1316

THEORY AND PRACTICE OF ENGINEERING. Book

sometimes the fronts of the housearc cased with courses of woodentiles with ornamental edges, placedover each other like plates ofmail.

Circular Chambers are obtainedin a roof by keeping up the tie-beams, but at the expense of theirutility : in such cases the externalwalls should be of sufficient thick-ness to resist the outward pressureof the feet of the rafters, which can-not be sufficiently held in by the ribsof the curves, or the braces attachedto them.

Where semicircular ribs are in-troduced, with their lower endsframed into the girders of the floorbelow, a part of this objection maybe removed, and a very convenientapartment obtained: the outer wallsshould only be carried up to such aheight that they can have their

crowning-plate affixed or tied to the

braces, which are framed into thecollar above : this kind of roof isvery common on the Continent,and is evidently derived from an-cient examples; to light theseapartments a dormer window isconstructed, which has its top alittle below the collar, or hammer-beam, as it is sometimes termed ;the chambers being appropriatedfor rest were called dormitories,from whence these windows pro-bably derived their name.

In many of our parish churches

we find roofs whose length of rafter

is equal to the entire width of the ^ '-

building; at about half their height Iis thrown in a collar, on which Istands a tree-post, strutting up theprincipals: beneath the collar areusually thrown in two braces, which are framedinto it and the rafters; such a roof, with atie-beam to hold in the plates which receivethe rafters, endures for ages.

Where the tie-beara is omitted, for thepurpose of giving a polygonal characterto the ceiling, it is necessary to have thewalls of considerable strength; over theirentire width is a capping of timber, on whichthe principals rest, and to which they arefirmly pinned.

The walls of our churches where these roofsare introduced are generally 3 feet in thickness,but if the width of the span be great, buttressesare required at every 10 or 12 feet, to resist thethrust which it occasions : by spreading thefeet of the rafters the weight of the roof bearsupon the entire thickness of the masonry, andfurther strength and protection areobtained byan additional short rafter, which projects to theouter face of the wall, and carries the eaves outsufficiently to throw off the water, and preventits doing injury to the construction.