Corrosion of trusses/truss plates under pool? 2

[rcast044]

Hi guys,

Our truss guys are wondering how to protect the trusses/truss plates from corrosion due to the pool being under the lanai (see attached picture below, blue lines represent trusses). I didn't think this was an issue. It is my understanding that any moisture will dry due to the pool being in an open lanai. Any answer would be appreciated.

 

[r13]

Depending on humidity around your area, the potential for the truss plates to catch moisture is high. I think you need to pay attention to rust prevention.

 

[KootK]

Former metal plate connected truss designer here.

Here are some corrosion protection options: Link

My preferred solution to this would be to not do it. Under repeated, lumber moisture changes, the plates tend to work themselves loose even if they don't corrode. Under pools and hot-tubs, my order of preference is:

1) Solid sawn lumber.

2) LVL

3) I-joists

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496) Metal Plate Connected Wood Trusses

 

[XR250]

I'm with KootK on this - super bad idea.
My field experience with MPC trusses is that they love to creep under sustained high loads - especially with elevated humidity levels. LVL's and I-joists tend to do the same. I would stick with dimensional lumber or steel.

 

[rcast044]

Hi Koot,

I was reading the article you provided about corrosion in farm buildings. It mentioned proper ventilation reducing corrosion problems significantly. The lanai will always be open to the outdoor air. Do you think this will prevent corrosive problems?

 

[rcast044]

This is just a typical 1-story home. The trusses are bearing on CMU walls on each side of the lanai. The pool is on the ground. XR250, you mentioned sustained high loads. I don't believe these truss should be under sustained high loads.

 

[XR250]

Sorry, my ADD self misread your post. I'm working on a pool right now that is intended to sit on wood framing so I must be projecting my project onto yours!

 

[KootK]

So you have wood framing beside your pool but not under it then?

Do you think this will prevent corrosive problems?

It's difficult to say. I'll stick to my answer even with the framing being beside the pool if that's the case. I don't like metal plate connected wood trusses outside the building envelope, really, and any water tracked over from the pool only makes things worse.

 

[XR250]

I get the impression that the trusses form a roof over the pool?

 

[r13]

You are essentially having an open structure that is heavily influenced by the environment it subjects to. No matter it is wood or metal structure, you shall pay attention to fluctuation of whether weather, evaporation of pool water, and cyclic wetting and drying of materials. It sounds like a nice/expansive residence, why play cheap on protection rather than paying a little more for durability?

 

[rcast044]

XR250 is correct. The trusses form a roof over the pool.

To retired13, it isn't about playing cheap, it is about convincing others of a potential issue with sound data if it is actually a concern. Telling contractors to do something they don't normally do builds poor repertoire if you don't have sound judgment. I just want to make sure given my example it is of sound judgment.

 

[r13]

The "potential" problem isn't belong to the contractor, when bad things happen, they are long gone but your name and your company. In this case, seems the contractor (the truss guy) is more concerned than you do though.

 

[rcast044]

Yes, the potential problem DOES belong to the contractor, as well as everybody involved. We have worked with them for over 20 years. Not sure why you came into this post just to insult my concern of the whole thing when I am asking a curious question if it is required.

 

[Celt83]

Sitting directly over the pool you'd have a strong corrosion potential from the chlorine and other chemical treatment of the pool. This could be compounded by using wood framing as it would tend to absorb some of the moisture evaporating off of the pool.

In an application like this I'd probably only use pressure treated lumber and if that can't get there then wolmanized PSL's. I think there are surface treatments now for LVL's for moisture protection but don't have any experience with them but that may be an option if the manufacturer would warranty the installation.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[r13]

If you feel it was an insult, I apologize. For your case, the "potential" is very difficult to judge, unless you know the local practices very well. I'll listen to the contractor who had stayed with me and served well for 20 years.

 

[rcast044]

So it looks like I am thinking a vapor barrier in the attic with a special coating post-installation on the plates to minimize any corrosion problems. Appreciate all the help guys.

 

[r13]

I think there must be something fit your need in Simpson Strong-Tie. here is the link you can browse through. Link

 

[KootK]

Yeah, if it's trusses above, and open walls to the side, I suspect that you'd be fine. Treated lumber and/or some corrosion protection for the plates seems reasonable. In you attempts to tighten up the vapor barrier, keep in mind that solid ventilation of the attic is surely a friend here.

 

[EdStainless]

There are multiple things going on in this application that can cause trouble.
-The humidity levels will be higher than in normal installations, this will lead to more swelling/shrinking of the wood members.
-There will be may times where in the evening the roof sections starts to cool off and the ground and water below are still warm, this will result in condensation on the roof members. This will lead to saturation possibly.
-The Cl effects from pools are well known and extensively documented. There are special rules for 'above pool' structures. The Cl smell that you get from pools isn't just Cl, it is chloramines. These are volatile organic compounds that are very corrosive. This is why stainless steels are forbidden from overhead loads in pool applications.
Just my opinion:
In short being over a pool and under roof is worse than just being open to the weather.
Making this from steel that is galvanized (after all cutting, drilling and welding) would be an option, though Galv with a urethane top coat would be a good option. You have to assume that this will be wet all of the time and treat it like a coastal application with heavy sea salt exposure.

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P.E. Metallurgy, consulting work welcomed

 

[XR250]

Here is one I looked at today. 19 ft. span, 14" trusses, 24" O.C. in a humid crawlspace. The deflection in the first panel point is noteworthy. The lumber was not soft and the plates were still attached pretty well.

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