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Wood Trusses for Car Wash/Gas Station

Wood Trusses for Car Wash/Gas Station

Wood Trusses for Car Wash/Gas Station

(OP)
Has anyone ever designed wood roof trusses for a car wash?  5/8" "Green Board" will be attached to P.T. 2x4's attached to the bottom chord of the trusses.  Should I be concerned with moisture content even with green board attached?  Thanks in advance.  

RE: Wood Trusses for Car Wash/Gas Station

Don't know for sure - hard to know how much water would get up to the trusses but I'd assume there would be significant vapor in the air - I would call a roof truss manufacturer to get their view on it - but at the very least, you'd want them to use the Wet Service factor on the wood design found in the NDS.

RE: Wood Trusses for Car Wash/Gas Station

Some of these treatments are water soluble, so be careful to use one that is not going to leach out.

There is a wet use factor in the timber code also that may be applicable in this case.

csd

RE: Wood Trusses for Car Wash/Gas Station

If you have gable ends in the car wash, you might consider forced air fenting with a couple of fans.  Roof should be vented anyway, but you can never be too careful.  Where in the country will this be?

Mike McCann
McCann Engineering

RE: Wood Trusses for Car Wash/Gas Station

(OP)
It is near Cincinnati Ohio.  
The attic space of the roof will be vented.  My concern is how well green board actually prevents moisture from penetrating into the attic space and if the moisture content in the attic space will exceed 19%.      

RE: Wood Trusses for Car Wash/Gas Station

I don't think greenboard works as a vapor barrier, only that it's meant to  be used in moist environments.  I would think you would have the same concerns as trusses for pool enclosures -- high moisture content.

RE: Wood Trusses for Car Wash/Gas Station

Check out page 7 of this:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch03.pdf


For the moisture content of wood to be >19%, the Relative Humidity would have to be >90% for extended amounts of time.  With proper ventilation, that probably wouldn't occur.

RE: Wood Trusses for Car Wash/Gas Station

ctcray - helpful article.  But in a car wash, you aren't talking about humidity in the air, but direct moisture spraying on the wood trusses from the equipment below.  That is a much higher moisture condition than 90% relative humidity.

RE: Wood Trusses for Car Wash/Gas Station

I guess I would trust the proposed greenboard and treated 2x4 girts to prevent direct contact with water.  

I've had people approach me about using treated lumber trusses to create exterior footbridges. In researching the idea, our engineering rep at Alpine basically said truss plates should not be used in environments where the trusses will see cycles of wet and dry (ie, rain) because the teeth are much more likely to loose their "bite" from the local shrinking and swelling associated with wetting and drying. I got the impression that it would be much more so than the NDS Cm reduction for nailed connections.

RE: Wood Trusses for Car Wash/Gas Station

I would not trust only green board to isolate the attic space from moist. It is only as good as the joints. Could you fasten a top quality, heavy duty vapor barrier (retarder) to the underside of the trusses before applying the green board?

Additionally, I would tell the Owner (in writing) that the longevity of the wood truss system depends upon the quality of maintaining the attic space at nature moisture levels and if joints, green board and vapor barrier are not maintained to near perfection, he may find himself replacing the roof. Although the initial capital costs are lower, maintainance and replacement costs may exceed those of say a precast roof.

RE: Wood Trusses for Car Wash/Gas Station

(OP)
Thank you all for your replies.  ctcray...  thanks for the literature.  

RE: Wood Trusses for Car Wash/Gas Station

From my experiences with MPC tooth withdrawal due to moisture cycling, I would suggest a bolted timber truss design using wet service reductions, purlin roof support with no ceiling.

RE: Wood Trusses for Car Wash/Gas Station

Did the trusses in a car wash last year.  Its funny, my first question to the builder/customer was how are the trusses protected?  The building designer of record did a good job in research.  He found and spec. in the plans a manufactured ceiling assemble system designed and rated for high moisture applications.  Sorry don't remember the trade name but could find it if you need it.  I do remember it was a thick plastic    back insulated panels that attached to a special channel that was attached to bottom side of the trusses.  Don't know if it had a NOA letter or not,  But I did not hear of any issues with plan review at the building dept.  

RE: Wood Trusses for Car Wash/Gas Station

Pressure Treated 2x4's?  Are you using stainless steel plates?  ACQ is allegedly really corrosive, if that is the treated lumber you are talking about.  The plates will push off as mentioned too from shrinking and swelling.

RE: Wood Trusses for Car Wash/Gas Station

This is a question I faced a few months ago and while this forum is dedicated to trusses, I tried to talk the carwash Owner into using a precast concrete roof, with no luck. Time will tell if it was a good choice.

All five of the previous carwash projects I've dealt with in the past used precast concrete roof plank. One of the buildings has been operational for over 40 years, the equipment has been changed twice but the building is still functioning.

Let's face it, each material has its' place in the market. IMHO precast concrete roof plank suit carwashes better and will last longer than wood trusses.

RE: Wood Trusses for Car Wash/Gas Station

Think about light gauge galvanized steel trusses.

RE: Wood Trusses for Car Wash/Gas Station

As capnbobbo1 pointed out, I would educate the owner and specify either steel joists or light gage steel stud trusses. Wood trusses would be pretty much submerged in water, and that is looking for trouble.

RareBug

RE: Wood Trusses for Car Wash/Gas Station

Contrary to capnbobbo1's advice, I would not use light gauge steel trusses in this very humid environment.  The protection you can get for these sections is minimal, you would have major corrosion issues within a few years.  The advice by CTruax to use bolted timber trusses or advice by others to use precast concrete are appropriate.  Also could use rolled steel beams with timber purlins.

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