Substances which conduct heat very slowly, such as slate, make better coverings than the metals; the former preserve an equable temperature, while the latter conduct the heat in summer, and the cold in winter, to the interior of the building.
The pitch, or inclination of the sides of a roof, is determined chiefly by the nature of the covering.
Thus thatch, which would easily allow wet to penetrate it. must be laid at a steep angle, so as to throw the water off at once ; whilst, on the other hand, hard and impervious slates may be laid at a much smaller angle, and sheets of metal may be nearly flat.
The pitch is, moreover, varied greatly to suit different styles of architecture, and also according to climate. Some writers have gone so far as to prescribe an exact pitch for every variation in latitude.
The following remarks by the late Professor Robinson are of a more practical character: -
"A high-pitched roof will undoubtedly shoot off the rains and snows better than one of lower pitch; the wind will not so easily blow the dripping rain in between the slates, nor will it have so much power to strip them off;" and further - "A high-pitched roof will exert a smaller strain upon the walls, both because its strain is less horizontal, and because it will admit of lighter covering; but it is more expensive, because there is more of it, - it requires a greater size of timbers to make it equally strong, and it exposes a greater surface to the wind."
The pitch of a roof is expressed either by the angle which its sides make with the horizon, or by the proportion which its height in the centre bears to the span.
Thus the roof shown in Fig. 77, p. 39, may be said to have a pitch of 26½ degrees or ¼.
The subjoined table, taken chiefly from Tredgold, gives the inclination for roofs covered in different ways. The weights of various coverings are also given, but these will vary considerably according to the quality and thickness of the material used.
Kind of Covering.
Inclination of sides of Roof to
Roof in parts of Span.
Weight on a square (i.e. 100 square feet) of roofing in lbs.
30 to 40
80 to 120
Corrugated Iron, l6' BWG1
Sheet Iron, 16 BWG .
18° to 20°
550 to 850
Slates (large) .
900 to 1100
,, (ordinary) .
550 to 800
450 to 650
Slabs of Stone
" (Pan) .
,, (Taylor's Patent) .
Zinc (1/32 in. thick).
Boarding (¾ thick) .
" 1 "....
N.B. - The additional pressures to be taken into account in practice are the following: -
Pressure of wind . . . 2500 to 5000 lbs. per square of 100 feet. do. of snow, in this country 500 lbs. per square.
Tiles of burnt clay are made in several different forms, a few of the more important of which will be described.
1 BWG stands for Birmingham wire-gauge - a measure of thickness (see Part III.) 2 Plain tiles are used on roofs of any pitch from 30° to 60°.
Plain Tiles are slabs of burnt clay, either rectangular or in various patterns, as at p,p, Fig. 99, P1. II., generally about 10½ inches long, 6½ inches wide, and about ½ inch thick. They are slightly curved in their length to make them lie close.
They are laid on battens 1½ inch x ½ inch, or on laths of oak or fir, being hung from them by wooden pins driven through holes near the upper edge of the tiles. Sometimes the tiles are hung by projecting nibs, of which there are generally two or three upon their upper edges. Sometimes only every third or even only every tenth course is nailed.
The arrangement of the tiles is similar to that of slates - the tail of each rests upon the tile below for a length of about 6 inches, the gauge being 4 inches (often 3½ inches) and the lap over the head of the tile next but one below about 3 inches.
Weather Tiling.1 - Plain tiles are often used vertically to protect walling. Battens are nailed upon the wall, and the tiles hung upon them in somewhat the same manner as for roofs, - each tile being bedded in mortar so as to make the covering warm and weather-tight.
Plate II. shows in Figs. 99, 100, part of a plain tiled roof and of a wall with hanging tiles. Figs. 101-106 show various forms of tiles which are necessary to make good work, as shown in Fig. 99.
Fig. 107 shows the method of securing a tile by a pin which should be preferably of oak or otherwise of heart of Memel cut with a knife out of any dry stuff. Fig. 108 shows a tile secured by a nail which should be of copper or of malleable iron.
Ridges may be as shown in Fig. 99. Sometimes the ridge tiles have longitudinal grooves along their upper edges, into which detached ornamental "fleurs" are fitted. Sometimes they have ventilating openings in them.