CARPENTRY. 



527 



Cv>ntruc. drawDv, E 10, F x, Gy, which make respectively 

 to / 1, s'2, /:i, ill ; and draw the curve M>vwxy, 

 ^ which will give the under side of the angle nt>. 



Draw 111 parallel to BG, at a distance from BG 

 ;. equal to the thickness of the stuff, which must be 

 equal to the thickness of the angle rib, together with 

 the wasie or greatest recess of the hollow : where 

 the lines 1 n, 'Jo, :*/>, 4- G, cut the line BG, draw 

 ordinatcs parallel to DJ>, Ew, FJ, G/// also through 

 the puintK, where the lines i , /. <>, //, tnG, cut HI, 

 draw another set of ordmates parallel to D v, E tv, 

 Gi/, making both set of ordinates of the same 

 height with the ordinates Dv, E>, For, G y, and 

 truce a curve through the extremities of each set : 

 the first curve shows the concourse of the two sides, 

 or angular line ; the next, the ranging upon the side 

 of the rib which faces the arc AB ; and the outside 

 one, the ranging upon the side of the rib which faces 

 the arc B, or ranges with the large opening. The 

 under edge of the angle rib is first cut out at right- 

 angles to the sides, and the lines are drawn upon each 

 respective side, according to what is here shown. 

 In order to find the place of the angle corresponding 

 to its seat on the plan, take the stretch of the parts 

 B v, v ic, w x, xy, and extend them in a straight line 

 from B to D v , from D v to E w , from E w to F x , 

 from F x toG y ; draw the ordiuates D v , E w o, 

 F*/>, which make equal to D w. E o, F^>, and draw 

 the curve B, n, o, p, G, which, being bent under the 

 angle rib, will give the line of concourse. 



In practice, it would be better to get out the an- 

 gle rib in two or three parts, otherwise the waste of 

 stuff wouJd be enormously great : this is shown at 

 the other half. The process is the same as what has 

 been described in the first half. 



On Roofing. 



ofing. A roof is that part of a building raised upon the 

 walls, and extending over all the parts of the inte- 

 rior, in order to protect its contents from depreda- 

 tion, and intense changes of weather. 



Roofing, in carpentry, is the timber work which 

 is necessary to support the external covering. 



The most simple form of roofs is that consisting 

 of a level plane ; but this description of roofs ia only 

 adapted to short bearings, and is not calculated to 

 resist torrents of rain or moisture from penetrating 

 the interior. The next simple form is, that which 

 consists of an inclined plane ; and though well cal- 

 culated to resist the injuries of the weather, and to 

 admit of greater strength than a level disposition of 

 the timbers, it is far from admitting of the most 

 strength from a given quantity of timber, and occa- 

 sions a vyant of correspondence in the symmetry of 

 the fabric, or otherwise an unnecessary height of 

 walling. The best figure is that which consists of 

 two sides equally inclined to the horizon, termina- 

 ting at the summit over the middle of the edifice in a 

 horizontal line or ridge ; so that the section made by 

 a plane perpendicular to the ridge, is every where an 

 isosceles triangle, the vertical angle being the sum- 

 mit of the roof. This form is very advantageous in 

 aving timber, as it may be executed with the same 

 Kantliugs, to span double the distance that the single 



loping roof may ; or in buildings of the same dimen- Con*re 

 sions, the scantlings of the timbers will be greatly ' 

 reduced. '_r 1 -u^- 



The ancient Egyptians, Babylonians, and other 

 eastern nations of remote antiquity, constructed their 

 roofs flat, a practice which is still adopted by the 

 present inhabitant* of those climates. The ancient 

 Greeks, though possessing a mild climate, but some- 

 times liable to rain, would soon find the inconvenience 

 of a platform covering ; and accordingly raised the 

 roof in the middle, declining thence towards each side 

 of the building, with a gentle inclination to the hori- 

 zon, forming an angle Irom 13 to 15 degrees, or the 

 height from one eighth to one ninth part of the span. 

 In Italy, where the climate is still more liable to rain, 

 the ancient Romans constructed their roofs from one- 

 fifth to two-ninth parts of the span. In Germany, 

 where the severities of the climate are much more in- 

 tense than in Italy, the ancient inhabitants, as we are 

 informed by Vitruv'us, made their roofs of a very high 

 pitch. When the pointed btile of architecture wa 

 introduced into Europe, high pitched roofs were con- 

 sonant to its principles ; and therefore formed exter- 

 nally one of the mos>t striking features of the Gothic 

 stile, the prevailing proportion being the equality of 

 the rafters, and the breadth or span of the roof, or the 

 rafters the sides of an equilateral triangle, of which the 

 spanning line was the base. During the middle ages, 

 this form prevailed not only in public, but also in pri- 

 vate edifices, from the most sumptuous mansion to the 

 cottage of the labourer ; and this equilateral trian- 

 gular roof continued in vogue till the expulsion of 

 th-- pointed -tile, by the introduction of Roman ar- 

 chitecture, introduced by the celebrated Inigo Jones. 

 When the rafters were made three-quarters of the 

 breadth of the building, the proportion was called 

 true pitch, which still prevails in some parts of the 

 country where plain tiles are ued ; and subsequent* 

 ly, the square seems to have been considered as true 

 pitch. But in large mansions, erected in the Italian 

 stile, roofs of a pediment pitch, covered with lead, 

 were introduced. In the present day, where good 

 slates are to be obtained in abundance, the roofs may 

 be covered from the pyramidal equilateral Gothic, 

 down to the gentle inclined Greek pediment. In 

 respect to the present practice, the proportion of the 

 roof depends on the stile of the architecture of the 

 edifice, the ordinary height being from one-third to 

 one fourth part of the span. 



There are borne advantages in high pitched roofs ; 

 they discharge the rain with greater rapidity ; the 

 snow does not continue to be so long on their sur- 

 face ; they may be covered with smaller slates, and 

 are not so liable to be stripped by heavy winds as 

 low roofs ; but low roofs have less pressure on the 

 walls, and are much cheaper, since they require 

 shorter timbers, and consequently smaller scantlings. 



The roof is one of the principal ties to a building, 

 when executed with judgment ; as it connects the 

 exterior walls, it also binds the whole into one mass, 

 and secures the work from a state of decay, which 

 would soon ensue trom the injuries of rain and frost, 

 so as to rot the timber, destroy the connection of 

 bond in the walls, and cause the building ultimately 

 to fail. 



