Roof Framing Simplified

ROOF FRAMING SIMPLIFIED The details of roof construction are com paratively simple, and are pretty generally understood. It is in the laying out of the dif ferent members, finding their proper lengths and cuts, that the difficulties of roof framing arise. Owing to the many different styles and pitches of roofs, this is considered a very com plicated matter by a great many otherwise good mechanics, who accordingly resort to certain unsatisfactory "cut and try" methods and "rules-o'-thumb" to lay out the work.

The steel square is the 'carpenter's best assistant for laying out all framing; but it is for roof work that its use is most essential. By the use of the steel square—after a very few of the fundamental principles of roof framing are well understood—the whole subject becomes clear to an astonishing degree.

Roof Pitches and Degrees. Fig. 51 contains a whole volume on roof framing. The fractional pitch lines for the common rafter are shown for each inch in rise up to the full pitch; and their lengths are expressed in decimal figures to the one-hundredth part of an inch; while to the right of the blade, the same are expressed for the corresponding octagon and for the common hip or valley for a square-cornered building, 91 which are reckoned from 13 and 17 on the tongue respectively. However, neither is abso lutely correct, though near enough so far as the Fig. 51. Roof Pitches and Degrees on the Steel Square.

cuts are concerned, the greater deviation being in the hip for the square-cornered building. It lacks .0295 of being 17 inches, and represents the rim of the hip to a 12-inch run of the com mon rafter. Its true length being 16.9705 inches, this is the length from which we have reckoned for the lengths of the hips, instead of 17, as is the usual custom. This may seem a trifling difference; and so it is in a short run and low pitches; but suppose it is for iron con struction. To begin with, the shortage of each foot in run with the common rafter is .0295 inch; added to this is the gain it would have in the pitch, which would be .015 of an inch by the time

it got up to the full pitch for the common rafter. This, added to the .0295 to start with, would be a difference of .0445 inch to the foot in run with the common rafter. Now, suppose the run to be 18 feet; 18 times .0415 equals .8 plus, or of an inch difference; or, if no account were made of the gain in pitch, the .0295 inch in the run would amount to over half an inch in the length of the hip alone.

This is a common error; and while it is not much, and probably would never be noticed in wood construction, it is well to know this dis crepancy and guard against it when the occasion demands, and for that reason we give the cor rect amounts. The shortage in the octagon is not so pronounced. Instead of it being in the run, it is the tangent that is lacking the same amount, it being 4.9705 instead of 5 inches. This, coming as it does, cannot affect the length of the rafter nearly so much as in the above.

We explain this shortage better by referring to that part of the illustration showing the plan of a combination square and octagon frame with the heel of the steel square resting at the center. From this it will be seen that the two outer circles catch the corners of the frame and seem Fig. 52. Roof Pitches.

ingly intersect the tongue at 13 and 17, the figures used on that member for the seat cuts; but the true length of the run of the hip is 16.9705, and that for the tangent of the octagon is 4.9705.

In connection with this illustration we also give a table of decimal equivalents to the one twenty-fourth part of an inch, for convenience in finding their values in common fractions.

What Determines the Pitch? This is a sim ple question; yet there seems to be a wide difference of opinion as to what determines the incline given the roof. Custom has long since settled upon the rise given the roof in proportion to the span; thus, a one-fourth, one third, one-half, etc., pitch, must have a rise in that proportion to the span. Reckoned on this basis, a full pitch has a rise equal to its span. See Fig. 52.