Lumber Species and Grade

• The four most common species of wood that are used as structural lumber are southern pine (SP), spruce pine fir (SPF), hemlock fir (Hem-Fir) and Douglas fir -- larch (D. Fir-L). The abbreviations are standard and are stamped on the lumber. When consulting span tables for proper header length or spacing between trusses, joists and deck beams, these abbreviations will be used. You will also find grade stamps that say SS, #1, #2 or #3. "SS" stands for select structural and is the highest grade, with a tight grain and very few, very small knots. Number #1 may have knots up to 1 1/2 inches across and no more than one knot every 3 linear inches. Number #2 can have 2-inch knots within 2 linear inches. Number #3 lumber can have knots as wide as 2 1/2 inches every 1 3/4 linear inch. Select structural is the strongest, and the other grades progress downward to #3, the weakest.
  
  

Space and Span

• Two spacing considerations must be taken into account when using 2-by-10 lumber structurally: spacing and span. Span refers to the space along a single board's length between supporting structures. Spacing refers to the distance between parallel boards. For example, in a floor, the 2-by-10 joists might be 16 inches apart (spacing) and 10 feet long (span).
  
  

Load Bearing Variables

• Trusses bear the load of a roof, both the roof surface and shingles as well as, in some cases, snow and ice. Ceilings joists bear the weight of the ceiling plaster or drywall, as well as suspended lighting and insulation. Floor joists and deck beams bear the weight of the walking surface, as well as everyone and everything that is on the floor. The relation between lumber species, lumber grade, lumber dimension, spacing and span is additive; that is, stronger wood or a larger dimension will lower the maximum necessary spacing and increase the maximum span; or smaller spacing and shorter spans means you can get away with weaker lumber. Because the overall relation between these variables changes with a change in any one variable, builders use calculation tables.
  
  

Calculations

• Joists and rafters made with 2-by-10 lumber are equally spaced boards, facing-edge up, that support the weight of a floor (joist), a ceiling (joist) or a roof (rafter). It is always wise to consult one of the readily available online span calculators before planning your structural spacing. In the case of a joist or rafter, for example, if you are using #3 SP 2-by-10, and local code says you can space floor joists no more than 16 inches apart, then the maximum recommended span is 12 feet 2 inches. If all the calculations are the same, except you are using SS SP 2-by-10, then your span can be as high as 17 feet. If you use the same SS SP 2-by-10, and you elect to space them 12 inches apart, then the span can be extended to 18 feet 5 inches.