221. A roof is intended to cover and protect a building from the effects of the weather, and also to bind and give strength and firmness to the fabric. To effect these purposes it should neither be too heavy nor too light, but of a just proportion in all its parts to the magnitude of the building. Mr. Ware observes, " that in practice roofs are generally made too heavy; and that he will do a most acceptable service to his profession who can show how a roof may be constructed with the smallest quantity of timber; by which an unnecessary load will be taken off the walls, and a large and useless expense saved to the owner." *

The timber roofs of our ancestors, in the styles of building called Norman and Gothic, were generally made without horizontal ties at the feet of the rafters, and were intended to be supported by the walls as an arch is supported by its abutments. The heavy walls they were in the habit of erecting in the Norman style, and the skilful disposition of buttresses in the Gothic, rendering ties unnecessary; besides, a tie beam would have been wholly incompatible with their mode of finishing the interior of a building.

Their principles of construction bear a closer analogy to masonry than to modern carpentry. It is true they sometimes erred in placing too great an oblique pressure against the walls, but in general we have more to admire than condemn in those celebrated buildings. The fashion of

* Ware's ' Body of Architecture.' timber-framed roofs, as applied to great halls, originated about the reign of Edward III. They became common about the year 1400, and spans of considerable extent were roofed in a most judicious manner. The timber roof of the Gothic architects was generally executed in oak, and ornamented with bold and graceful mouldings, having richly carved ornaments at the joinings. The most elaborate specimens are the halls at Christ Church, Oxford, and Hampton Court; that at Trinity College, Cambridge, is somewhat inferior: each of these is 40 feet span.* The span of the roof of Westminster Hall is 66 feet.†

In the old Gothic buildings the roof is always of a high pitch; its outline forms a striking feature, and in general is in graceful proportion to the magnitude of the building: sometimes, however, it presents too extensive a plain surface, of which we have a notable instance in the roof of Westminster Hall. A high roof is in perfect unison with the aspiring and pyramidal character of Gothic architecture; but in the opposite, though not less beautiful style of the Greeks, it becomes a less conspicuous feature; indeed, many of the Grecian buildings were never intended to be roofed at all. Yet when a roof was necessary it was not attempted to be hidden, but constituted one of the most ornamental parts of the structure.

Of timber roofs we have no examples in Grecian buildings; but the beautiful stone roof of the Octagon Tower of Andronicus Cyrrhestes,‡ and that of the Choragic Monument of Lysicrates,§ are sufficient to show that they were more inclined to ornament than to hide this essential part of a building.

222. In carpentry, the term roof is applied to the framing of timber which supports the covering of a building. The pitch of a roof, or the angle which its inclined side forms with the horizon, is varied according to the climate and the nature of the covering. The inhabitants of cold countries make their roofs very high, while those of warm countries, where it seldom rains or snows, make their roofs nearly flat; but the practice even in the same climate has varied considerably. Low roofs require large slates" and the utmost care in execution; they are cheaper, since they require timbers of less length and of smaller scantling. Formerly the roofs were made very high, perhaps with the notion that the snow would slide off easier; but where there are parapets a high roof is attended with bad effects, as the snow slips down and stops the gutters, and an overflow of water is the consequence; besides the water in heavy rains descends with such velocity that the pipes cannot convey it away soon enough to prevent the gutters overflowing. In high roofs the action of the wind is one of the most considerable forces they have to sustain, and it is supposed to have been with a view of lessening their height that the Mansard or curb roof was invented. The quantity of room lost by a curb roof, the difficulty of freeing the gutters from snow, and the ungraceful effect of the roof itself, are objections that are not compensated by the small difference of the expense between it and a common roof, especially now that experience has proved that roofs may be made much less in height than our ancestors were in the habit of making them.

* Dalhuway's 'Observations on English .Architecture,' p. 188. † Idem, p. 189. ‡ Stewart's ' Athens,' vol. i. § Idem.

223. The height of roofs at the present time is very rarely more than one-third of the span, and should never be less than one-sixth. The usual pitch for slates is when the height equals one-fourth of the span, or when the angle with the horizon is 26 1/2 degrees. Near the sea, or in very exposed situations, the height of the roof should be one-third of the span, for if less the rain and snow will be driven under the slates by the wind,

The pediments of the Greek temples make an angle of from 12 to 16 degrees with the horizon; the latter corresponds nearly with one-seventh of the span. The pediments of the Roman buildings vary from 23 to 24 degrees: 24 degrees is nearly two-ninths of the span.

224. The coverings used for timber roofs are copper, lead, iron, tinned iron, slates of different kinds, tiles, shingles,* and thatch of reeds or straw, the relative degree of slope which each should have being determined by the mode of laying or forming the joints. Taking the angle for slates to be 26 1/2 degrees, the following Table will show the inclination that may be given for other materials, and the weight of each material on a superficial foot of the inclined surface: -

225. Snow just fallen. is variously stated by different authorities to weigh from 8 to 14 lbs. per cubic feet. The quantity which falls at a time varies according to climate; in some places it is less than an inch, and in others as much as several feet in thickness. In the climate of New York it is usual to estimate the depth fallen at 2 1/2 feet. In England it would not be quite so much.*

* Shingles are now very little used in this country, though formerly they appear to have been much used. See Neve's ' Builder's Dictionary,' art. Shingle; Britton's ' Archit. Antiq.,' vol. ii., p. 79.

226. The pressure of an ordinary gale of Wind in England against a vertical surface is from 25 to 35 lbs. on the square

. foot. Occasionally pressures over 50 lbs. have been known. The highest ever recorded in this country was during a sudden gust on the 27th December, 18G8, by Mr. John Hartnup, F.R.S., at the Liverpool Observatory, Birkenhead, which amounted to 80 lbs. on the square foot as registered by Osier's Anemometer.

227. In England it has been usual to calculate on a pressure of 40 lbs. to the square foot for wind, snow, and other occasional forces acting on the inclined surface of a roof. Experience has proved this allowance to be ample, and that it might be reduced when the roof is sheltered from the effects of powerful winds, etc. The instances are very rare where roof timbers have been broken by the wind, but more frequently they have been stripped of their covering by its getting underneath, or from the vacuum caused by its passage over the building.