Chap. XXII.

TIMBER ROOFS.

1321

Mansard 's Roof\ as it is termed, is curbed,for the purpose of keeping down the heightof a building, or obtaining a more capa-cious apartment within it; the whole is usu-ally arranged within a semicircle, but itssystem may be said to consist of four piecesdr rafters hung together in such a mannerthat, if inverted, they would naturally assumethe form given to them.

The tie-beam in this case, which holds thewall plates together, are the common joists ofthe building, and the principals and struts Fig. 2111

above the collar, which hold in the curb-plates, are proportioned to their uses.

Many granaries and storehouses of considerable extent are constructed in a very simplemanner, and have their roofs in a single span: the entire width is divided into four parts,

MANSARD S ROOF.

two of which are given tothe middle. Upright postswith braces framed into thejoists, which extend throughthe entire width, support themiddle of the rafters: as theyare equally balanced, theyhave but littleoutward thrust.

It is the ge-neral practice inEngland, wherethe span of the

roof exceeds 25 feet, to adoptthe king-post and tie-beam,and not omit the struts.WTiere it is required that thebacks of the common shouldlie even with those of theprincipal rafters, the purlinsmust be framedinto the side ofthe latter, orstrapped up withiron. "Whenthe tie-beam is

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Fig. 2112.

SECTION OF A WAREHOUSE.

placed at a height above the level of the plates on which the rafters pitch, it is called acollar: the chief object in framing a roof should be to make it neither too heavy nor

too light, though the formeris often preferred to thelatter, from the idea thatweight upon the walls

adds to their stability, solong as it is not sufficientto crush them.

Fig. 2113.

noOF WITH TIE-BEAM.