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Open Web Truss Size and Spacing

Posted by golfer15 (My Page) on
Sat, Nov 16, 13 at 15:26

Builder has proposed using 14" open web trusses 19.2" on center for all floor joists except above the family room due to a long span of about 20 feet. There, the engineer recommended the same open web truss size, but 12" on center. As an alternative, the engineer proposed 18" open web trusses at 16" on center in the family room and 19.2" on center everywhere else.

Builder wants to go with 14" OWTs everywhere. I am concerned that the smaller joists may not provide as much sound dampening and sturdiness as the 18" OWTs.

I am also concerned that the smaller floor joists may not accommodate larger AC ducts. What do you think?


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RE: Open Web Truss Size and Spacing

Sound would not be affected. Sound treatment should occur on the bottom and top of the trusses.

The greater depth would be much stiffer and better avoid a bouncy feeling. Bounce is not always considered in the design of a floor structure so you need to ask the engineer (not the builder) what the maximum deflection at the design load is of each option. A really stiff floor would deflect no more than L/720 (.33 inch).

The openings for ducts would be larger but you need to find out the size of the largest duct needed. The smaller truss should allow at least an 8" round duct and the larger one should allow at least a 10" round duct which is 60% larger. The smaller trusses can sometimes be designed to allow a larger duct at a specific location. If the ducts have not been designed you probably should not be selecting the trusses.

16" spacing makes it much easier to position and install recessed lighting but if you are concerned about sound transmission you might want to limit ceiling penetrations or put them in sound/fire boxes.

What is the downside of the larger trusses?


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RE: Open Web Truss Size and Spacing

I also do not think that it will make a lot of difference as far as sound transmission goes. You need to ask the engineer these things.

If cost is not a concern than it is almost always better to have a deeper stiffer floor. (within reason)

If you need sound proofing that could be designed in.


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