Pressure Treated Plywood for Coastal Shed with Roofing but Exposed Underside? 3

Check with roofing suppliers; you shouldn't need sheathing with standing metal roof. Because it's coastal, you might want to use PT material anyway and stainless or HDG fasteners due to the corrosion potential of PT material.

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-Dik

The original thinking was to use the wood sheathing for diaphragm action as the standing seam roofing normally has none. That said, I'm certainly not opposed to using some other form of metal sheathing if it does have some nominal, demonstrable diaphragm capacity.

Coastal makes me concerned primarily with the hardware. Elevated humidity...is condensation on the underside of the roof likely? Frequent fog? If not, then I wouldn't be overly worried. If frequent wetting is likely, then I would go with an APA Exterior grade panel or its equivalent. Exposure 1 would be a minimum.

Are you specifying stainless steel truss plates? I've seen a few 10-15 year old open wood truss structures with frightening looking MPCs.

Koot:

The building will almost certainly see condensation on the underside of the deck - I'd treat it. The presence of the wood panel will, to some extent, ameliorate the condensation since it has a much lower thermal conductivity than the metal roofing. Metal panels on purlins (no sheathing) will be dripping all the time. As others have noted, SS fasteners and hardware are also necessary. Also, the membrane is important to prevent the wood panels from rotting from the top side. You are right - these buildings seldom have sheathing - metal panels on purlins is the norm.

Regards,

DB

NB: The condensation will be dripping on whatever is stored under the roof.

This seems a conversation to have with the owner or client to understand how important service life is to them. There are a ton of exposed wood in this area. Architects love the wood look. The last time I bought plywood at the lumber store the cost difference for treated was not very great. I do not think this would be a hard sell if they do not object to plywood sheathing in general.

The use of standing seam is not that common in my experience for this type of building. Most use typical metal sheets with lap joints and exposed fasteners. The driving factor for this with strapping is cost, ease of installation, and the willingness of some to overlook detailed diaphragm design. In your case, I expect you will need at least a 1"x4" straps over the plywood to engage your screws. Screwing thru plywood is not great, and coordinating with the trusses does not work out with typical roof sheets. We have used 2"x4" straps in many cases over plywood given the poor quality of 1"x_ stock.

KootK said:

That said, I'm certainly not opposed to using some other form of metal sheathing if it does have some nominal, demonstrable diaphragm capacity.

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The Post-Frame Building Design Manual published by NFBA ( does have tables providing diaphragm capacities and stiffness for some typical framing setups using both 26 gauge and 29 gauge sheathing with exposed fasteners (based on past tests performed). If you do go that route it would probably be more economical to space trusses out to match your post spacing and use 2x purlins with joist hangers between the trusses.

DBronson said:

Metal panels on purlins (no sheathing) will be dripping all the time.

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I know this is a major issue for enclosed buildings, would it still be an issue for an open building?

There are also products like this, available. Although, in a costal area with moist air present all the time this might cause even more issues.

Thanks for the help thus far guys.

phamENG said:

Elevated humidity...is condensation on the underside of the roof likely? Frequent fog?

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Quite likely, yes.

phamENG said:

Are you specifying stainless steel truss plates?

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Is there such a thing as a stainless steel, toothed truss plate? Or do mean a stainless steel nailing plate? Either way, the answer is no, I have not yet specified anything like that. And I suspect that it would be cost prohibitive if I were too. Back in the late 90's, when I was doing MPC truss design, this would come up from time to time. Nine times out of ten it would get value engineered down to no protection at all. And when there was an appetite for it, it took the form of tar painted over the plates or something. That was still costly and, obviously, hideous.

Brad805 said:

In your case, I expect you will need at least a 1"x4" straps over the plywood to engage your screws.

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I had overlooked that aspect entirely, thank you.

dauwerda said:

If you do go that route it would probably be more economical to space trusses out to match your post spacing and use 2x purlins with joist hangers between the trusses.

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Ughh.. the thing actually started that way and I changed it to trusses. The seacan truss bearings factored into the decision. I didn't want the heavy gravity reactions nor, more critically, big uplift reactions on the seacans.

Brad805 said:

There are a ton of exposed wood in this area. Architects love the wood look.

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I've got some rather skilled wood people reviewing this project and have been debating with then when it is and is not appropriate to use pressure treated wood. Their stance seems to be to only us pressure treated when:

1) Wood is at or very near the ground.

2) Wood is in contact with masonry or concrete.

3) Wood would get wet and not have the opportunity to try readily.

In their opinions, nothing here requires pressure treating so long as I'm using dimension lumber and not OSB/LVL.

Any opinions on that (anyone)? Originally, I'd planned to have everything pressure treated.

I would agree rather skilled people (I assume agreeing with them may benefit you outside of the office as well, wink wink).

That's pretty much how I've assessed the need for treated lumber.

Are the walls going to be clad?

KootK said:

Is there such a thing as a stainless steel, toothed truss plate? Or do mean a stainless steel nailing plate?

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Yes. Probably still expensive...but even a G90 coating won't hold up. Down in North Carolina for exterior applications on the beach they prohibit carbon steel less than 3/16" thick on residences, period. 3/16" and up can bemust be galvanized, anything smaller has to be stainless.

jayrod12 said:

I would agree rather skilled people (I assume agreeing with them may benefit you outside of the office as well, wink wink).

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Ha! In reality, my wife probably values my dissent more than my acquiescence in most things. And, given the nature of me, that's surely for the best.

jayrod12 said:

Are the walls going to be clad?

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No sir. She shall blow up from the inside like a big soggy balloon when those pacific winds blow.

I understand that stance and given the other components are not treated it is logical. This would be a case where I would look at the cost difference. Below are current prices for plywood. If it were my building it would take me very little time to decide, but I have worked with many very tight clients that would object to the extra $4-$5k. You have made your argument, and put in print your preference. I am not aware of a code clause that would require you to treat this.

If you go down the road of stainless steel screws and special plates, that would be a big time killer. While understandable, it would lead to some discussions when the contractor starts trying to procure the materials.

Having lived "coastal" (read -- 20 ft from the ocean) RUST is an ever present risk. I've had fairly nice hardware, albeit from HD, inside the house, showing rust one year after installation.

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KootK said:

The seacan truss bearings factored into the decision. I didn't want the heavy gravity reactions nor, more critically, big uplift reactions on the seacans.

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That's a fair point. How do you have the trusses connecting to them right now? My old boss did something similar to this for a client, only steel framing was used instead of wood. I believe he actually put a beam across the top of the containers that spanned to the stacking points as that is truly where the containers are designed to take loads. The trusses then framed into the top of said beam.

@Brad805

2x4's are pretty ridiculously priced right now also.
3/4 Advantech is like $87 last time I checked.
I framed my house in November and I thought I was getting bent over paying $57 for Advantech - now it seems like a bargain!

Wow. Where is reinforced masonry these days? It sounds like it really is getting cheaper to build a block house than a wood house.