Pitches and Roof Framing

Lengths, Cuts and Bevels of Common Rafters. —In Fig. 234 is shown a part of a very con venient table for the length, cuts and bevels for the common rafter up to an 18-foot run and with a rise from 1 to 24 inches to the foot. We have only given the figures for two of the pitches (1-3 and 3-8).

The figures on the blade represent the rise to the foot run. The fractional figures to the left of the blade represent the pitch of the common rafter. The figures on the tongue represent the run in either feet or inches. Therefore, if we wish the length of the common rafter, for a 16-foot span for the pitch, we take one-half of the 16 for the run on the tongue (8) and look in the square opposite and we find 6 feet which repre sents the rise and 10 feet represents the length of the rafter. If it be an 8-inch run, then the figures found there would represent as many inches.

This table is very convenient in finding the com mon difference in the length of jacks; as they are but a part of the common rafter, their lengths may be readily found as follows: If they are spaced one foot from centers, then the length of the first square will be the common difference. The length of the second jack will be that found in the second square, and that for the third will be found in the third square, etc. If the jacks are spaced on 24-inch centers then find the lengths in every other square. If they be placed on 16-inch centers, look under the 16th run and consider the length found there as inches. If they be placed on 18-inch centers, look under the 18th run, etc.

There are twelve scales in this table. Each figure in the run representing a scale and all giving the same cuts. Thus, the seat and plumb cuts for the common rafter are as follows: Take the number of any run on the tongue and its rise found in the square above and oppo site the desired pitch, on the blade will give the seat and plumb cuts. Take the number of any run on the tongue and the corresponding length of the rafter on the blade will give the side cut of the jack and also the face cut across the roof boards to fit in the valley or over the hip. The blade giving the cut on the former and the tongue in the latter.

Take the length of the rafter for any run on the blade and its rise on the tongue and the latter will give the miter cut across the edge of the roof boards, which is the same as the miter for the square hopper. The figures for the full scale are found in the twelfth run for the above cuts as before illustrated.

To Find the Side Cuts for the Polygonal Jacks. —Take the tangent on the tongue (see Fig. 230) and the length of the common rafter found in the twelfth run on the blade and the latter will give the cut.

Lineal Board Measure.

This table also con tains a complete lineal board measure for any width of board up to 18 inches, as follows: Let the figures on the tongue represent the width of the board and those on the blade the length. The top figures in the intersecting squares opposite these numbers will be the con tents of the board in feet and inches, as follows : A board 6 inches wide and 9 feet long con tains 4 feet 6 inches. A board 17 inches wide and 9 feet long contains 12 feet 9 inches. A board 15 inches wide and 8 feet long contains 10 feet, etc. A board 14 inches wide and 20 feet long, the answer would be found in the intersect ing square opposite the starting points, as at X.

Thus it will be seen that a completed table of this kind would come very handy as a ready reckoner. The reader will notice in the pitch that the lengths are without fractions. This oc curs at three places on the blade-5, 9 and 16, respectively. Thus in the 1-3 pitch, the lengths end in fractions and are expressed in twelfths. For an 8-foot run the length would be 8 feet 7 5-12 inches. For an 8-inch run the 5-12 may be dis carded as it represents less than half of a twelfth of an inch. The answer would then be 9 7-12 inches.

Length of Rafters for Odd Runs. We

wish to call special attention to the table at Fig. 234, in the convenience of same for finding the lengths of rafters for odd runs, such as feet, inches and fractions of an inch in the run, as the figures stand for either feet, inches or fractions of an inch. The fractions being expressed in the same denomi nations (twelfths), permits of a sliding scale, as follows: for an example, suppose the run is six feet and one-half inches with a one-third pitch. In the intersecting square opposite the rise and run, we find seven feet two and six twelfths which answers for the six feet. For the six inches in the run, read the above figures as so many inches and twelfths of an inch, and for the half inch, read the above figures again as so many twelfths and fractions of a twelfth of an inch. The whole may be expressed thus : For the six feet . . 7 feet 2 and 6-12 inches For the six inches 7 and 2-12 inches For the six-twelfths 7-12 inches Answer 7 feet 10 and 3-12 inches Then 7 feet 10 3-12 inches is the correct length. The last two figures (8 and 6) are dropped because they represent too small a denomination to be retained. Remember these figures repre sent twelfths (not tenths) and we only carry to the next column when the sum exceeds twelve, otherwise the operation is just the same as in simple addition. If the run was, say 5 feet 7 and 9-12 of an inch, the figures would be expressed thus : For the five feet 6 feet and 1-12th inches For the seven inches 8 and 4-12 inches For the nine-twelfths inch 10-12 inches Answer . . . . 6 feet 9 and 4-12th inches Then 6 feet 9 and 4-12 inches would be the correct length for the common rafter. This may seem like getting the lengths and cuts down to a small point. So it is. To many it may seem useless. In this, we have been accused of split ting hairs, but we would rather see split hairs than see rafters wedged up with a "dutchman" and with gaping joints at the bearings, for what is the use of using good material and leaving yawn ing joints with the bearings oftentimes at the tip ends of the rafters where the wood is thin and this cut to pieces with nails in the vain effort to make it "good enough?" If we make poor joints why not use poor lumber? Sorry to say we are forced sometimes to use poor lumber, but there is no occasion for poor joints. Make the cuts to get the full bearings and thus save all the strength there is in the material in bracing power. This table refers only to the rise and length of the common rafter. It could be so enlarged as to in clude the corresponding octagon hip and common hip or valley, thus making a very handy table for ready reckoning purposes.