How Thick Is the Floor Between Two Levels?

Floor Joists

• The floor joists comprise the bulk of the floor thickness. Standard floor joists are usually 2-by-10 or 2-by-12 dimensional boards, installed on edge. The actual width of those boards, however, isn’t identical to what their name implies. A 2-by-10 joist is really only 9 1/4 inches wide and a 2-by-12 is only 11 1/4 inches wide, so depending upon which size joist the blueprints call for, they will add the respective measurement to the thickness of the floor.

Subfloor

• Subflooring only adds 3/4 inch to the floor thickness. There are two main types of subflooring, plywood and oriented strand board, or OSB. Both come in 4-by-8 sheets and feature tongue-and-groove installation. Construction adhesive is used between the joists and the subfloor but it does not measurably add to the thickness of the floor.

Floor and Ceiling Finish

• The type of flooring you choose will add some height to the floor. Hardwood floors and standard carpet with pad add about 3/4 inch to the floor height. Vinyl flooring is usually the thinnest choice for floors, and 1/8-inch vinyl installed over 1/4-inch underlayment will add only 3/8 inch to the floor height.

  Standard drywall, installed on the ceiling below the floor, adds a minimum of 1/2 inch to the floor thickness. There are thinner types of drywall, but they are not adequate for ceiling installation. In some cases, such as a “party” ceiling, which separates individual apartment units, building code may require double drywall layers to reduce fire spread and to damper noise between floors.

Total Thickness

• When accounting for joist width, subflooring, flooring and standard ceiling drywall, a floor framed with 2-by-10s will be very close to 11 1/4 inches thick. The same floor, if framed with 2-by-12s, will be 13 1/4 inches thick.