228 Rules for the scantling of Roofing timbers. [April, 



As the weight on each of the beams is proportioned to the distance 

 between them ; and as the strength of the beam is proportioned to its 

 breadth : the breadth in inches, as marked in the first column of the 

 table, must be multiplied by the distance in feet between each beam, 

 measured from centre to centre for the breadth of the beam ; or, if the 

 breadth of the beams are given, the distance in feet between them isfound 

 by dividing their breadth by the breadth in the fkst column of the table. 



Examples. 



A room, 22 feet by 33 feet, has to be roofed in, the timbers provided, 

 for which are round, 18 inches diameter in the middle, and 25 feet 

 long. It is required to know the most economical manner of cutting 

 them up, the scantlings of the beams, and their distance apart. 



The stiffest beam that can be cut out of an 18 inch tree is 9 X 15, 

 or if cut into two timbers, 4^ x 15, to ascertain if this timber will 

 be so thin as be liable to break side- ways, the rule for this purpose 



.6 X 22 13.2 



will be applied as follows: — — = -■ - = 3. 4 in. the least 



1 v/i5 3.87 



breadth ; the beams A\ x 15, are therefore not too thin. By refer- 

 ring to the table, under 22 feet length of bearing, a depth of 15 inches 

 requires a breadth of \\ inches. The breadth of the timber, 4^, being 

 divided by \\, gives3 ft, the distance from centre to centre of the beams ; 

 this distance gives 11 spaces, or 10 beams, or 5 timbers in the 33 ft. 



The timbers of the dimensions above stated could be cut into two 

 beams 12.7 X 6.4, having a greater section than that given above, 

 15 X 4|; but on a reference to the table in the column of 22 feet 

 length, and 12 - 9 in depth, the breadth is 1\ inches, and 6*4, divided 

 by 2'5, gives 2 feet 8 in distance from centre to centre, if beams re- 

 quiring 12| spaces, or 12 beams, or 6 timbers. 



2nd Example. 

 Beams 8| X 12, having been provided for a roof of 22 feet span — 

 required to know the distance they are to be placed apart. In co- 

 lumn of 22 feet span, opposite a depth of 12 inches, is a breadth of 3 

 inches, and 8^ divided by 3, gives 2 feet 10 inches as the distance 

 from centre to centre, at which the beams ought to be placed. 



3rd Example. 

 Proposed to roof a room 18 feet wide, with timber placed 1 foot 3 

 inches from centre to centre, so as to be covered with tiles instead of 

 burgahs, the deepest timber procurable being 9 inches, required the 

 breadth of the beams. 



In the column of the span of 18 feet, and a depth of 9 inches, the 

 breadth is 4 inches, which multiplied by l£, gives 5 inches for the 

 breadth of the beam. J. T. 



