GENERAL CONSIDERATIONS 47 



Fig. 24 is 20 feet wide and the ridge is 7 feet 

 above the eaves, the value of the angle, 

 known as A, may be found by the following 

 formula: Tang. A equals Tang. A- 

 equals Tang. A=-7oo or A=35 degrees. 



Should the house be of uneven span it is 

 only necessary to measure the distance 

 corresponding to a (Fig. 24) and apply 

 the same formula. When this is not con- 

 venient, a plumb bob may be dropped from 

 any part of the roof, as at c, and the distance 

 measured from the roof to the point c 1 , where 

 it cuts a horizontal line or straight edge from 

 the point where the roof joins the wall. This 

 distance may be substituted for b in the 

 formula, and the distance from c 1 to the in- 

 tersection of the roof and wall may be sub- 

 stituted for a. To avoid error the triangle 

 thus formed should be as large as possible 

 and care taken to see that the lines are per- 

 fectly vertical or horizontal, as the case may 

 be. By referring to the following table the 

 angles in degrees and minutes formed by 

 roofs on houses of various widths and heights 

 of ridge may be quickly found. The figures 

 in the left-hand column correspond to half 

 the width of even-span houses or to the dis- 

 tance represented by a in the above formula. 



