Common quality requirements for floor joist material are good bending strength, stiffness, freedom from warp, and good nail-withdrawl resistance. Joists are generally 2 inch thick lumber. All species of softwood framing lumber are acceptable provided they comply with size and grade requirements for the span. Because all species are not equal in properties of strength, weaker species are to be used in larger sizes in order to provide the adequate stiffness for a given span.
The requirements for subflooring boards are not as exacting; however, godd stiffness, nail-holding power, and ease of workability are desirable. Generally, 1 inch boards 4, 6 or 8 inches in width are used for subfloors.
Number 3 Common is the minimum grade you should use in species like Douglas Fir, Southern Cypress, Western Hemlock, and Southern Yellow Pine.
Number 4 Common is the minimum grade for subflooring of White Fir, Western Larch, Spruce (except Sitka), Ponderosa Pine, Northern White Pine, and Red Pine. All subflooring should be of uniform thickness.
All framing lumber and subflooring should be well seasoned. Lumber 2 inches thick or less should have a moisture content not to excess 19 percent for conventional construction and 12 percent for shop fabricated construction.
As noted above, joists are selected to meet strength and stiffness requirements. Strength requirements are dependent upon the loads to be carried. Stiffness requirements are intended to limit plaster cracking under live loads. Stiffness is also of importance in limiting vibrations from moving loads- often a cause of annoyance for occupants.
After the sills have been leveled on the mortar beds and anchored to the foundation walls and piers, the joists are located and spaced in accordance with the design. (Sixteen inch center to center spacing is the most commonly used.)
Any joists having a slight bow edgewise should be so placed that the crown is on top. A crowned joist will tend to straighten out when subfloor and normal floor loads are applied. The largest edge knots should be placed on top, since knots on the upper side of a joist are on the compression side of a member.
A header joist is end-nailed to each joist with twenty fourpenny nails at the corner and two at intermediate joists. In addition, the header joist and the end joist parallel to the exterior walls in platform construction are toenailed to the sill with sixteenpenny nails spaced 16 inches on center. Joists are lapped a minimum of 4 inches at girders and nailed together with one sixteenpenny nail on each side.
Floor joists should be doubled under all bearing partition walls; and if spacing is required for heat ducts, solid blocking should be used used between the joists.
Subfloor should consist of either
- square-edge or tongue and groove bards no wider than 8 inches and not less than 25/32 inch thick
- Or Plywood
Tongue and groove end-matched boards may be used, but they should be applied so that each board will bear on at least two joists and so that there will be no two adjoining boards with end joints occurring between the same pair of joists. Subflooring is nailed to each joist with two eightpenny nails for widths under 8 inches and three nails for 8 inch widths. In square-edged boards, joints should always be made over the joists.
Subflooring may be applied either diagonally or at right angles to the joists. When subflooring is placed at right angles to the joists, the finish floor should be laid at right angles to the subflooring. Diagonal subflooring permits finish flooring to be laid either parallel or perpendicular to the joists.
The joist spacing should not exceed 16 inches on center when finish flooring is laid parallel to the joists or where parquet finish flooring is to be used; nor should the spacing exceed 24 inches on center when finish flooring at least 25/32 inch thick is at right angles to the joists.
Where balloon framing is used, blocking should be installed between ends of joists at the wall for nailing the ends of diagonal subfloor boards.
In areas where rain may occur during construction, square edge boards should be laid with open joints for drainage. Tongue and groove boards should have holes at suitable intervals to allow runoff of rain water.
The following thickness and joist spacings are suggested by the Federal Housing Administration for plywood subfloor when used as a base for wood finish floors, linoleum, rubber, or ceramic tile: