Floor Joists and Subflooring

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.

Floor Joists

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.

Tables of permissible spans for floor joists are published by the American Forest & Paper Association's (AF&PA) American Wood Council (AWC). Click here to access the tables online.

Joist Installation

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.

floor joists and subflooring

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