What Size of Wood for My Second Floor Ceilings?

Structural Framing

• Behind the exterior portion of a home lies its structural framework. A home’s structural framework consists of walls, beams and floors. These areas transfer the weight of a house down to its foundation structure. Floor levels, in particular, sit on top of underlying walls and columns. Flooring systems transfer the weight of furniture, people and any overhead structures to underlying columns and walls. In effect, a home’s structural framing consists of vertical and horizontal weight-transfer systems. So the design of a second floor ceiling depends on the amount of weight involved and the overall layout of a home’s structural framework.

Wood Types

• Different types of wood have different weight-loading capacities. Wood types consist of different species -- such as Douglas Fir and pine -- and different grades, including Select Structural, and No. 1 and No. 2. Grade designations indicate the quality of wood. Ultimately, different types of wood have different strengths, which may make one type more suitable for a particular project than another.

  Builders and carpenters use floor joist span tables to determine which designations will work best for a particular project. These tables assign specific wood types according to the dimensions needed and the amount of weight load involved. So the size of wood needed for a second floor ceiling will depend on weight loads and the length and width of the floor being built.

Wood Spacing

• The dimensions for a second floor ceiling in terms of length and width will call for a certain size of wood as well as for a certain amount of spacing between wood pieces. Wood spacing formulas use the center portion of a piece wood — also known as a floor joist -- when calculating the number of pieces needed to cover a floor level. Since larger pieces of wood can hold more weight, the bigger the floor joist the fewer pieces needed to cover an area. Using a floor joist span table, a builder can determine the number of pieces needed to cover an area based on the size of the floor joist. For example, on a floor joist span table for Douglas Fir wood, a 14-foot-long second floor area would require 2-by-10 wood pieces with 16 inches of spacing between each piece.

Support Structures

• In cases where a floor plan includes larger dimensions than those found on a floor joist span table, additional support structures are needed to meet the necessary weight-carrying requirements for the floor. Support structures commonly used include wood beams, load-bearing walls and column supports. Load-bearing walls and column supports sit underneath the actual second floor ceiling and must be constructed to meet the ceiling’s weight-carrying requirements.

  Wood beams typically run down the center of a floor plan to connect and support two adjoining rows of floor joists. Much like floor joist pieces, wood beams also come in a range of different sizes and grades. Fortunately, beam span tables provide the needed information for determining the type and size of beam to use. A second floor area spanning 28 by 10 feet, for example, would require a 4-ply, 2-by-10 beam to support two adjoining rows of joists based on a standard Douglas Fir beam span table.