Framing a Floor
Floor joist layout is important when framing a floor. There are a number of factors that determine the size (height and width), the span (length), and spacing of floor joists.
Framing a Floor
The design of a peer and beam foundation starts with the floor framing plan. Floor joist spans and beam lengths determine the location of peers and footings. This article focuses mostly on the floor's wooden structure.
Floor Joist Spacing
Floor joist spacing is determined by the joist layout plan you choose. The two diagrams below show optional joist plans that can be used to frame the same size (16' x 24') floor plan.
Using the Floor Joist Span Table (Live Load = 40 lbs/ft2 - S. Pine), we can see that, in order to span 16 feet, 2x12 Southern Pine (#2 grade) joists, spaced 16 inches apart, are required. These floor joists have a maximum span of 16-6 (16 feet + 6 inches), when spaced 16 inches apart. Using our example floor plan, a total of 19 16-foot 2x12 joists would be required.
When framing a floor, I prefer to slightly over-design my floor joists. While it is within the building code, 16 foot 2x12s are a little close to the maximum allowed span (16-6).
The second joist framing plan (below) shows a properly supported girder or beam placed in the middle of the 16-foot dimension, thus creating two 8-foot spans. Using the same span table as above (Live Load = 40 lbs/ft2 - S. Pine), we can see that 2x8 joists can be used (with floor joists spaced 24" apart) for the required 8' span, with a maximum span of 9-8, resulting in a total of 13 16-foot 2x8 joists - a considerable cost savings. You could, alternatively, choose to purchase 26 8-foot 2x8 joists, in lieu of the 16-footers. Of course, the cost of the beam/girder would need to be factored in, to determine the overall cost of this floor joist layout.
I prefer the second floor joist plan, when framing a floor. 2x12s are heavy, especially 16-footers, and are hard to work with, compared to 2x8 material. Smaller dimensional lumber (2x8s in this example) are considerably less expensive (per foot) than the larger 2x12 material. Longer lengths also tend to be more expensive.
Squaring a Floor Frame
If the floor framing is not perfectly level and square, the difference will haunt the builder throughout the entire construction process, right down to the last piece of trim. If the floor is not level or square, neither will be the walls or ceiling or roof.
The most common method for squaring a floor frame is to use the Pythagorean Theorem (A2 + B2 = C2), or C = sqrt(A2 + B2).
One of the nice things about the Pythagorean Theorem is that there is an easy-to-remember 3-4-5 proportion that you can use to square anything. Basically, if side A equals 3, and side B equals 4, then side C (the long side) will equal 5.
Any combination of numbers proportional to 3-4-5 will work (3-4-5, 6-8-10, 12-16-20, 30-40-50, etc.).
For example: On any frame, measure 12' from the 90° corner and make a mark on the frame. Measure 16' from the same 90° corner along the adjacent framing, and make a mark. The distance between the two marks should be 20'. If they are not, the frame is out of square and should be adjusted.
Another option for checking square of a frame is to cross-measure the opposing corners. If the frame is square, the numbers will be equal.
Floor Joist Framing for Partitions, Openings, and Overhangs
If your floor framing will have overhangs that extend beyond the foundation, or will be supporting wall partitions, or will have openings from floor to floor, special framing will be required. These aren't the only scenarios where double-framing might be required, but these are the most common.
Note, in the diagram below, some floor joists and related framing members are doubled. Wall partitions require double floor joist framing beneath. Overhangs, where floor joists extend beyond the foundation, are doubled. Openings in the floor, such as stair wells, require floor joists to be doubled on either side of the opening and require doubled headers at each end.
Floor Joist Layout
Where wall partitions attach to the floor, additional framing may be required. The diagram below shows two common methods used when framing a floor to support partition walls. The design on the right, with the 2x spacers, works best if you plan to route plumbing through the floor into the wall.
Framing Floor Joists for Partition Walls
The diagram below shows typical floor joist layouts - one with 16" O.C. (on center) and the other with 24" O.C. spacing. Whatever the floor joist spacing, the process is the same - use a tape measure to layout the joist header (at the desired spacing) and mark an X on the side of the line where the joists will attach.
Laying Out Joist Headers for Floor Joists
Note: When additional framing is required for openings or partition walls, it should be added to the floor joist layout, while maintaining the layout of regular joist spacing.
When I design any structure, I like to use dimensions that make the most out of the materials that are being used. This helps to minimize cost and waste. The 16' x 24' floor plan described on this page, makes use of almost the entire length of the 16' joists, and every square inch of the 4x8 sheets of plywood decking.
First Floor Framing
The first floor level floor framing, in this example, ties to the sole plate that is bolted to a foundation wall.
For building code information regarding residential floor construction, go to the Wood Floor Framing section of the International Residential Code.
Second Floor Framing
The second floor level floor framing attaches to the double-top-plate of the first floor walls.
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