known. Suppose the building is 20 feet, 8 inches

(fig. 2-17). This shows the run, rise, and pitch of

wide and the rise of the rafters is to be 13 inches per

rafters of the seven most common pitches of roof

foot of run. The total run of the rafters will be 10

(fig. 2-17). The figures are based on the length of the

feet, 4 inches. Look in the fist line of figures; under

horizontal measurement of the building from the

the 13-inch mark appears the number 17.69, which is

center to the outside. The rafter table and the outside

the length in inches of a rafter with a run of 1 foot

edge of the back of the square, both the body and

and a rise of 13 inches. To find the line length of a

tongue, are in twelfths. The inch marks may

rafter with a total run of 10 feet, 4 inches, multiply

represent inches or feet, and the twelfth marks may

17.69 inches by 10% and divide by 12 to get the

represent twelfths of an inch or twelfths of a foot.

answer in feet. The 17.69 inches times 10% equals

The rafter table is used in connection with the marks

182.79 inches, which, when divided by 12 equals 15

and figures on the outside edge of the square. At the

3/12 feet. Therefore 15 feet 3 inches is the line

left end of the table are figures representing the **run**,

length of the rafter. The remaining procedure for

the **rise**, and the **pitch**. In the fist column, the figures

laying out the rafters after the length has been

are all 12. These may be used as 12 inches or 12 feet

determined is described in **a **above.

as they represent the **run **of 12. The second column

of figures represent various rises. The third column

of figures, in fractions, represents the various pitches.

appears on the back of the blades of some squares