This arrangement is useful for diminishing the height of a roof, the level platform being covered with lead to compensate for the want of slope. It should be observed, however, that even this part is not per fectly level, the centre being slightly elevated to throw off water. A similar saving of height is frequently obtained by means of a roof in which each sloping face consists of two planes of different degrees of inclination. This form, which is denominated a curb roof (or, from its inventor, a Mansard( roof), is very common in London, because affords more space for the formation of bedrooms in the roof than the simpler forms. A curbed roof may be hipped or not, according to circumstances. Pig. 4 represents it hipped at one end only, as the last similar to a valley, but in a horizontal instead of an inclined position, the term gutter is applied instead of valley.
A further distinction, which it may he well to mention before entering upon the details of construction, is that between roofs with dripping eaves, and those in which the water is collected in gutters. In the former erase the roof projects several inches, or even feet, beyond the walls, and the water running from the roof either drops at once on the ground, or is collected in troughs fixed under the margin of the eaves, and conducted by them to descending pipes. This arrangement has a clumsy appearance, and is perhaps unnecessary where a sufficient projection is given to the eaves, though it is essential to the dryness of I the walls when they are of the diminutive size often adopted by modern builders. In gutter roofs the timbers do not extend to the outside of time walls, which are carried up as parapets, of a reduced thickness, to such a height as to conceal the roof either wholly or partially. The gutters, which are troughs of wood covered with lead or other metal, are laid at the bottom of the slopes, just within the parapets, and have a gentle inclination (usually about an inch in ten feet), to cause water to tam freely towards the pipes. In extensive roofs it is well to use two or more falls instead of one, that the elevated end of the gutter may cover as little of the roof as need he. Similar troughs are often used in the valleys. Gutters are generally made wide enough for a man to walk along them, and should be suffi ciently capacious to avoid all risk of overflowing during a sudden heavy fall of rain.
The degree of slope given to the inclined faces of a roof varies according to the covering material employed, as well as to the climate. The ancient Grecian temples had very low, or pediment roofs, varying from about to about the height being from one-ninth to one. seventh of the span. In Roman buildings the inclination Is somewhat greater, being usually or or from one-fifth to two-ninths of the span. The general introduction of the pointed style of architec ture led to the use of very high-pitched roofs, a very common proportion being that in which the length of the rafters is the same as the span, so that they formed an equilateral triangle. In comparatively
modern domestic architecture in this country, it has been considered desirable for the length of the rafters to be three-fourths that of the span, and an angle of is still considered by some to be the best pitch when plain tiles arc used. As builders can, in the present day, obtain excellent covering materials, the pitch may be made of any required degree, down to the low Grecian pediment, and it therefore depends on the style of architecture and the taste of the builder ; the most common height being from one-fourth to one-third of the span. High roofs discharge rain the most rapidly, and do not retain snow so much as those of low pitch ; but where they have gutters they are liable to become choked by snow sliding into them, and to overflow from water running into them faster than the pipes can convey it away. Steep roofs may be covered with small slates, and, are less likely to be stripped by violent winds. Low roofs, iu consequence of their superior lightness, are less expensive, the timbers not only being shorter, but of proportionately smaller scantling, and they press less injuriously on the walls. The following table, extracted from Tred gold's Elementary Principles of Carpentry,' shows the proper angle for roofs covered with the materials specified in the first column, the last column indicating the comparative weight of each kind of covering :— gore. showing, like the previous diagram, the plan, and side and end elevations.
tirch are the principal forms of roof used in covering simple rectam Leder buildings, but they require many tnodifieatlons to suit irregu larities of shape or combinations of rectangular forma. Thus, lu figs. 5 which represent the junction of different roofs or portions of nailing at right angles with each other, the lilies a a a indicate mays, nr the junction of two planes in such a manner as to form hollows the rever u of hips. When two faces of a roof join so as to form an angle In describing the timber-work of an ordinary roof, each of the planes of which it is composed may be considered to be bounded by a frame, the parts of which have the general name of bordering pieces. Those which join the wall are the stall-plates ; that at the meeting of two faces, parallel to the wall-plates, is the ridge-piece ; and the inclined bars extending from the wall-plates to the ridge-piece are rafters, those which forum the salient angles in hipped roofs being distinguished as hip-rafters. The support necessary for the external covering is given by a series of rafters or inclined bare, extending from the wall-plates to the ridge-piece. and placed parallel with each other at equal distances. In a hipped roof, the rafters near the ends, being parallel with the others, are necessarily diminished in length, extending from the wall-plate to the hip-rafter instead of the ridge-plece. All such pieces, being shorter than the length between the wall-plate and the ridge-piece, are called jack rafters.