ROOF. 



Example 3. Fig. 6. 



Let one of the corners of a floping body, A B C D, reft 

 opon the top of a wall at D, which is quite level ; it is re- 

 quired to find the pofition of a notch, cut out of the upper 

 end C, fo that the body may reft upon a wall made to fit 

 the notch. 



Let the fmall part, F C H, be fo cut, that C H may- 

 be parallel to the horizon, then the body will be fup- 

 ported by the two walls at C and D. For draw D I, 

 G K, and C L, perpendicular to the horizon, then thefe 

 lines being produced, they may be fuppofed to meet at 

 an infinite diltance. To find the preffure on the walls : 

 join D C, and produce the vertical line K G to meet it 

 in E ; then if G be fuppofed to be the weight of the body, 



the preffure on D will be — J ^~ s — , and the preffure 



„ DE x G 

 °" C * -DC - " 



Example 4. Fig. 5. 



Let the body A B C D lie with its upper end againft 

 the vertical face of a wall at C ; it is required to find the 

 pofition of a plane fupporting the lower end D, fo that the 

 body may be at reft. 



Draw the vertical line G E, and C E, perpendicular to 

 the face of the wall C L ; join E D, and draw D F per- 

 pendicular to E D, then D F is the pofition of the plane 

 required. Complete the parallelogram E H I K ; then the 

 preffure on D, and on C, and the weight of the body, are 

 to one another as E H, E K, E I. 



Example 5. Fig. 4. 



To fupport a body A B C D by two props at two 

 given points, E and I, the direction of one of the props, 

 E F, being given. 



Draw the vertical lise G K, produce F E to K, and 

 draw KIH; and I H is the prop required. On the ver- 

 tical line G K, take K M to reprefent the weight of the 

 body ; and on K M, as a diagonal, defcribe the parallelo- 

 gram K L M N ; then K L is the compreffion of the prop 

 E F, and K N the compreffion of the prop I H. 



In order to be underftood by the reader, it will be necef- 

 fary to explain fuch terms as are ufed in the fubfequent part 

 of the article, by way of definitions. 



Wall-plates are pieces of timber laid on the wall, in order 

 to diftribute the preffure of the roof equally, and to bind 

 the walls together. Thefe are fometimes called railing- 

 plates. 



Trujfes are ftrong frames of carpentry, generally of a tri- 

 angular torm, tupporting the covering. Thev are difpofed 

 at equal diftances, and are ufed when the expanfion of the 

 walls is too great to admit of common rafters alone, which 

 would be in danger of being bent or broken by the weight 

 of the covering, for the want of fome intermediate lupport. 

 They are vari.jufly confiructed, according to the width ot 

 the building, the contour of the roof, and the circumftances 

 of walling below, &c. 



Tie is any piece of timber connected at its extremities to 

 two others, acted upon by oppofite preiiuies, which have 

 a tendency from each other ; or to extend the tie, as rope 

 or chain. 



Straining-piece is any piece of timber, connected at its ex- 

 tremities to two others, acted upon by oppofite prellures, 

 which have a tendency towards each other. 



Hence a tie acts contrary to a ftraining-piece. A chain, 



rope, or fmall bar of iron, may be ufed fur the former ; b-: 

 the latter mull always be inflexible, being in a ftate ef com- 

 preffion. 



Principal rafters are two pieces of timber in the fides of 

 a trufs, fupporting a grated frame of timber-work over 

 them, on which the flating or covering refts. 



Purlins are horizontal pieces of timber, fixed upon th** 

 principal rafters. 



Tie-beam is a horizontal piece of timber, connected to two 

 oppofite principal rafters ; it anfwers a twofold purpofe, 

 viz. that of preventing the walls from being puflied out- 

 wards by the weight of the covering, and of lupporting the 

 ceiling of the rooms below. — N.B. The tie-beam, when 

 placed above the bottom of the rafters, is called a collar- 

 beam. 



Common rafters are pieces of timber of a fmall feftion, 

 placed cquidiftantlv upon the purlins, and parallel to the 

 principal rafters : they fupport the boarding to which the 

 flating is fixed. 



Pole-plates are pieces of timber refting on the ends of the 

 tie-beams, and fupporting the lower ends of the common 

 rafters. 



King poft is an upright piece of timber in the middle of a 

 trufs, framed at the upper end into the principal rafters, 

 and at the lower end into the tie-beam : this prevents the 

 tie-beam from finking in the middle. 



Q;i?en-po/ls, two upright pieces of timber framed below 

 into the tie-beam, and above into the principal rafters, 

 placed equidiitantly from the middle of the trufs, or its 

 extremities. 



Strutts are oblique (training-pieces, framed below into the 

 king-polls or queen-poits, and above into the principal 

 rafters, which are fupported by them ; or fometimes they 

 have their upper ends framed into beams, which are too long 

 to fupport themfelves without bending. — N.B. Strutts are 

 often called braces. 



Puncheons are fhort tranfverfe pieces of timber, fixed be- 

 tween two others for fupporting them equally, fo that when 

 anv force is on the one, the other refills that force equally, 

 and if one break, the other will alfo break. Theie are fome- 

 times called finds. 



Straining-beam is a piece of timber placed between two 

 queen-polts at their upper ends, in order to withitand the 

 thrult ot the principal rafters. 



Straining-fill is a piece of timber placed at the bottom of 

 two queen-pofts, upon the tie-beam, in order to withilar.d 

 the force of the braces, which are acted upon by the weight 

 of the covering. 



Camber-beams are horizontal pieces of timber, made on 

 the upper edge floping from the middle toivards each end, 

 in an obtufe angle, for diicharging the water. They are 

 placed above the ftraining-beam in a truncated roof, for 

 fixing the boarding on which the load is laid ; their ends 

 run three or tour inches above the floping plane of the com- 

 mon rafters, in order to form a roll for -fixing the lead. 



Auxiliary rafters are pieces of timber framed in the fame 

 vertical plane with the principal rafters, under and parallel to 

 them, for giving additional fupport, when the extent of the 

 building requires their introduction. Thefe are femetimes 

 called principal braces, and fometimes cufliion rafters. 



Joggles are the joints at the meeting of itrutts with king- 

 polls, queen-pofts, or principal rafters ; or, at the meeting 

 of principal rafters with king and queen-pofts : the beil 

 form is that which is at right angles to the llrutts. 



Coding, or cogging, is the particular manner of fixing the 

 tie-beams to the wall-plates : one method is by dovetailing, 

 and the other is by notching the under fide of the tie-beam-. 



