Chap. XXII.
TIMBER ROOFS.
1330
of the first row, it being requisite that the planks at the ends should either be half oronce and a half the length of the others. \Vhen the two planks are properly adjusted,they are then to be united, by first cutting the mortises, through which the ties are to bethreaded, which are to unite the principal ribs with each other; these ties are to be of thesame thickness as the two planks, and their width four times as much ; they are alsomortised on each side the ribs, into which are introduced the wooden keys or pegs, the sizeof which are proportioned to the ties.
Fig. 2153.
SHED ROOP.
As some objection has been made to this construction, on account of the labour requiredin cutting the number of mortises, making the longitudinal ties, and introducing the pins,that the whole may be firmly united, it may be well to consider if mych of this labourmight not be dispensed with, andthe construction not be weakened :it might no doubt be effectedby securing the ties either to theupper or lower parts of the ribs, orboth ; the boarded covering pinnedon to them, or properly spikeddown, would almost serve the pur-pose, and thus avoid an expenseof labour, which is rather calculatedto weaken the ribs, by so frequentlycutting mortises into them. Whenthe ribs can be cut out of 8-inchplank their thickness is to be 1 inch,and this is sufficient for a roof of25 feet span; for one of $8 feetthe width of the planks must be 10inches, and their thickness 1^ inch ;for a span of 60 feet they areto be 13 inches, and their thickness2 inches; for 90 feet 2^ inches, andfor 100 feet 3 inches.
At Liboume , in the departmentof the Gironde in France , is ariding-house with a roof, erectedin 1826, under the direction ofA. R. Emy, colonel of engineers, which is an improvement upon the principles ofPhilibert Delorme . The internal diameter at the springing is 68 feet, the clear widthbetween the walls a foot more, and the roof springs 25 feet above the floor. The systemadopted for the construction of the ribs was first suggested in 1811 by M. de SaintPhar, who proposed to make the arches of a bridge of bent planks: in the riding-houseroof neither mortise nor tenon is used in the arrangement of the bent timbers: and