Figure 3, the construction of the End Gallery and Children's Gallery A, Bressummer, 10 inches by 8 inches. B, Girders, 10 in. by 8 in. C, Binders, 13 in. by 3 in. D, Bridging-joists, 4 in. by 3 in. E, Truss, 6 in. by 4 in. F, Carriage, 8 in. by 5 in. G, Bearer, 7 in. by 4 in. H, Puncheons, 3 in. by 3 in. J, Binders to Ceiling-joists, 7 in. by 5 in. K, Ceiling-joists, 3 inches by 24 inches. M, Oblique Girder, 12 in. by 8 in. N, Carriage, 8 m. by 6 in. O, Binder, 10 in. by 3 in. P, Bridging-joint, 4 in. by 3 in. Q, Binder to Ceiling-joists, 7 in. by 5 in. R, Ceiling-joist, 8 in. by 24 in. S, Plate, 12 in. by 8 in. Figure 4, the longitudinal Trusses of the West-end Gallery. T, Trusses, 6 in. by 4 inches. V, Strainers, 5 in. by 44 in. X, Abutment-piece, at each end, 9 in. by 6 in. ROOFING. 100. The Roof is that part of a building which is raised upon the walls, and extends over ail the parts of the interior, in order to protect its contents from depredation, and from the severities and changes of the weather. The Roof, in Carpentry, consists of the timber-work which is found necessary for the support of the external covering. The most simple form for a roof is that consisting of a level plane; but this description of roof is adapted only to short bearings, and is not at all calculated to resist or prevent the torrents of rain or moisture from penetrating into the interior. The next simple form is that which consists of an inclined plane; and, though well calculated to resist the injuries of the weather, and to afford greater strength than a level disposition of the timbers would supply, it is far from admitting of the utmost strength that a given quantity of timber is capable of affording; and 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, called the ridge of the roof; 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 pan 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. 101. The antient Egyptians, 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, found the inconvenience of a platform covering for their houses; and, accordingly, raised the roof in the middle, declining H 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 the roof 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 with a rise of 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 the usual proportions of the Gothic roof, the length of the rafters was 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, with little variation, 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 to be used till the pointed style began to decline, and Italian architecture, in a great measure, superseded it. The celebrated INIGO JONES was chiefly instrumental in introducing Italian architecture; then a change in the proportions of the roof took place, and 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, however, the square, or angle of 45 degrees, seems to have been considered as the true pitch: but, in large mansions, constructed in the Italian style, roofs of the same inclination as the pediment, called a pediment pitch, were introduced, and covered with lead. At the present time, where good slates are to be obtained in abundance, roofs may be covered with them of any pitch, from the pyramidal Gothic down to the gently-inclined Greek pediment. Therefore, with regard to the present practice, the proportion of the roof for slates 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 have less pressure and stress 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 judgment; 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 effects of rain or frost, as moisture would operate in rotting the timbers, and frost in destroying the connexion of the walls, and the whole would ultimately fall to ruin. 102. The several timbers of a roof are termed principal rafters, tie-beams, king-posts, queenposts, struts, collar-beams, straining-sills, pole-plates, purlins, ridge-piece, common-rafters, and camber-beams. The uses of these will appear from the description of them; but, in the first place, we must describe the nature of the roof itself. The usual EXTERNAL FORM of a ROOF has two surfaces, which generally rise from opposite walls, with the same angle of 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 a triangle with two equal sides; the base being the extension from the one wall-head to the other. This extension is called the span of the roof. TO FRAME the TIMBERS of a ROOF, so that their external surfaces shall keep this position, is the business of the Carpenter; and ingenuity is displayed in making the strongest roof with a given quantity of timber. All long beams, or pieces of timber, from their weight, when supported at the two ends only, become concave on the upper side; and this concavity is the greater as the distance between the props is the greater. It is, therefore, the 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 the more upright it is, or 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 ends. Now these supports must themselves be supported from some base or other; but, if the resting points or places be 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 supported 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. The supported points, in Carpentry, are those points or places where there are walls, or columns from the ground, or two timbers meet together in an angle; and no roof or piece of framing is good where the end of one piece, used as a prop to another, presses upon a third piece of timber, between its supported extremities. The pressing end of every prop ought to rest upon some supported point. 103. PRINCIPAL RAFTERS are the two pieces of timber, in the truss of a framed roof, that form the two equal sides under the covering. It is evident that the greater the opening is, the more supports each principal rafter will require. A piece of timber may be supported either from some supported point above it, or some supported point below it: if the support be above the piece to be supported, then it is evident that the connecting piece will act as a string; but, if below the piece to be supported, it is evident that the prop must be an inflexible beam or piece of timber. Therefore, as from the nature of a roof the principal rafters cannot be supported from above, they must be supported from below, by walls, piers, or posts, made of some fit material, as stone, brick, iron, or timber, of sufficient thickness; and, because the pressure occasioned by the weight of the covering is uniformly distributed over the principal rafters, these principal rafters must be supported at a certain number of equidistant points, which will depend upon the distance of the walls. 104. A TIE-BEAM is a piece of timber for connecting the feet of the principal rafters, in order to prevent them from spreading, by the weight of the covering. The tie-beam is therefore used as a tie, and, in respect to the strain from the pressure of the rafters, is in a state of tension. In order to prevent the sagging of the tie-beam, in very wide houses, it must be supported in one or more places in its length; and if it cannot be supported from the ground, it must be supported from some other supported point or points in the roof itself. Such a point we have where the two principal rafters meet each other; and this one point will furnish as many supports to the tie-beam as we please: while, from each of these supported points in the tie-beam, the middle parts of the principal rafters may be supported. For, when once a frame is formed, the timbers may be made to strengthen one another, so as to form a strong and perfect whole. It is not easy to give a direct rule for the disposition and position of supporting timbers: but we ought to prefer such a disposition as will keep the bearing timbers within a proper limit as tc length, and the angles should be as direct as possible. Oblique or acute angles occasion very great strains at the joints, and should therefore be avoided. One grand principle is, in every frame or roof, to resolve the whole frame into the least number of triangles, affording direct connections between all the points of support, which must be considered as the elements of the framing. Four-sided figures must be avoided, if possible; and this may be done by introducing diagonals, which will form them into triangles; for, without this, a four-sided figure will be moveable round its angles, and has no stiffness except what depends on the strength of its joints. Sometimes it may be necessary to divide a quadrangular piece of framing into four triangles, by means of two diagonal pieces, particularly when this figure occurs in the middle of a roof. Our plate XXI, fig. 2, shows a beautiful specimen of this arrangement, designed by Mr. Smirke, whose high reputation as an architect is well known. The principles of framing being once understood, a little practice in designing will soon enable the Carpenter to judge of the proper disposition of timbers, so as to produce a good design, if not the best possible. 105. A KING-POST, or PRINCIPAL POST, is a vertical piece of timber, extending from the place where the two principal rafters meet to the tie-beam, for the purpose of supporting the tie-beam in the middle. The KING-POST is, therefore, in a state of tension; and, consequently, it may be a slender bar of wrought iron, or any tenacious material that will not be liable to extension when stretched or drawn in length. The principal rafters are frequently supported from one or more points in the king-post: but it is evident that both the rafters must be supported exactly in the same manner when the supporting points in the king-post are between its two extremities; so that the principal rafters may produce equal and opposite pressures on each side of the king-post. Each of the principal rafters may be supported in many points, either from one point in the king-post, or from as many points as the number of points to be supported; or, as has been said before, either from one supported point, or from as many supported points, in the tie-beam, as the number of points to be supported; and, in short, the principal rafters may be supported from any supported point whatever, from the king-post, or tie-beam, or from both. This very circumstance points out the vast variety there may be in designs for roofs. 106. QUEEN-POSTS are two pieces of timber, equidistant from the middle of the truss; the one suspended from the head of one of the principal rafters, and the other from the head of the other, with a level piece of timber, called a straining-beam, between them. The Queen-posts, therefore, divide the internal space of the frame into three compartments, of which the two extreme ones are right-angled triangles, and the middle one a rectangle. The use of the queen-posts is similar to that of the king-posts; viz. for furnishing a general support for the principal rafters, at different points between the ends, by connecting timbers, but their chief object is supporting the tie-beam at more places between its extremities. 107. STRUTS are those props which support the principal rafters, in one or more points, so as to divide them into equidistant parts. Struts are generally disposed in pairs, equally inclined to the vertical line, which divides the truss into two equal and similar parts; and which, therefore, divides the two beams into two equal lengths. Struts are necessary in roofs where the span is great; and the greater the span or distance of the walls, the greater the number of struts will be required; for, in this case, more points in the principal rafters will have to be supported. 108. A COLLAR-BEAM is the piece of timber framed between two principal rafters, and usually employed where there are no king-posts. 109. A STRAINING-BEAM is the piece of timber framed between the heads of the queen-posts; and is necessary where the roof is to have a platform, or flat for walking upon, or wherever rooms are required in the roof. queen 110. A STRAINING-SILL is a horizontal piece of timber, disposed between the feet of the posts, to counteract the efforts of the struts, in pushing these feet nearer to each other, when, on account of rooms, the space cannot be filled with diagonal braces. Having thus noticed the several parts of a truss, it may be proper to observe that all kingposts, queen-posts, and tie-beams are ties; and may be formed by an iron tie incapable of farther extension than is sufficient to bring it to a straight line. A chain, or a slender bar of iron, will therefore answer the same purpose, in some of these cases, as a piece of timber, or other such inflexible material. It is also to be remarked, that all collar-beams, principal rafters, and struts, are to resist compression; and are, therefore, necessarily constructed of an inflexible material, such as wood, or a stiff piece of iron. It may be further observed that, in complex frames, such as the centring to large arches or bridges, the same timbers, in different stages of the work, sometimes perform the office of ties, and sometimes that of struts; and, in the transition from one office to the other, must be sometimes in a neutral state. The material employed in such situations must, necessarily, be inflexible. This is to be recommended not only here, but in every doubtful case, or where it is uncertain whether the part of the truss requires to be a tie or a strut. 111. A POLE-PLATE is a beam over each opposite wall, supported upon the ends of the tiebeam, or upon the feet of the principal rafters, to receive the ends of the common rafters. 112. PURLINS are horizontal pieces of timber, supported by the principal rafters. Their office is to support the common rafters in the middle parts of their length. 113. A RIDGE-PIECE is a beam at the apex of a roof, supported by the king-posts, or by the heads of the principal rafters, and supports the upper ends of the common rafters. COMMON RAFTERS are inclined pieces of timber, parallel to the principal rafters, supported by the pole-plates, the purlins, and the ridge-piece. They support the covering, the material of which is sometimes large slates, extended from rafter to rafter, but more commonly either boards, or laths, are nailed upon the common rafters, and the slates, tiles, &c. fastened on with nails or pins. 114. JOGGLES are the joints at the meeting of struts, king-posts, queen-posts, and principal rafters; where there is a piece left to form the abutment. The usual form for the joggles is that which is at right angles to the lengths of the struts or rafters, or at right angles to the tenoned piece: but this position cannot, at all times, be obtained, from the want of sufficient substance of timber: in that case, the joint is made either oblique, or the upper part in a line with the side of the piece which has the mortise, and the lower part perpendicular to the sides of the |