ROOF. 



is entirely occafioned by the braces G H and NO: the 

 camber-beam C D is no otherwife affected than by its own 

 weight. Let it now be fuppofed, that the angles C and D, 

 N° 4, are braced at I K, L M. In this difpofition, the 

 puncheons B A and E F are not affeaed in refpeft of 

 tranfverfe ftrains ; the rafters CB, D E, and beam CD, 

 would all become concave on the outfide ; and the points 

 B and E, at the bottom of the rafters and top of the pun- 

 cheons, would be puftied out beyond the perpendicular of 

 A and F, at the bottom : here it is neceflary to obferve, 

 that the effect produced in this cafe on the rafters C B and 

 D E is contrary to the effeft produced in N° 3, by the 

 braces being difpofed in the lower angles. Ladly, fuppofe 

 that all the angles are braced, as in N° 5, it is evident, fince 

 the braces H G, N O, produce a contrary effect to the braces 

 K I, L M, thefe bending the rafters downwards, and thofe 

 upwards, that the rafters C B and D E will become nearly 

 llraight, or afl'ume an undulated line : the puncheons B A 

 and E F, receiving the force of the braces H G and N O at 

 the points G and O, muff dill be bent, fo long as the under 

 ends G and O of the braces do not coincide with the under 

 ends A and F of the puncheons : in this cafe, there is no 

 other remedy than by giving the puncheons a fcantling fuffi- 

 cient to withftand this tranfverfe drain, or horizontal thrufl, 

 at the points G and O : however, the fliape of the contour 

 may be pretty well fecured by introducing two abutments, 

 H I and M N, N° 6 ; thefe, by being bolted through the 

 two ends, will add greatly to the diffnefs of the rafters 

 B C and D E : the bolts that go through the upper ends 

 may alfo ferve for the braces I K and L M : the fhape of 

 the horizontal beam C D will likewife be very much pre- 

 ferved by the piece K L, bolted in three places, one at each 

 end, into the braces I K and L M, and another in the mid- 

 dle : the contour of a roof, thus fupported, would be quite 

 unchangeable, if the rafters were inflexible ; but as this is 

 not the cafe, and as they are afted upon tranfverfely by the 

 braces, the trufs will, therefore, in fome degree, be ex- 

 panded at B and E, and confequently occafion lateral pref- 

 fure on the walls ; it will therefore be unfit for an oblong 

 building, without other precautions for this purpofe. By in- 

 ferting parabolic curves in the fides B C and D E, as in 

 Plate XLVI. fg. 1, it will be effe&ually prevented. 



In roofs of this defcription, joggle-pieces of wood fliould 

 never be ufed, as the dirinking of them would tend greatly 

 to alter the outline of the rafters. 



Having laid down fuch principles as will enable the work- 

 man to judge of the ftrength and drain of timbers in the 

 framings of carpentry, it will now be neceflary to proceed 

 to fliew the mode of condrufting roofs to anfwer various 

 purpofes ; to give fome practical obfervations relative to 

 their drength, and to (hew the various modes of joining 

 timbers, the forms of draps, &c. 



Plate XLVII._/Ff. 1. is the roof of the chapel of the royal 

 hofpital at Greenwich, condruded by Mr. S. Wyatt. 



Indies 

 Scantling. 



A A, The tie-beam, 57 feet long, the fpan of the 7 



walls being 51 feet - - J 



B, Queen-pods - - - - 9 X 12 



C, Braces - - - - 9x7 



D, Straining-beam - - - 10 X 7 



E, Straining-piece - - - 6x7 



F, Principal rafters - - - 10x7 



G, Camber-beam for the platform - 9x7 

 H, An iron rod, fupporting tie-beam - 2x2 



The trufles are feven feet dear ; the platform is covered 



H 



with lead, which is fupported by horizontal beams fix by 

 four inches. 



The timbers of this trufs are well difpofed, and perhaps 

 contain lefs wood than mod roofs of the fame dimenfions. 

 The iron rod feems of no other utility than to prevent the 

 middle compartment of the beam from fagging. 



Fig. 2. is the roof of St. Paul's, Covent-Garden, defigned 

 by Mr. Hardwick, and condru&ed by Mr. Wapfliot, in 

 1796. 



Inches 

 Scantling. 

 A A, The tie-beam, fpanning 50 feet 2 inches 16 x J 2 



B, Queen-pods - - - - 9x8 



C, Straining-beam - - - 10 X 8 



D, King-pod, 14 inches at the joggle - 9x8 



E, Strutts - - - - 9x8 



F, F, Auxiliary rafters, at bottom - 10 x 

 H, H, Principal rafters, at bottom - 10 X 85 

 I, Studs fupporting the principals - 8x8 



This roof is much better condrufted than the original one 

 by Inigo Jones. A trufs of the prefent defign contains only 

 98 cubic feet of timber, whereas that by Inigo Jones had 

 273, and was very inefficient at the joggles, and had fome 

 of its timbers very ill difpofed : the interior trufs is well con- 

 trived for fupporting the exterior, which reaches feven feet 

 beyond the walls. The tie-beam has perhaps too much 

 camber, being fix inches ; for fince it afts as a dring, it will 

 lengthen in the fettling of the roof. 



Fig. 3. is the roof of Drury-lane theatre, 80 feet 3 

 inches fpan, and the truffes 15 feet apart: conftru&ed by 

 Mr. Edward Gray Saunders. 



Inches 

 Scantling. 



A, Beams - - - - 10 x 7 



B, B, Rafters - - - - 7X7 



C, King pods - - - - 12x7 



D, Strutts - • - - 5x7 



E, Purlins - - - - 9x5 



G, Pole-plates - - - - 5*5 

 H, Common rafters - - - 5x4 

 I, Tie-beam - - - - 15x12 

 K, Pods to ditto - - - 15 x 12 

 L, Principal braces to ditto - 14 and 12 x 12 

 M, Strutts - - - - 8 x 12 

 N, Straining-beam - - - 12x12 



The principal beams are t ruffed in the middle fpace with 

 oak braces, five inches fquare. This was requifite on ac- 

 count of its width, which is thirty-two feet, that the floors 

 might carry the work-fhops neceflary for the ufe of the 

 theatre. This trufs is mod admirably conftru&ed, it is 

 hardly equalled for drength, diffnefs, and lightnefs by any 

 other, and will fafely bear a load of nearly 300 turns, which 

 is four times more than ever it is likely to be loaded with. 



Plate LVIII. jig. I- is the defign of a trufs-roof for a 

 church, with a nave and two fide aides. The beam of the 

 middle compartment is kept from fagging in the middle by 

 two drutts, reding at the bottom on pods over the columns, 

 and connected together at the top by a draining-piece. As 

 the pods firmly fupport the middle of the rafters, keeping 

 thefe points always dationary, there will be little danger of 

 lateral preffure from a roof of this conftru&ion. All that 

 is wanted further is to give the rafters diffnefs, by fixing 

 other pieces from the two fixed points of the tie-beam, and 

 from the two pods. 



Fig. 2. is a defign for the truf6 of a curb-roof, with a door 

 in the middle. The rafters are fupported at equal diftances, 



from 



