Most of us don’t think too much about the floors we walk upon – unless they are not level, squeak when we walk on them, or are too bouncy.
Traditionally wood floors have been framed with dimensional lumber (2×6, 2×8, etc.), usually spaced 16 inches on center. Often floor joist span limitations are not based upon the strength of the lumber (the ability to carry a load), but upon deflection criteria. Building codes limit floor deflection to L/360, where “L” is the length of the span, in inches.
The “stiffest” (by MOE – Modulus of Elasticity values) commonly used framing lumber is Douglas Fir. A #2 grade Douglas-Fir 2×12, 16 inches on center will span 18’1” when carrying standard residential loads. An L/360 deflection event, would cause the center of one of these floor joists to deflect as much as 6/10ths of an inch!
Lumber is organic, so it varies in consistency from one board to the next. It also varies in size, and it is not unusual for a dimensional variance of over ¼ inch, from one end of a board to another. Combine this with the probability some of the boards will be crowned the wrong way, and it means an uneven floor will result.
One of our friends lives in a fairly new home. In the hallway between her kitchen and the sleeping areas, there is a good ½ inch dip in the floor – more than noticeable when walking across it!
I first used floor trusses in my own pole building almost 20 years ago. My trusses were designed so they were only 1-1/2” thick, but the 30 foot span trusses are only 24 inches in depth. They allowed me to create some unique interior areas, without the need for columns inside the building.
When we built our home in South Dakota, we utilized floor trusses again – here to span 48’ (yes 48 feet)! We live upstairs in a gambrel building, with a clearspan ½ court basketball court size garage downstairs!
A few years ago, my oldest son Jake needed a new garage – his mom convinced him the design would be so much better with a mother-in-law apartment upstairs. We used 4×2 (2x4s turned flat) floor trusses to span the 24 foot width!
I’d forgotten how fast a trussed floor can be done – until Jake used them for the support of the second floor in the addition he is now putting on his home. In a matter of just a couple of hours, the entire 24 by 32 floor was framed and ready for sheathing. All of the ductwork can be run through the open truss webs, making for nice clean ceilings downstairs.
Considering a second floor loft in a pole building? Don’t want posts down below to get in the way of full space utilization? Then floor trusses may be the answer.
Make sure to allow adequate height for the thickness of the trusses. As a rough rule-of-thumb, I plan upon one inch of thickness, for every foot of span. While it will nearly always be less, it is better to design for having a couple of extra inches, than not enough.