Most wood frame floors utilize nominal 2-inch thick joists placed on a centerbearing beam and covered with sheathing. Floor trusses have recently become more popular and post-and — beam crawl space floors have been in use in some areas for years.
Floor design is based on a number of factors, such as:
• Design load
• Lumber species, size, and grade
• Clear span between supports
• Floor sheathing materials and thickness
• Fastening techniques
Utilization of full span capacity of lumber joists can often save between 6 and 8 percent of joist framing costs.
If allowable spans for joists presently used exceed the spans shown on the floor plan by 1 foot or more, important savings may be realized by changing either the joist grade, spacing, or size.
Built-up wood girders are usually more cost effective and easier to work with than steel girders.
Floor sheathing adds stiffness to the floor and, depending on thickness and fastening methods, can result in reducing the size and/or increasing the spacing of framing members.
When a plywood subfloor is properly glued and nailed to the joists, the subfloor and joist act together as a composite T-beam and as such will span a greater distance than if the subfloor is fastened with nails only. Glue-nailing is also effective in reducing floor squeaks and stiffens the floor. A single layer tongue-and- groove system is usually less costly than a separate subfloor and under- layment system.
Bridging or blocking between floor joists at mid-span has been proven ineffectual in almost all cases. An extensive series of tests conducted by the NAHB National Research Center in the early 1960’s proved that mid-span bridging adds nothing but cost to most floor systems. Major model codes
require bridging only in floors with joists exceeding a depth-to-thickness ratio of 6 to 1 based on nominal dimensions. In other words, joists up to and including 2x12s do not require mid-span bridging.
Another common but unnecessary practice is to double floor joists under nonbearing interior partitions. In fact, it is not necessary to locate nonbearing partitions over any joist when 5/8-inch or 3/4-inch plywood subfloor is used. The weight of the partition does not warrant extra support. Load-bearing interior walls usually run perpendicular to joists and do not require added support. If loadbearing walls run parallel to floor joists, a supporting girder sj’stem is required.
Wood frame floors must be anchored to the foundation to resist wind forces acting on the structure. In conventional construction practice, 2×6 sill plates are attached to the foundation with anchor bolts, and floor joists are toe-nailed to the sill plate.
Metal anchor straps are available for embedding in the foundation concrete or mortar that do not require holes in the sill plate. Such straps are less exacting and do not interfere with joist or band joist framing as anchor bolts often do.
Sill plates can be eliminated altogether if the top of the foundation is sufficiently level and accurate. Joists, however, must rest on a solid surface and not over the cores of concrete block. Anchorage can be provided by anchor straps attached to the joists and firmly set in foundation concrete or mortar.
Floors built on pressure treated wood foundations do not require separate sill plates or anchoring devices.
Joists are toe-nailed directly into the foundation wall top plate.
Stairwell framing costs can be reduced in the design stage by positioning stairwell openings parallel to floor joists. Double joists (trimmers) on each side of the opening are not necessary where the header which they support is located within 4 feet of the end of the joist spans. A single header is generally adequate for openings up to and including 4 feet in width.
The band joist (sometimes called the rim or header joist) typically is the same size as floor joists. The major function of the band joist is to keep the floor joists vertical. Therefore, if wall studs and floor joists are aligned, a nominal 1-inch thick board or plywood band is adequate.
In certain cases, the band may be eliminated altogether. When a structural exterior wall sheathing is used, it can extend over the ends of the floor joists where the band would
normally be. Joists must be temporarily braced until the wall sheathing is applied.
The typical wood frame floor has two joists lapping over the center girder, each joist acting independently from the other. This system creates problems with plywood sheathing layout, one side of the floor being 11/2 inches out-of-kilter with the other.
The allowable span of floor joists may be increased by maintaining continuity over the center bearing — that is, if the joist is continuous rather than lapped over the girder.
If two joists of unequal length are spliced together so that the splice occurs at a point off center, an increase in stiffness of up to 40 percent is possible. This off-center- spliced-joist technique can result in reduced lumber size, increased spacing, or both. In addition, subfloor layout is greatly simplified.
EXAMPLES FROM THE DEMONSTRATION PROJECTS
However, the added cost of end trimming and splicing the joists must be considered. A Research report titled Off-Center-Spliced Floor Joists (143-51 is available from NAHB. 15th & M Streets NW, Washington, DC 20005.
Floor trusses are becoming more popular as alternatives to conventional wood frame floors. They are usually more costly but have the advantage of greater spans between supports, thereby creating greater floor plan flexibility. There are several different floor truss designs, some with open webs and some with a continuous plywood web. Open web trusses have the added feature of providing plumbing, wiring, and sometimes HVAC chases without drilling or cutting.
The major disadvantage of single-span floor trusses is the height of the trusses. In order to span greater distances, the truss must be much deeper than conventional 2x framing. This adds extra costs to sheathing, siding, stairs, etc. These, plus the extra cost of the floor itself, should be analyzed versus the value of the clear span.
Innovative floor framing methods were used on several JVAH sites. Some were unique while others were known methods not normally used in the local area.
As discussed in the Foundation section, Wayne Hood built the Plenwood system in his demonstration homes. The wood frame floor portion of the system consisted of 2×8 floor joists spaced 24 inches on center with glue-nailed 3/4-inch plywood.
John Phillips constructed one of the most innovative wood frame floors in the demonstration. Typically, in the Lacey area, the foundation/ floor system consists of a conventional spread footing, a cast-in-place concrete foundation wall, a post-and — beam center bearing girder, 2×10 joists spaced 16 inches on center, and a two layer sheathing/underlayment system.
Phillips designed and built a system that used 2x6s, spaced 24 inches on center, spanning about 8 feet between post-and-beam supports. Band joists were eliminated. Floor framing did not come into contact with the perimeter foundation at all. The first interior support girder was placed about 4 feet from the foundation wall and the joists cantilevered about 2 feet toward the foundation.
The 2-foot gaps at the ends of the floor joists and the 2-foot spacing where the joists are parallel to the foundation were easily spanned with 3/4-inch tongue-and-groove glue-nailed waferboard. Floor construction costs were reduced by an average of $852 when compared to the conventional floor.
Christian County, Pup Robertson also built an innovative
Kentucky floor system that saved $295 per
house. Instead of the conventional 2×8,16-inch on-center joists over a single center girder, he used two box girders spaced about 8 feet on center and 2×6 joists at 24 inches on center. He used 3/4-inch glue-nailed T&G plywood instead of the more conventional 1/2-inch underlayment.
Tom Webb designed and built a cantilevered front porch deck by continuing the interior stair landing joists through the front door opening. This eliminated deep porch footings and foundations which are very susceptible to frost heave in Fairbanks.