Floor Construction

Supporting Members
The distinction between "beams," and "sills," and "girders" is a little muddeled and is probably subject to local custom. So don't get caught up in the confusion. Just role the punches, and be prepared to use the terms interchangeably.

In any case, this is the big stuff that carries the weight of the floor (and whatever it may support) to the foundation members - walls, piers, etc. The supporting members may be wood or metal or some combination of the two. Sizing is determined by the load to be carried, the type of beam, and the span.

Some examples are:
simple (or "built-up") beam
Usually two or more 2x8 or 2x10 nailed together.

flitch beam
Two 2x’s (pronounced "two-buys") with a piece of steel plate or plywood between. A typical example would be two 2x10’s with a 1/2" x 9" steel plate. These pieces are bolted together.
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box beam
This (fairly uncommon) beam is constructed with a 2x top and bottom, and with plywood sides. The sizing of the material, depth of the beam, and load it will carry determine its span capability.

laminated beam
This beam is made up of several pieces of wood (usually 2x4's) glued together. This is a factory made item.
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steel i-beam
Steel is often used when longer spans are desired. A 2x wood plate is usually fastened to the top of the steel beam to make it easier to attach joists.
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Microllam©
Microllam© is a registered trademark of Trus Joist, a Weyerhaeuser Company.This beam l resembles a very thick piece of plywood. It comes in various thicknesses and widths.When ordering, you specify the length you need. As a manufactured item, it can be cut to any desired length.
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floor joist construction
Floor joists are usually wood 2x’s. The size depends on the type of wood, the span (length between supports), and the anticipated load they will carry.
Spans for various species and grades of wood can be found in published span tables.
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TJI
There is a manufactured joist on the market called a TJI, also a registered trademark of Trus Joist.
It consists of two 2x4’s (top and bottom) with a plywood web between. It’s advertised benefits are no-squeek floors, consistent quality, lightness, and ease of installation.
Depending on the market, the TJI may or may not be less expnsive than 2x’s.
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Floor Truss
Another type of joist is the floor truss. Its advantages would include the ability to achieve greater unsupported spans, keeping in mind that the greater the span the greater the depth of the truss that would be required.
Another plus for floor trusses is that the trusses are "open," allowing wiring, plumbing pipes, and possibly some HVAC ducts to be run through the trusses. With solid joists, holes must be drilled in the joists for wiring and pipes, and HVAC ducts have to be boxed in under the joists - intruding into your "living" space.
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Floor joists are usually at 16" or 24" on center - depending on the size of the joist and its "span." At the foundation wall they usually sit on a sill plate or mud plate - a treated 2x placed flat and bolted to the wall.
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Treated wood has been impregnated with special salt solutions under high pressure to retard rotting in the presence of moisture.
The joist which is placed at the end of the other joists is called the rim or header joist.
Joists can extend over the foundation wall, or, in the case of the second floor of a two story home, over the first floor wall, creating an overhang. This is called a cantilever.

Bridging
Bridging, or "cross bridging" is some kind of lateral inserted between floor joists, to prevent lateral distortion, twisting, or warping.
It is usually comprised of 1x4's installed to form an "x" or "solid bracing" of 2x's nailed in at the mid-point of the span - or more often in long spans.
There are also metal bracing systems available.
Many people swear by one method or another. But the truth is that none of the national codes require any bridging any longer. If your local building inspector requires bridging, or your framing sub insists on it, just roll with the program. There will be more important battles in the course of your project!

Hangers
By code, floor joists must be supported at the end by a minimum of 1-1/2" of wood or by a joist hanger. These metal straps are ubiqutous in home construction now and are required by many local codes.

Flooring
Subflooring and finish-flooring are sometimes combined into one piece. This would typically be a plywood product, perhaps tongue and grooved. It would be attached to the floor joists with glue, nails, screws, or a combination thereof.When separate layers are used, the sub layer can be plywood (typical) or 1x lumber (very rare these days). Since two layers are being used the plywood is thinner - usually 1/2".

The top layer (finish floor) can be plywood, particle board, or hardwood. A layer of 15 pound “felt” paper is often laid between the layers as a vapor barrier.Plywood with a waterproof glue is preferred in wet rooms (kitchen and baths). This is because particle board tends to decompose when wet.

Slab floors are common in many areas, especially those with warmer climates and stable soils.
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If you find that slabs are common in your area, you may consider this kind of foundation/floor for your new home project.
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On the plus side, slabs can be faster to install. They may even be less expensive, depending on current wood and concrete prices.
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Many people think that slabs are a colder floor than a wood floor. This is not necessarily so if the slab is properly insulated. Slabs are also good candidates for radiant heating. On the negative side, slabs do not have the "give" of wood floors, and thus can be harder on the feet.
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Slab construction is a pretty big subject, so we have dedicated a special page to the subject.