I built my first home and am about to start another but I am unsure about whaat to use for the floor support system; I joints or floor trusses. I like the space that the floor trusses have for running ducts, wires, pipes. I also see that there are I joists that have pre-cut openings but they seem much more limited. I also wonder about strength and longevity.
The home design is not anywhere near done so I do not have spans or other structural details at this time. I am looking for advantages and disadvantages of these two floor support systems.
Thanks!
Mick Simpson
Replies
Floor joists
My opinion:
Sawn lumber of ideal quality is getter scarcer so engineered wood is becoming a better option.
Wood "I" joists and wood trusses can span greater distances than sawn lumber of a comparable depth, but at a cost in comfort. Long span floor joists, while of adequate strength, tend to behave like a drum. Someone walking across the room can cause ripples in a glass of water and can cause tall appliance or floor lamps to wobble. The floor may be safe, but the discomfort of vibration may prove disconcerting to some.
Glued & screwed floor sheathing can reduce this to some degree but my preference is to install bridging or blocking between the joists at 1/3 or 1/4 points to help stiffen the floor.
This can be designed around
One reason engineered floors can seem bouncy is that they're often designed to just meet the code minimum of L/360 deflection, with a standard safety factor built in. You can spend a little more and have the floor designed to a higher standard - L/480 or L/640. I framed a house once that was designed to L/720. You could land a C-130 on that floor.
If you have the height...
I would recommend the open web joists. Allows for ducts and sewer lines to move around freely...the sub's will love you for using them. If you're a good negotiator, you should be able to get a better price from them. Typically, they will be deeper (taller). Taller also means they are (or can be) stiffer.. We just did a house 24' wide and two stories over a basement...first and second floor clear spaned the 24' with 18" deep trusses. Also the trusses were made with 2x4's laid flat...not like a roof truss where they are laid on edge. This left triangular spaces 15" tall throughout. Because they are engineered for each job, you can also ask for open spaces to be left if you need larger ducts or whatever.
You can also get them made with "top chord bearing" meaning that at the wall connection, only the top chord of the truss is resting on the wall. This can come in handy if you want the first floor to be close to the outside grade. Normally, the grade must be 6" away from wood framing. If you had 2x10 floor joists, the grade would be 18" or so below the floor level. With top chord bearing trusses, grade could be 10".