I'm thinking of using them to make a bonus room over the main part of our house. I need to sit down with a truss designer to see if they would be practical.

I got them in my last house for the room over the garage and have them in the house I am building now over the master bedroom (the attic space will be my craft room) and over the garage (storage). The master bedroom is 20' wide and I ended up with a 12' wide room. The ceilings are a little lower than I had hoped; they are around 7', but for a craft room, it's fine. My trusses are spaced 24" OC and I wish I had closed that spacing up to 16" OC to give the attic floor a little more strength. I am probably going to install 3/4" oak floors up there to give it a more solid feel. I am very happy with the extra space I got for the little extra I paid for those trusses.

There is a pull-down stairway to access the attic over the garage, but I will build a 6" wide stair case to the craft room after the final inspection. Because of the layout of my room, the stairs wont meet code. The rise and run will be a little off. Still safe, but not within code. If you want stairs to get up to that attic space, make sure your truss engineer calculates what it will take to cut the bottom cord of the truss to permit a stairway.

Do check with your structural engineer before converting trussed space into living space. Some types of trusswork are simply not designed to handle that sort of load. Same for attic flooring, be sure the beams can handle the additional weight and placement of flooring and furniture.

flgargoyle ~ you might like the attached link re trusses. (I do like things explained in plain English.)

Here is a link that might be useful: re trusses

If you intend to make the attic into a habitable space the "attic truss" usually doesn't work well in most climates.

The point of a truss is to be able to make the individual members smaller (and therefore cheaper)and to be light enough to be easily delivered to the site and lifted by crane. Usually insulation is installed at the bottom chord. Unfortunately, for a habitable attic space the small rafter chords leave less space for roof insulation, possibly less less than required by code.

In a cold climate, for a steep roof, I often find it is necessary to use site installed 2x12 rafters to accommodate the required roof insulation and rafter venting.

Be sure to tell the truss designer that you intend to use the space for more than storage so the bottom truss chord can be made stiff enough for comfortable occupancy. A bouncy floor is OK for storage but not for people.

If the roof pitch is steep enough and the usable space is large enough, attic trusses could cost more than site built rafter framing. There is a good reason why conventional trusses have so many intermediate bracing chords.

M.A. brings up a good point concerning insulation. Ours were engineered to exceed the snow load requirements, but i had to fur down the top chords in order to get the required sized insulation with air space. This brought down the headroom a tad but the headroom depends on the pitch and span of the trusses.

There is a point in the design of a roofing system when the span gets large enough, the roof pitch gets steep enough, and the habitable space gets large enough, that the bending moment induced in the center of the rafter chord is so great that the chord becomes almost as large as a conventional rafter. The same thing occurs in the bottom chord. In other words, as the truss becomes increasingly inefficient it becomes similar to a conventional site built framing system with kneewall and ceiling braces.

When the raw material, transportation, and installation costs get large enough, you might be paying more for the installed trusses than for conventional site built framing. But don't expect the truss supplier or even the installer to tell you that.

You should price such hybrid framing systems carefully with contractors who have adequate experience with all methods.

Here's a sketch of what I want to do for my second floor. I'm just afraid that the floor joists will have to be huge/expensive to span that distance. Most of the house will have internal walls underneath, but the last 16' is over a big open room. I'm thinking of going with 6" SIPs, so coupled with the mild SC climate, insulation isn't much of a concern. Actually, I think the SIPs I'm looking at are self-supporting for the roof- just need to figure out how to hold the floor up!

I see you are in South Carolina? If you are near the midlands, give the folks at Hoover a call. They do awesome work and can design them any way you want them. I am not sure you will be able to get 16' wide though with a 24'span, but ask them, let them know what you want and see what they come up with. You may even be able to do it via E-mail. I am fairly certain they deliver anywhere in the state.

I'll give them a call, although I'm in the extreme upstate, out past Travelers Rest.