Exterior Plywood Beams

[phamENG]

This is not my project, just something I came across that looked both neat, but also a little concerning and I thought I'd post it here for a general discussion. The idea of a deck builder fabricating curved beams out of plywood in his shop and then installing them outside sets off alarm bells for me. APA has their "Supplement 5: Design and Fabrication of All-Plywood Beams," but they don't mention exterior exposure of the beam (just reference to adhesive exposure ratings).

How do others feel? Are you okay with is as long as the adhesive (both in the plywood and between lamination) is rated for exterior use with frequent/prolonged wetting AND the plies in the plywood have been preservative treated? Or is it more about using a moisture barrier on the top surface of the beam (between the tread and the stringer and the riser board and stringer, properly lapped) and the PVC trim on the side. I'd tend to be okay with the second in less humid environments, but here in the Mid-Atlantic and the South East (US) where you almost need gills to breathe I'd be worried.

If you're good with this kind of construction, how long do you think it would last if properly maintained? What kind of maintenance would you expect?

Link
PDF of the article (pop up trying to get you to sign up to their newsletter is really annoying):

 

[STrctPono]

Good post phamENG.

I have a vested love interest with outdoor decks, curved/spiral staircases, and wood working. Oddly enough, I think about wood exposure in exterior environments quite often as a lot of my wood working projects have been outdoor pressure treated, cedar, redwood builds. The article mentioned that he used "pressure treated" wood but never mentioned what type of pressure treated wood or whether it was a CDX or ACX plywood. To me, that is the key component. The issue is two fold. First, whether or not the adhesive is exterior rated. If, for instance, he used an exterior grade plywood such as CDX then I wouldn't see any concerns with the design being outdoors from a structural perspective. Second, the pressure treating is there to resist rot, decay, insect attack which can become a structural issue (but is more a serviceability issue) if left unchecked. If he used a CDX borate based treated plywood then I would say that this is incorrect. If he used a CDX with exterior rated treatment then I would say that he did it correctly. I am unfamiliar with what the different plywood or treatment options are in New Jersey. It appears that he then painted the stairs which is the best option and further protects the wood and preservative. It the curved stringers were designed with adequate capacity then I see no issue with what he is doing. Would I stamp it if someone brought me the design.... probably not.

How do others feel? Are you okay with is as long as the adhesive (both in the plywood and between lamination) is rated for exterior use with frequent/prolonged wetting AND the plies in the plywood have been preservative treated? Or is it more about using a moisture barrier on the top surface of the beam (between the tread and the stringer and the riser board and stringer, properly lapped) and the PVC trim on the side. I'd tend to be okay with the second in less humid environments, but here in the Mid-Atlantic and the South East (US) where you almost need gills to breathe I'd be worried.

I think that it is both if you want a good system. The waterproof glue between the laminations is important but then I also agree with you that the moisture barrier on the outside (i.e. paint and/or deck wrap on top of the stringers) is what I think would make this system as close to bullet proof as possible.

When I built the spiral staircase in my home some years back in my research efforts I had ran across some other wood working article that talked about some students who had built a grand interior curved staircase in the exact same fashion. Difference being that it was interior.

 

[Craig_H]

I might consider it, depending on climate. I'll admit that I have very little knowledge of the different types of exterior-grade plywood, mostly because I almost never come across them in my course of work. StrctPono's description was informative. I think it's important to note that plywood's exposure grade was likely expecting it to be "on flat" rather than "on edge" as it is in this application. I would hazard a guess that the plywood is WAY more vulnerable to moisture damage with all its laminations facing the sky. That said, there may be ways to keep the moisture mostly out of these components in the first place:
- Standoff connectors where the plywood comes into bearing contact with anything else that could be wet (protection from wicking)
- Protect the top flange from top-down moisture using a robust flashing tape. I'd find a tape that self-seals around fasteners, or perhaps a fluid-applied flashing product. In a dry climate, I have seen solid-sawn (untreated) decks survive over 60 years, and still look remarkably new, with a detail like this in place. I suspect that there's a tipping point where the ambient humidity would be too high for the wood to dry quickly between repeated wettings, and where this would no longer work.
- Give water that falls a path to drain. Drain it away from the stringers. I'm picturing very slight slopes to your individual treads, a drip edge routered into the lower ends etc. Bonus points if the decking is impervious (such as the composite products that are around these days).

 

[phamENG]

STrctPono - thanks. You clearly know more about treatment options for plywood than I do, so thank you for sharing some insights there. I was taught that plywood should never be left outside. Full stop. So I've always found alternatives to using it in exterior exposures. I saw this article and figured they were either crazy or there was a serious deficiency in my knowledge of plywood's capabilities. Looks like the latter is a little closer to the truth. I'm intrigued by your response, though. You say that using proper ratings and moisture barriers could get you to a good system. But above that you say you wouldn't stamp it. Why is that?

Craig - I'm with you on the climate question. Hopefully somebody can chime in with some ideas about where that tipping point is. I think with proper detailing - such as the standoffs and flashing you mention - the flat/side issue could be mitigated.

 

[r13]

This blog provides several grades of exterior plywood. Read on. Link

 

[STrctPono]

But above that you say you wouldn't stamp it. Why is that?

Maybe if I knew the Contractor doing the work and felt comfortable about their abilities and craftsmanship then I would. I probably wouldn't stamp it with the idea that the homeowner was going to be soliciting it out for bid and not knowing who I got. Same reason for why I wouldn't stamp a design with mortise and tenon joints unless I knew the Contractor.

On a side note, there are some things that I have built that I also would not show on drawings and stamp. I built a pavilion a few years back in my backyard and wanted it to be completely out of redwood. Getting a 16ft long 4x12 redwood beam would have cost me over $1300 fr one! I ended up going and buying my own visually graded all-heart redwood 2x4's, planing them down, gluing them up with a water-resistant PVAglue and making my own architectural glulams. They are overdesigned, are sheltered from the rain, and are coated with the best alkyd based sealer money could buy. I know it works and satisfies my needs but I would never stamp a drawing expecting a Contractor to do this.

I was taught that plywood should never be left outside. Full stop.

You are correct for 95% of plywood out there but there are some types that can handle. There are marine grade plywoods too. Heck they build boats out of them.

 

[STrctPono]

Protect the top flange from top-down moisture using a robust flashing tape

There may be something to your theory regarding the laminations being exposed on the horizontal surface. I'm a big fan of flashing tape (bituminous based products in general) and think that they certainly add that extra layer of protection.

If the Contractor did all this, in my environment (HI) and maintained the paint with regular coatings I would expect those stairs to easily last 30-40 years before you had to make your first repairs.