18 ROOF. 



short straight pieces, arranged as shown in fy. 17, and held together 

 by bolts or wooden keys. When curved timber can be obtained it is 



ROOF. 



170 



to be preferred, as it reduces the number of joints. For small roofs 

 timbers may be bent into the required form, as it is found that a piece 

 of wood the thickness of which does not exceed j^th part of its length, 

 may be bent into a curve rising one-eighth of its span without impairing 

 its elasticity. Two such pieces may be laid together, and bent by 

 twisting a rope attached to their ends, ai is done in tightening the 

 frame of a bow or pit saw ; and, being bolted together while curved, 

 they will spring back but little when the rope is relaxed. Another 

 mode of forming such a rib is to take a piece of wood whose thickness 

 is about one-sixtieth of ita length, and cut along the middle with 

 a thin saw from each end, leaving about eight feet in the centre solid. 

 The beam may then be bent, and bolted or pinned together as before 

 described. In either case the rib should be bent about one-fourth 

 in. .rv than it w intended to remain, to allow for springing back. A 

 parabolic curve is the form moat recommended; but a circular arc, 

 rising half the height of the roof, will answer the purpose. J-'iy. 18 



fig. II 



represents the truss of a truncated roof strengthened by a curved rib, 

 the suspended pieces being, when the rib is formed in the manner first 

 described, placed at each joint, and each consisting of two pieces, one 

 <w each aide of the rib, notched to it and the beam, and fastened by 

 bolt* and straps. 



One of the advantages of this mode of construction is that the tie- 

 beams may be suspended from any number of points, which is im- 

 portant in large spans, where the beams have to be formed of 

 piece* scarfed together. [SfAimxo.] Diagonal braces, though un- 

 necMmry with parabolic curves, may be added to meet ac 

 train*, a* shown by the dotted lines in the cut. This principle of 

 oonatmction, with an arc composed of several pieces of timber, was 

 followed in one of the largest roofs ever built that erected in 17K1 

 orer a riding-house at Moscow. The span of this roof, which ha* been 

 said to be' the most extensive wooden roof in the world, is stated by 

 1.1 at 235 feet, the slope bein s about 19', and the external 

 dimensions of the building 1920 by 310 feet. He state* that it had 

 sunk so much that it was proposed to add a second curve for additional 

 strength. 



A simple and economical roof, invented by Mr. A. H. Hold*worth, 

 apd rewarded by the Society of Art* in 1820, is supported by curved 

 nbs of timber applied in a different manner. A detailed description is 

 given in tin- :>Ui volume of the Society's 'Transactions;' but fy. }'.' 



ifficiently explain the principle of its construction : a i* a beam 



wrring a* a tie-beam, and also to support the upper floor of the build- 



'. * are curved rib*, formed in a similar manner to those just 



dmcnbed, the lower ends of which are firmly secure.! to 'the tie-beam 



Tl- ].rinci|>al rafter* rest on these ribs, and their lower ends bear 



i - resting on the wall*, these piece* being fastened by 



strong iron straj to the curved rib*, to counteract the outward thrust 



f the rafters. By this arrangement the whole of the interior of the 



roof, wUeb i* usually encumbered with king-post*, queen post*, braces, 



ftc., a rendered available for useful purpo.es, in addition to which it 



effects a considerable saving of timber. 



Wr-uiKht-iri.n *traps of various forms are very useful, when judi- 

 ciously applied, in strengthening the joints of a roof. They should 

 fixed with regard to the unavoidable tendency of the timbers to 

 shrinking, so that while they may, hi some cases, counteract or lessen 

 t, they may so far yield to it as to prevent a strain which 

 ii| .on a timl.fr, l,eing entirely thrown, by iu alteration of 

 ipon the strap. Tie-beams are often suspended to the trussing. 



posts by means of straps, so arranged as to allow the beam to be keyed 

 up to ita true position in case of the roof sinking. When this is not 

 the case, the ties are sometimes drawn up into a slightly convex or 

 cambered form, to meet the same contingency. Height may be gained 

 inside a building by disposing the timbers aa in fg. 20, the want of a 



Fig. 20. 



continuous tie-beam being compensated for by an iron strap to unite 

 the ties to the bottom of the king-post at a ; but it is evident that the 

 safety of the plan must depend wholly ou the straps, which alone 

 counteract the outward thrust of the rafters. 



In roofing a church with a nave and side aisles, the continuity of 

 the tie-beams may be dispensed with, intermediate support being 

 obtained from columns. It is however necessary to guard carefully 

 against any lateral strain to the columns. 



Many of the high-pitched roofs of old Gothic churches and halls are 

 very ingeniously contrived, but they often throw great pressure on the 

 walls, owing to the absence or elevated position of the ties ; thereby 

 rendering very solid walls and buttresses necessary. The Norman roof 

 is an ingenious but complicated contrivance for the construction o'f 

 roofs of large span with small pieces of wood. Fig. 21 shows this 



arrangement, in which all the rafters abut on joggled king-posts, of 

 which there are several, their relative position being maintained by 

 diagonal braces. The timbers of this kind of roof are often left visible, 

 being so carved as to have an ornamental effect Such a roof may be 

 mad* to exert very little injurious pressure on the walls. 



When the space covered in is of an irregular shape, it is. best to 

 arrange the inclined planes of the roof in a similar manner to those of 

 Millar building, leaving a level platform in the centre, corre- 

 spoitding to the plan of the inclosed space. Where the space covered 

 is circular, elliptical, or polygonal, although the construction of the 

 .y appear more complicated to the eye, it is. in fact, simpler amt 

 easier than that of a quadrangular building, the strain of the roof beinjr 

 more equally distributed. The nearer a roof approaches to a circle in 

 plan, the stronger it will be, the parts deriving that mutual support 

 from each other which forms the distinguishing character of the dome. 

 Dome* of wood, of great size, have been made without trussing, simply 

 by forming the timber* into curved ribs abutting on the wall-plates, 

 which then form a circle, and are kept in their proper positions by hori- 



.ntal circles framed with them at intervals. As the ribs approach 

 the upper part of the dome, the intervals between them diminish in 

 width, to allow for which every second or third rib is discontinued at 

 intervals, the en.ls of the ribs thus discontinued being received by the- 

 horizontal circles, which may be compared to purlins, the ribs taking 

 the place of rafters. The wooden dome formerly existing at the Halle 

 aux Bics, at Paris, was a remarkably bold example of this kind, being 

 200 feet in diameter, and having a large opening in the centre. It waa 

 built at the suggestion of M. Moulineau, and, having been destroyed 

 by fire, has been replaced by a similar structure of iron, but of smaller 

 dimennions. 



When the roof approaches the circular form, but not sufficiently to 

 have the character of a dome, it may be considered as consisting of 

 several trusses resembling those of an ordinary roof, but so contrived 



as to intersect each other in the centre ; the king-post being common 

 to all the trusses. Fly. 22, representing .1 design for a polygonal roof. 



