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capable of affording : it occasions an inequality, and a want of uniformity and correspondence in the proportions of the fabric, and an unnecessary and unpleasant height of walling. The best figure for a roof is that which consists of two equal sides, equally inclined to the horizon, terminating in the summit, over the middle of the edifice, in a horizontal line, or the ridge of the roof, as it is called: so that the section made by a plane, perpendicular to the ridge, is every where an isosceles triangle, the vertical angle of which is the top of the roof. This form is very advantageous, as it regards saving of timber; for it may be executed with the same scantlings, to span double the distance, that the simple sloping roof admits; or, in buildings of the same dimensions, the scantlings of the timbers will be very much diminished.
The antient Egyptians, Babylonians, and other eastern nations, of the remotest antiquity, constructed their roofs flat, as do likewise the present inhabitants of these countries. The antient Greeks, though favoured with a mild climate, yet sometimes liable to rain, soon found the inconvenience of a platform covering for their houses, and accordingly raised the roof in the middle, declining towards each side of the building, by a gentle inclination to the horizon, forming an angle of from 13 to 15 degrees, or the perpendicular height of from one-eighth to one-ninth of the span.
In Italy, where the climate is still more liable to rain, the antient Romans constructed their roofs from one-fifth to two-ninth parts of the span.
In Germany, where the severities of the climate are still more intense than in Italy, the antient inhabitants, as we are informed by Vitruvius, made their roofs of a very high pitch. When the pointed style of architecture was introduced into Europe, high pitched roofs were thought consonant with its principles; and they therefore formed, externally, one of the most striking characteristics of the Gothic style.
In their usual proportions, the rafters were equal to the breadth or span of the roof, or the rafters were the sides of an equilateral triangle, of which the spanning line was the base.
During the middle ages this form prevailed, not only in public but in private buildings, from the most stately and sumptuous mansion down to
the humble cottage of the common labourer; and this equilateral triangular roof continued in request until, finally, the pointed style came into disuse.
When the celebrated Inigo Jones introduced Roman architecture, the rafters were made three-quarters of the breadth of the building; and this
proportion, which was called true pitch, still prevails in some parts of the country where plain tiles are used; subsequently, also, the square seems to have been considered as true pitch: but, in large mansions, constructed in the Italian style, roofs of a pediment pitch, covered with lead, were introduced.
In the present day, where good slates are to be obtained in abundance, roofs may be covered with them, from the pyramidal equilateral Gothic down to the gently inclined Greek pediment.
With regard to the present practice, the proportion of the roof depends on the style of the architecture of the edifice; the usual height varying from one-third to one-fourth part of the span.
There are, doubtless, some advantages in high pitched roofs, as they discharge the rain with greater rapidity; the snow does not lodge so long on their surface; also, they may be covered with smaller slates, and even with less care, and are not so liable to be stripped by high winds as the low roofs are ; but the low roofs bear less weighty on the walls, and are considerably cheaper, since they require shorter timbers, and, of course, smaller scantlings.
The roof is one of the principal ties to a building, when executed with judgement, as it connects the exterior walls, and binds them together as one mass; and, besides the protection it affords the inhabitant within, it preserves the whole work from a state of decay, which would soon inevitably ensue, from the violence of rain or frost, which would operate in a way of rotting the timbers, of destroying the connection in the walls, and would cause them ultimately to fall.
The several timbers of a roof are, principal rafters, tie-beams, king-posts, queen-posts, struts, collar-beams, straining-sils, pole-plates, purlins, ridgepiece, common-rafters, and camber-beams. The uses of these will appear from The usual EXTERNAL FORM of a Roop has two surfaces, which generally rise from opposite walls, with the same inclination; and, as the walls are most commonly built parallel to each other, the section of the roof made by a plane perpendicular to the horizon, and to one of the walls, is an isosceles triangle; the base being the extension from the one wall head to the other. This extension is called the span of the roof.
TO FRAME TIMBERS, so that their external surfaces shall keep this position, is the business of trussing; and the ingenuity of the carpenter is displayed in making the strongest roof with a given quantity of timbers.
All beams, or pieces of timber, from their weight, when supported at the two ends only, acquire a concavity on the upper side; and this concavity is the greater as the distance between the props is the greater. It is, therefore, a grand object to prevent this bending as much as possible. The curvature will take place whether the position of beams be horizontal or inclined; but the same beam will have less curvature, as the angle, to which it is inclined to the horizon, is greater. For, it is evident that, when a beam is laid level, and supported at its extremities, its curvature will be greater than when inclined at any angle, however small; and, again, if it stand perpendicular to the horizon, its curvature will be nothing; that is to say, its curvature will be nothing when the angle of inclination is the greatest.
The curvature which timber obtains by bending is called sagging. To prevent timber from sagging, as much as is possible, it must be supported at a certain number of intermediate points or places, besides the two extreme points or places. Now these supports must themselves be supported from some base or other ; but, if the resting points or places is upon the surface or surfaces of other timbers, the greatest care must be taken that they do not fall between the extremities of the supporting timbers intended to support the other. That is to say, the lower end of every piece of timber, used as a prop, must rest upon some fixed point; or, otherwise, the propping piece of timber must be so disposed that the pressing forces at each end must be equal to each other.
These are the general principles upon which the strength of roofs depend.