536 



CARPENTRY. 



Construe- the lowest timber connecting the two sides for pre- 



tive venting the lateral pressure of the roof from pushing 

 Carpentry. 



caus j ng i t to p ress vertically. 

 When the tie beam is placed at the bottom, it is cal- 

 led a footing beam. The tie beam, besides its use in 

 the truss, is necessary in supporting the ceiling floor. 

 The tie beam is generally placed at the bottom of 

 the truss ; but in some buildings, in order to get a 

 more lofty story, it is placed in a higher situation ; 

 but this disposition of the tie beam is dangerous, if 

 very remote from the wall heads, as the weight of the 

 roof will bend the inclined upper sides of the truss so 

 as to make them concave outwardly. The want of 

 the tie beam, or its not being placed at the bottom, 

 is a great fault in the construction of Gothic edifices, 

 as the roofs in most of these edifices have almost 

 ruined the walls. 



Principal rafters, are the two inclined pieces of 

 timber in opposite sides of the roof fastened each to 

 each end of the tie beam, which prevents their thrust- 

 ing out the walls by the weight of the roof. The 

 principal rafters are generally of equal lengths, and 

 form a triangle with the tie beam when the roof con- 

 sists only of two sides without a flat, and when the 

 tie beam is laid on the wall plate. 



King post, is a piece of timber in the middle of a 

 truss, suspended at the upper end by the principal 

 rafters from the apex of the roof, so that the king 

 post divides the internal space into two equal com- 

 partments, each a right angled triangle. The use of 

 the king post is to furnish a general support for the 

 principals, and suspend the tie beam between the ends, 

 so as to keep it from sagging. 



Queen posts, are two truss posts, equi- distant from 

 the middle of the truss, the one suspended from the 

 head of the one principal rafter and the end of a beam 

 on one side of the middle, and the other qu*n post 

 from the head of the other principal and the other 

 end of the beam on the other side of the middle ; so 

 that the queen posts divide the internal space of the 

 frame into three compartments, of which the two ex- 

 treme ones are right angled triangles, and the middle 

 one a rectangle. The use of queen posts is the same 

 as king posts, viz. for furnishing a general support 

 for the principals at different points between the ends, 

 by connecting timbers, and supporting the tie beam 

 between its extremities. 



Struts, are oblique pieces of timber, or braces, branch- 

 ing upwards from one, two, or several points in king 

 posts and queen posts, to as many points in the principal 

 rafters, or to as many points in the external frame. 

 Struts are generally disposed in pairs, equally inclin- 

 ed to a vertical line passing through the middle of 

 the plane of the truss, and the directions of each pair 

 tend to the same point in this vertical line. Struts 

 are necessary in large trusses consisting of long tim- 

 bers, which have to support the covering at various 

 points in the external frame. 



Collar beam, is the piece of timber framed between 

 the heads of two qneen posts. A collar beam roof 

 is necessary, where there is to be a platform, or 

 wherever rooms are required to be formed in the roof, 

 and to be of greater extension than the distance be- 

 tween the trusses. 



Straining silly is a horizontal piece of timber, diapo- 

 3 



sed between the feet of queen posts, to counteract the Construe- 

 efforts of the struts in pushing the queen posts nearer l ' ve 

 to each other. 



Having thus mentioned the several parts of a truss, ^ 

 it may be proper to observe, that all king posts, 

 queen posts, and tie beams, are ties, and therefore 

 a string incapable of farther extension than sufficient 

 to bring it to a straight line, Or a chain, or a slender 

 bar of iron, will answer the same purpose, as well as 

 a piece of timber or other such inflexible material. 

 Aloo, that all collar beams, principal rafters, and 

 struts, are straining pieces, which are therefore neces- 

 sarilv constructed of an inflexible material, such as 

 wood, or a stiff piece of iron. It may be farther ob- 

 served, that in complex frames, such as centerings to 

 large arches, os bridges, in the act of building, the 

 same timbers, in different stages of the work, some- 

 times perform the office of ties, and sometimes that 

 of straining pieces, and in the transition of office must 

 be sometimes in a neutral state. The material em- 

 ployed in such situation, must necessarily be inflex- 

 ible : this is not only to be recommended here, but 

 in every doubtful case, or where it is uncertain whe- 

 ther the part of the truss requires to be a tie or a 

 straining piece ; since all, or most inflexible materials, 

 may not only be employed as ties, but as a straining 

 piece also. 



Pole plate, is a beam over each opposite wall, sup- 

 ported upon the ends of the tie beams, or upon the 

 feet of the principal rafters. 



Purlins, are horizontal pieces of timber, supported 

 by the principal rafters. 



Ridge piece, is a beam at the apex of a roof, sup- 

 ported by the king post, or by the heads of the prin- 

 cipals. 



Common rafters, are inclined pieces of timber parallel 

 to the principal rafters, supported by the pole plates, 

 the purlins, and the ridge piece for supporting the 

 covering, the material of which is sometimes large 

 slates extended from rafter to rafter, and sometimes 

 smaller slates nailed upon boarding, battening, or 

 hung to lath, and these nailed upon the common raf- 

 ters. 



Joggles, are the joints at the meeting of struts, king 

 posts, queen posts, and principal rafters, and indeed 

 all the joints of a roof may be termed joggles. The 

 best form of the joggles is, that which is at right 

 angles to the lengths of the struts or rafters, or at 

 right angles to the tenoned piece ; but this position 

 cannot at all times be obtained, for the want of suf- 

 ficient substance of timber ; in this case, the joint is 

 either made oblique, or the upper part in a line with 

 the side of the piece which has the mortice, and the 

 lower part perpendicular to the sides of the tenoned 

 piece ; or the joint is sometimes made partly parallel, 

 and partly perpendicular to the morticed piece. 

 When the joint is oblique, the force of the tenoned 

 piece in a direction of its length, causes the end to 

 slide upon the abutment towards the side which con- 

 tains the obtuse angle, but this is in some degree 

 counteracted by the resistance of the tenon on the 

 lower end of the mortice. However, with regard to 

 the stress of the timbers in a frame, the abutting joint 

 is of little importance. M. Perronet, the celebrated 

 French engineer, formed the abutments, and conse- 



