cheaper, since they require shorter timbers, and of

^ h less

r Oof

scantling. When executed with judgment, the

e Xte r : ° ne ^e principal ties to a building ; as it binds theact lik° r w alls to the interior, and to the partitions, which

PUi

Roo e f ®trong counterforts against them.

are of various forms, according to the nature of theTh e ’ an< ^ the law of the horizontal and vertical sections.r °"r of° St s ' m P^ e f° rm of a roof is that which has only onearranged in an inclined plane, which throws^ a n~(o ' e , nt ' r ely to one side. This is called a shed-roof, or«ot) 8 i ' -The most elegant roof for a rectangular building,h'cWa two rectangular planes, of equal breadth, equallyCotisen ’ ant ^ terminating in a line parallel to the horizon ;being." entl y> ’ ts form is that of a triangular prism, each sides 0 ^? Ual 'y inclined to the plane of the wall-head. This

Celled* ^' e P* an ' 3 a trapezium, and the wall-heads properly*° term' ^' e ro °^ cannot be executed in plane surfaces, so as°f bei n ^ nate in a level ridge. The sides, therefore, insteadtbit p a 5 1 f lanes > are made to wind, in order to have the sum-*° ffialr a , e ^ to the horizon ; but the most eligible method is° r f) at e . tae sides of the roof planes, enclosing a level space,th e ’ ln the form of a triangle, or trapezium, at the summitti>Cs e a °°‘ - Roofs flat on the top are said to be truncated:80 a 3 n ® chiefly employed with a view to diminish the height,tyr to predominate over that of the walls.iNes a l^ th e four sides of the roof are formed by inclined^ ' s sa ‘ c l to be hipped, and is, therefore, called a8tl gl es r °°f s and the inclined ridges springing from the■ a ar e walls are called the hips. Roofs of this descrip-! 8 in ,, re quently truncated; and when the plan of the wallse form of a trapezium, the truncation of the roof

Cf!, necessai 7-

I0of s

cul a:

or

'"•‘ar tk° n c * rcu l a r bases, with all their horizontal sections•'atyr, ’the centres of the circles being in a straight lineOt ' ? ot) r ° m t * le centre of the base perpendicular to theca ^ e< f revolved-roofs, or roofs of revolution.ap j.th'e plan of the roof is a regular polygon, or a circle,

»•«, a n( j' p | is > the horizontal sections being all similar to theh th e . the vertical section a portion of any curve convexb s a Sl< ^ e ’ the roof is called a dome.

.'}>l<li n „ 6 - the expense of lead in the roofs of rectangulara >ch ®thstead of the flat, a valley is sometimes used,a* ta Uier 68 ^e vertical section in the form of the letter M,8r ae of an inverted W : and henoe it has obtained the

Mroo f-

. 1 ' hg n I0Ns -—In order to be understood by the reader, it38e( lhem Ce8Sar y to ex P' a ' n su °h terms as are used in the^ ^ art °f th e article, by way of definitions.

t^hibutP pieces of timber laid on the wall, in order toPL ^alls t le P res sure of the roof equally, and to bindtogether. They are sometimes called raising-

• *****

8 »b:

,‘‘at f 0r ^’ ®trong frames of carpentry, generally of a trian->s t <li S t ’ su PP°rting the covering. They are disposed at^ • gre a t es ’ anc * are use< f when the expansion of the wallsCo '*1 (j a()n to admit of common rafters alone, which wouldSt^fihg, j’ er °h being bent or broken by the weight of theVi v ariL 0 , r the want of some intermediate support. TheyWp' n g) th'^ cons t ru cted, according to the width of thelib's; L, e oontour of the roof, and the ciroumstances of&c -

ac j P'ece of timber connected at its extremities to twotb a ?y ffonj a u P° n hy opposite pressures, which have a ten-each other; or, to extend the tie, as a rope or

ln y'Piece, a piece of timber, connected at its extre-

mities to two others, acted upon by opposite pressures, whichhave a tendency towards each other.

Hence, a tie acts contrary to a straining-piece. A chain,rope, or small bar of iron, may be used for the former ; butthe latter must always be inflexible, being in a state of com-pression.

Principal rafters, two pieces of timber in the sides of atruss, supporting a grated frame of timber-work over them,on which the slating or covering rests.

Purlins, horizontal pieces of timber, fixed upon the princi-pal rafters.

Tie-beam, a horizontal piece of timber, connected to twoopposite principal rafters. It answers a twofold purpose,viz., that of preventing the walls from being pushed out-wards by the weight of the covering, and of supporting theceiling of the rooms below. When placed above the bottomof the rafters, it is called a collar-beam.

Common rafters, pieces of timber of a small section, placedequidistantly upon the purlins, and parallel to the principalrafters : they support the boarding to which the slating isfixed.

Pole-plates, pieces of timber resting on the ends of thetie-beams, and supporting the lower ends of the commonrafters.

King-posts, an upright piece of timber in the middle of atruss, framed at the upper end into the principal rafters, andat the lower end into the tie-beam : this prevents the tie-beam from sinking in the middle.

Queen-posts, two upright pieces of timber framed belowinto the tie-beam, and above into the principal rafters,placed equidistantly from the middle of the truss, or itsextremities.

Struts, oblique straining-pieces, framed below into theking-posts or queen-posts, and above into the principal rafters,which are supported by them ; or sometimes they have theirupper ends framed into beams, which are too long to supportthemselves without bending. They are often called braces.

Puncheons, short transverse pieces of timber, fixed betweentwo others for supporting them equally, so that when anyforce operates on the one, the other resists it equally ; and ifone break, the other will also break. These are sometimescalled studs.

Straining-beam, a piece of timber placed between twoqueen-posts at the upper ends, in order to withstand thethrust of the principal rafters.

Straining-sill, a piece of timber placed at the bottom • oftwo queen-posts, upon the tie-beam, in order to withstandthe force of the braces, which are acted upon by the weightof the covering.

Camber-beams, horizontal pieces of timber, made on theupper edge, sloping from the middle towards each end, in anobtuse angle, for discharging the water. They are placedabove the straining-beam in a truncated roof, for fixing theboarding on which the lead is laid ; their ends run three orfour inches above the sloping plane of the common rafters, inorder to form a roll for fixing the lead.

Auxiliary rafters, pieces of timber framed in the samevertical plane with the principal rafters, under and parallelto them, for giving additional support, when the extent ofthe building requires their introduction. They are sometimescalled principal braces, and sometimes cushion-rafters.

Joggles, the joints at the meeting of struts with king-posts,queen-posts, or principal rafters ; or, at the meeting of prin-cipal rafters with king and queen-posts : the best form isthat which is at right angles to the struts.

Cocking, or cogging, the particular manner of fixing thetie-beams to the wall-plates : one method is by dove-tailing,