



ROOF 



ttootf. 



1U 



re inserted at Ue anriss formed by | 'Carpenter Mid Joiner's Companion,' which illustrates the'.use of slender 

 th. borfcooUl b.. that support the < 



JW. 1 J, i. representation of Yy mmple trow, from Nicholson's ' lower end* of the struts rest in stirrups attached to the vertical rods 



tt Wall-plain; *, Tie-beam ; t r. Principal rafters ; rf rf ,1,1, Purlins ; < r, Pole-platos ; //, Common rafter* ; ff, Km 

 , ColUr.bram ; i i, Qucn-p<wU; *, Straining-sill; / , Struts or braces; m m. Auxiliary rafters ; n, Kidgo-pircr. 



and the might bearing on the strut a is imparted, through 4 and r, to 

 the king-post. The tie-beam is suspended by bolts from each of the 



. i:. i: 



vertical nU, and the ends of the rafter- 1 to the tie-beam 



by iron straps panning round them, and bolted to the beam at rf, rf. 

 Trusses on the same principle may be mode of timber only. 



In curb roofs the upper rows of rafters are called rm-ti-raflen, and 

 the horizontal bars that receive the upper ends of the lower rafter*, 

 and the feet of the curb-rafters, are known as euili-plalei. The proper 

 position of equilibrium fur the rafters of n curb-roof may be ascertained 

 . simple means, within the reach of persons not possessed of 

 MiHiuirnt mathematical knowledge for determining it by calculation. 

 If the rafters are to be equally loaded, as in a roof entirely covered 

 with imc material, this position Will lx> exactly the reverse of that 

 which they would take by gravity, were they suspended in a chain or 

 festoon, the joints being flexible. If they are framed together in this 

 position of equilibrium, they will balance each other like the stones of 

 an arch ; and the tie-beams, posts, and braces will have no other office 

 to perform than that of resisting such irregular strains as might tend 

 to alter their arrangement. The rafters thus suspended would l';ill 

 into the position a licdt,fy. 13, a line drawn through the angles being 

 a catenarian curve ; and a b' r'rf' S, in the same figure, represents the 

 corresponding position in which they should be placed in an equally 

 loaded roof. If the rafters 6' c and r' <!' are to bear a greater weight 



than o' 4' and tl'r', they will, ii ]>rpit innately loaded when suspended 

 .11 a curve, fall in such a way as to increase the angles abc and c d c, 

 and diminish tied, thereby indicating their proper position in the roof. 

 When the roof is to be loaded unequally, and more on one side of the 

 lido* than the other, at it would In- if // c were to be covered with 

 lead, and the other planes with Mates, a corresponding weight added 

 to the centre of gravity of I, c will cause the bars to arrange themselves 

 u a l> e d t , fig. \ 4, the angles of which, being transferred to the roof, 

 give the position of equilibrium a' *Y rfY. This practical mctli' .1 of 

 finding the proper angles of a curb-roof may be applied under nil cir- 

 cumstances, the dimensions of the experimental bars being proper 

 tionaU to thoee of the rafters, and their centres of gravity being 

 loaded according to the pressure to be sustained by each plane of the 

 roof. The great advantage of curb-roofs consists in the space they 

 afford for chambers in the roof, such chambers being lighted by dormer 

 windows in the lower inclined face-. When the trusses of the roof 



form partitions between the bed-rooms, their posts and braces r. 

 arranged as to leave one or more doorways for communication Ix'tweeii 

 them. 



In roofs of very large span it is often desirable, in order t" avoid 

 running up to a great height, to form two or more ridges. "Wh. n 

 intermediate support can be obtained from partition walls, such con- 

 structions may be regarded as combinations of two or more distinct 

 roofs placed side by side. Fig. 15 is an example of a roof of large 



span without any intermediate support, ami having a large available 

 space between the tie ami collar beams. It represents the form of tin- 

 trusses, which were placed fifteen feet apart, of a roof of eighty feet 

 span, erected over Drury-Lane Theatre in 17'."!. 



It is sometimes necessary, in order to obtain additional height inside 

 a building, to raise the tie-beam above the level of the top of the walls. 

 In ;-inall spans this may be done by the simple arrangement called the 

 fr/" "'"''*'""<-' ,\..".'/- Pi), in which a firm union is effected between 



the beam and the rafters without the use of nails or pins. Such a roof 

 can only press injuriously on the walls by the rafters sinking into a 

 concave form, which however their lower ends are very liable to do. 

 In such a case additional strength may be obtained by inserting a 

 longitudinal truss, as in 11, ,/!;/. Hi, where r represents the end of the 

 truss, which should be (irmly built into the gables : (/ and e are side 

 views of two longitudinal trusses suitable for such a situation, the first 

 being stiffened by an arch of iron notched into the short vertical 

 and the second formed of timber only. Similar trusses are 

 occasionally introduced under the purlins. Hoofs without ties may be 

 greatly strengthened by the use of parabolic curves of iron, notched 

 into the rafters of each inclined face, and abutting on the wall-plate.--, 

 which in such a case arc firmly bolted together. The timbers of such 

 a roof may be framed together in planes, each having a distinct ridge- 

 piece, and the ridges being screwed or otherwise firmly connected 

 together. The curves may be cast in short segments, a.s they arc com- 

 pressed when in use.it being merely necessary to provide that the 

 joints should always abut on a rafter. Trcdgold, in his ' Elementary 

 I'rineiples of Carpentry,' recommends the use of similar curves, of 

 either wood or iron, in the trusses of an ordinary roof, by which the 

 derangement often arising from the shrinking of the king-posts and 

 i|ueen ]xts may be avoided. In this case the curves take the place 

 of the principal rafters, and, if made of wood, may be constructed of 



