Example:
Determine the B. M. in a girder 6" by 8" by 16'.
Solution:
Rule—Thickness in inches times width in inches, divided by 12" times
the length in feet gives a number equal to the number of board feet.
(6" × 8") / 12 × 16' = 64' B.M.
Stock less than 1" thick is figured as 1" thick. In commercial practice lumbermen make use of tables in determining quantities. Appendix III.
In estimating quantities, suitable allowance must be made for waste. This waste is incurred (1) thru loss when boards or planks are cut to required lengths. Standard lengths are 10', 12', 14', etc., and these will not always cut to advantage. (2) Waste is incurred in machining the stock, as dressing, edging, tonguing and grooving. Necessary allowances will be indicated herein.
Heavy Timbers. Girders, posts, etc., are determined by count.
Joists. To determine the number of joists required for a room or a building, count the actual number required beginning at a wall (¾ times number of feet in length or width of room when set 16" on centers), and to this add one joist to be placed against the second wall.
Cataloged or listed sizes are for lumber fresh from the saw. Shrinkage due to seasoning and surfacing one side and one edge so that the stock may have uniform thickness and width will give actual sizes as follows: 2" × 4" when sized on one side and one edge, will give 1⅝" x 3⅝"; 2" × 6", S-1-S and 1E, 1⅝" × 5⅝"; 2" × 8", S-1-S and 1E gives 1⅝" × 7½"; 2" × 10", S-1-S and 1E gives 1⅝" x 9½"; 2" x 12", S-1-S and 1E gives 1⅝ x 11½. When S-2-S and 2E or sized on 4 sides, stock will measure ⅛" less than indicated above.
Studs for Walls and Partitions.—Count one for each lineal foot of wall or partition, where specified 16" on centers. The extra studs are to be used in doubling corners, at doors and windows, and in gables. Barns and sheds will not require these extra studs.
Bridging.—Allow 25 lineal feet of 2" by 4" for each square of flooring.
Rafters.—On a plain roof, count actual number and add one, as in counting for joists.
Sheathing.—Calculate the exact surface to be covered, deduct openings; then, for unmatched sheathing or barn boards or fencing, as it is also called, add 1/12 for 12" boards, 1/16 for 10" boards and ⅛ where 8" boards are used. Four inch and 6" are seldom laid solid, being used mainly for roof sheathing for shingle roofs, and laid 2" apart. These additions are due to the fact that thru seasoning and dressing, a 12" board becomes 11½", a 10" becomes 9⅝", an 8" becomes 7¾", a 6" becomes 5¾" and a 4" becomes 3¾". The additions specified allow for waste in cutting.