Wood Truss for Car Wash 1

[Brad805]

I have been asked by a client to design a truck/car wash using wood trusses over the bays. I am not a fan of this at all, and I sketched out a precast alternate. The precast roof leads to more steps to create the roof finish they desire, and they are adamant about using wood trusses. The owner of the property has another car wash that used wood trusses, so they are not receptive to our concerns. At this point, I have not committed to complete the project, but I indicated we would investigate the solution briefly before stepping away from the project.

The general arch section is not any different than any typical residential application at this point. I have sketched the basics below.

Below is Technical bulletin from the Truss Plate Industry of Canada that discusses the use of wood trusses in a corrosive environment.
TPIC Bulletin

Wet v. dry moisture.

Currently they have not retained a mechanical engineer since that is not required due to the building size. I am not sure the air handling system will be stellar. I am trying to persuade them to add a mech eng since having a plan will be very helpful for the precast concrete aspects.

Questions:
1. Would you consider the attic space a moist environment and use the additional steps to protect the wood trusses? If so, I can probably just walk away at this point and save myself the headache.
2. Any experience with this? I found some discussions of others doing this, and a few plan submissions where they were doing so. The plans I found did not indicate anything special.

 

[XR250]

I think if I was going to use trusses, I would want a ceiling of 1/2" PT plywood covered with some sort of waterproof membrane or panel. That will also help with the stability of your gable end wall (assuming you have one).

 

[jayrod12]

Gross. Without a proper mechanical engineer on board I'd be walking away too. And you can pretty well guarantee that the moisture in the building fduring operation is in excess of 75-80%RH. Keeping that humidity out of the attic will be a challenge.

 

[dik]

There's no way you should be looking at this for wood trusses without a proper building envelope and building ventilation. Your truss plates will not hold up and will likely work their way out. Even with steel or concrete construction, a truck/carwash was is a specialty item.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Brad805]

Dik, I have done many car washes/wash bays, just not wood. Never before without mech eng or an arch who dug into the details.

I agree jayrod, that is the route I am leaning to right now. XR, I am waiting for details of his other carwash. I doubt it has PT plywood, but that is good idea.

 

[dik]

There's probably a really good reason for that.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Brad805]

100% Agree Dik. I see others have done it, so I said I would chat with others. I think I will walk on this one.

 

[RontheRedneck]

The situation doesn't bother me, as long as there's a vapor barrier in the ceiling and the attic is vented.

 

[phamENG]

Ron - do you do many designs in corrosive/wet environments? If it can't be completely controlled with a vapor barrier, do you have anything in design that you use to mitigate issues? Ever specify stainless truss plates? If so, what kind of cost increase does that cause?

 

[dik]

I would check with the truss plate institute; I understand that with high humidity, the truss plates can 'walk' loose. I don't know if it's a thermal issue, or moisture one. It may be 'urban myth'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[phamENG]

dik - Moisture issue. Wood shrinks and swells while the plate and teeth don't. Same thing as the nails backing out on my deck ever year. I just give my 5 year old a hammer and stand back...

I wonder if you could discount a certain number of teeth and run a couple screws in to prevent it from backing out. 99% sure that would void all truss warranties and probably hasn't been tested...but might be interesting.

 

[Brad805]

The TPIC bulletin has suggestions for the plates with different finishes and protection measures for a worst case scenario where they are in a swimming pool. Stainless seems to create several problems.

 

[dik]

With these 'heads up' notifications, it might be best to reconsider the application.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[EdStainless]

If you ever want to dig into the corrosion issues let me know.
The first problem tough is the humidity cycling just pushing out the plates like Pham said.
They make notched tooth plates, but that is just a token gesture in this case.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Brad805]

Yes, dik, I agree. I was interested to see what Ron and others might have to say given it is being done. I figured "wood truss" in the thread title would bait Ron.

 

[Brad805]

Ed, I think the batt insulation is not ideal either. Most want to use the spray foam these days, but I am not sure if that helps or harms the situation. We looked at another truss problem recently where the trusses were being damaged due to the moisture being exhausted by a mech vent. Here we will have the same situation each time the O/H door opens to let a vehicle to leave. You can watch the moist air path in the winter as it exits the bldg. It goes right up to the soffit.

 

[CrabbyT]

I think you should tell the client "I know you're smart, so don't be stupid. You'll be replacing the roof in a couple years if you use wood."

 

[Lomarandil]

Reminded me of a similar discussion a few years back: thread507-400820


----
just call me Lo.

 

[Brad805]

Crabby, I did that. My biggest problem is the wood does not even save any money in this case. I suggested extending the precast demising walls to the roof level (minor cost increase), and using either precast roof slabs or galvanized steel decking + galvanized beams as an alternate. I suspect his contractor is whispering how much more expensive the roofing will be for their own reasons. We are planning to step aside. This project is in an area where they can find a more agreeable engineer.

 

[CrabbyT]

Brad - you got tegridy.

Before you step away, have you considered asking the client if they'd let you solicit some quotes for beams and a galvanized deck? The lead time on bar joists is insane right now, but W-shapes are easy to come by. Seems like you could base the quotes on W8x31 purlins spaced at 10' O.C. and still come out ahead. I'm reasonably sure that 5 re-roofs would be more expensive than a steel roof, especially if you consider the downtime involved.

You might also hit em with the ol' "we don't have enough money to do it twice, but we have enough to do it right once"