My designer has specified 20" floor trusses at 19.2"OC for my new construction. The longest span is less than 27'. I want quality sturdy floors but am wondering if what is specified is overkill? Thanks

I often use 24" tall 4x2 wood floor trusses simply because the HVAC ducts need the space.  Generally speaking, a taller truss is stronger.  The selection of the truss should depend upon span, live and dead loads and deflection ratio.  I usually specify L/480 deflection ratio for stiff floors.  (Vibration is usually not a problem with a well designed wood truss.)  If you do not need the space between the top and bottom truss chords for utilities, then talk to your designer about a truss less tall than 20".  It may be possible for a shorter truss to carry the total load but the truss may need more webs or a double chord.  Installing taller trusses than needed adds to wall height which can cost more than shorter, enhanced trusses.

27' is a pretty long span. Even at L/480, that is over 5/8". The problem isn't net strength, but bounciness. A long span can act like a trampoline. How much would you save by going 'code minimum'? Probably not much. I'm in the overkill camp. I used 12" TJI's, which were spec for 24" centers. I put them on 16" centers. The cost difference was probably only a couple hundred bucks- not much compared to the total cost of the house. My floors are very solid. The drywall delivery stacked 44 sheets of drywall in one small area, parallel to the joists. Two joists were bearing the majority of 3600 lbs.! Essentially, I had a car parked in my living room. I kind of freaked out, and went down to the basement to see what was happening. I couldn't measure any appreciable deflection.

Thanks for the input. I am in the overbuild camp as well. I just read the manufacturer's span chart wrong.
Alton, Is 24" that much better than 20" for the utilities/ducts? Looking at the span chart, I can go with 24" at 24"oc as suppose to 20" at 19.2"oc. I assume the cost difference shouldn't be that much since lesser number of trusses will be required.

OP - are these for both the main and second floor? Are you doing any radiant slabs?

Remember that in the tji's there are several different families based on the size of the top and bottom chord. Sometime using a slightly more expensive truss will give you a more cost effective package.

Higher is almost always better from a structural point and wider spacing makes it much easier to put pipes through without a lot of couplings. Normally wider spacing requires thicker floor sheathing and as Alton indicates, also makes the building taller. Going from a 12" to 24" may impact how your front door looks and may impact the allowable hight of the building etc.

As with most things in the building industry, there are a lot of different right ways to do thing. A little though up front can save a lot of grieve later.

It will be a single story house with a walkout icf basement, so the floor trusses are for the main floor. The basement will have radiant slab. Allowable height is 35' above grade so no concerns there.

If you plan to hide the large ducts between the trusses and possibly use the built in chase, then any additional space is better for it.  I have seen large ducts partially below the basement ceiling when 18" tall 4x2 floor trusses were installed. 

19.2 spacing may work just fine if the ducts and insulation are not that big.  Although framing tapes are marked with the 19.2" spacing, some crews cannot seem to get it right.  Installing trusses at 24" o.c. is simpler and will require less.

Be sure to verify the thickness needed for the subfloor if wider spacing is used.

When laying out floor trusses, take into account the locaton of drains throuh the floor.  Sometimes, a truss cannot hit on the on center spacing if a drain from a tub, shower, commode, etc. is there.  In cases like this, an extra truss may be needed to compensate for the moved truss.

Another thing you probably already know:  If the length of the trusses vary, then install the right truss for that particular position.  Also make sure that the large chase aligns.  Chases can be mis-aligned and be useless, if the truss is turned around and the chase is located off center within the truss.  Also make sure the up side is up.  I have seen experienced framing crews get these things wrong.  Catching these mistakes after the subfloor is glued and screwed can be expensive to correct.  You probably do not need this level of detail from me but a lot of people with less experience may read this posting.  I would hate to see anyone make mistakes from a lack of knowledge.  I think this forum is good about helping everyone to tackle projects.

If anyone is planning an interesting and challenging project in my neck of the woods with alternative design and construction, then tell me about it.  I may be able to assist with answering questions through e-mail.  I try to limit my consulting practice to residences that are built with about anything except wood studs.  Since I retired from teaching in 2004, I do not charge for my time unless extensive work or travel is required.

My trusses are only 16", longest span 20'. The only thing I ran across was the need to duct the cold air returns, there is no way to pan them in. I have finished all mechanicals now with nothing below the trusses.