess I'm okay with painting sheathing, not as tedious I guess). But I don't think the inspector would buy it. I know there is WRB specific paint on products, but I like to use whats locally available and not that expensive.
Thanks guys. Your input is appreciated!
PT plywood isn't any better here in the PNW. It will delaminate in this usage scenario simply because it will be going from extremes of moisture accumulation to solar drying on one side. It just isn't resilient enough to be used like this.
And again, I have to point out that it will look like poop and reduce any chance of resale. Just use an exterior cladding that has already been proven for this application.
Besides the durability aspects noted above there is another critical problem...and how to put it politely? It'll look like poop.
As much as I like to see alternative approaches and testing theories, there are reasons that some things just aren't done.
junglepete,
I agree. There is a lot of merit to second storey cladding as maintenance free as possible.
Having the furring at 12"oc will certainly solve the building paper problem.
How are you thinking of managing the joints between the plywood sheets?
Good to hear that 12" OC furring can work over #15 paper. Thanks
I still have a bunch of DYMONIC® 100 caulk that I could use on the vertical seems of both the plywood and the Hardie. And will probably do vertical 1x2 (or 1x3) PT battens 24" OC over the plywood and seems. Galvanized "Z" chanel over the horizontal seams.
junglepete,
Sounds like a good plan. It's an aesthetic that may well work with your house.
Thanks Malcom, I very much appreciate the help.
If I go this route I'll post a photo of the final result.
if i were to use 15# felt, then I'd consider running the rolls vertically with the battens covering the seams. intuitively, it seems better sealed.
consider ripping the plywood into strips and running horizontally with some gap between rather than running the sheathing tight like T-111 style.
FYI i have a garden shed roof with exposed 1/2" sheathing as the roof. it is nailed 8" OC at the perimeter and it still has gotten a little wavy from exposure. a tighter nailing schedule on edges might have kept the panel flatter.
Vertical felt with the edges compressed under battens may work, but I would still worry about sideways capillary movement under the battens. I would rather rely on gravity.
I think I prefer the vertical for this build. Wide gaps can look cool. And some experts argue that it is drier, but others argue that it is not. And I would probably want a WRB specific to that application, with better UV durability and heavier wetting durability.
Seems to me that a roof, with much stronger sun exposure and stronger water pressure, would behave differently (hopefully) than siding under a 2' overhang.
Thanks for the input stamant.
Running underlayment vertically isn't a good idea. Lapping the edges horizontally will help move water out from behind the paper in the inevitable case where it will tear or otherwise be compromised. Think like shingles. It's a system that moves water.
1/2" sheathing left exposed as a roof further illustrates why plywood is not a good choice for siding. 8" OC nailing and it's still wavy. 3" OC nailing would just get smaller waves at the edges, but you'd still have issues and it won't look great. Those waves also indicate that the wood has been swelling and shrinking unevenly and will soon delaminate.