before we proceed to the scantlings of the timbers to be employed, the reader must be informed that the trusses to roofing, with whose nature he has now become acquainted, are placed only at certain intervals (which should not exceed 10 feet) apart, and are thus made to bear the common rafters and the weight of the covering, as well as to perform the office of suspending the tie beam by which the walls are kept together. Hence the rafters so framed in a truss are called principal rafters; and by the means of a purline A (fig. 688.), which lies horizontally throughout the roof's length on the principal rafters, they are made to bear all the superincumbent load. The purlines are in various ways made fast to the principal rafters, and upon it the common rafters are usually notched down. Their bearings are thus lessened, and less scantlings suffice for them. Fig. 688. They are received at their feet on a piece of timber (B in the figure), which runs longitudinally along the sides of the building. This piece of timber is called a pole plate, from being the uppermost plate in a building; at their summits they abut against a ridge piece D. When a roof slopes each way, the space enclosed between the intersection of the slopes is called a hip (fig. 689.); and the longest rafters in it, which are those at the angles, are Fig. 689. called hip rafters, and the shorter ones are named jack rafters, as A, A, A, &c. 2036. We have, at the beginning of this section (2007.), observed, that the use made of bolts must be always in a direction as nearly as possible counter to the strain which the pieces exert; the method, therefore, of introducing them will, on due consideration, be sufficiently obvious. Before proceeding to lay before the reader some few examples of roofs suitable to different spans, as well as of some of magnitude which have been executed, it may be as well to complete this portion of our labour, by giving some information on the scantlings of timber for roofing, in which a medium, founded on our own practice, is introduced between ignorant overloading, and fanciful theory. 2037. For roofs whose spans are between 20 and 30 feet, no more than a truss with a king-post and struts will be necessary, in which case the scantlings hereunder given will be sufficient. For a span of 20 feet, the tie beam to be 9 in. by 4 in.; the king-post, 4 in. by 4 in.; principal rafter, 4 in. by 4 in.; struts, 4 in. by 3 in. For a span of 25 feet, the tie beam to be 10 in. by 5 in.; the king-posts, 5 in. by 5 in., principal rafter, 5 in. by 4 in.; struts, 5 in. by 3 in. For a span of 30 feet, the tie beam to be 11 in. by 6 in.; the king-post, 6 in. by 6 in.; principal rafter, 6 in. by 4 in.; struts, 6 in. by 3 in. 2038. For roofs whose spans are between 30 and 45 feet, a truss with two queen-posts and struts will be required, and a straining piece between the queen-posts. Thus For a span of 35 feet, the tie beams to be 11 in. by 4 in.; queen-posts 4 in. by 4 in.; ⚫ principals, 5 in. by 4 in.; straining piece, 7 in. by 4 in. ; struts, 4 in. by 2 in. For a span of 40 feet, the tie beams to be 12 in. by 5 in.; queen-posts, 5 in. by 5 in.; principals, 5 in. by 5 in.; straining piece, 7 in. by 5 in.; struts, 5 in. by 24 in. For a span of 45 feet, the tie beams to be 13 in. by 6 in. ; queen-posts, 6 in. by 6 in.; principals, 6 in. by 5 in.; straining piece, 7 in. by 6 in.; struts, 5 in. by 3 in. 2039. For roofs whose spans are between 45 and 60 feet, two queen-posts are required, and a straining piece between them; struts from the larger to the smaller queen-posts, and struts again from the latter. For a span of 50 feet, tie beams, 13 in. by 8 in. ; queen-posts, 8 in. by 8 in.; small queens, 8 in. by 4 in.; principals, 8 in. by 6 in.; straining piece, 9 in. by 6 in; struts, 5 in by 3 in. For a span of 55 feet, tie beams, 14 in. by 9 in. ; queen-posts, 9 in. by 8 in.; small queens, 9 in. by 4 in.; principals, 8 in. by 7 in.; straining-piece, 10 in. by 6 in.; struts, 5 in. by 3 in. For a span of 60 feet, tie beams, 15 in. by 10 in.; queen-posts, 10 in. by 8 in.; small queens, 10 in. by 4 in.; principals, 8 in. by 8 in.; straining piece, 11 in. by 6 in.; struts, 6 in. by 3 in. 2040. The scantlings of purlines are regulated principally by their bearing; and though we have subjoined scantlings for bearings of 12 feet, such should be avoided by not allowing the distances between the trusses to exceed 10 feet. Thus For a bearing of 6 feet, the scantling of the purline should be 6 by 4. 8 feet, 10 feet, 12 feet, For common rafters the scantlings are as follow; 12 feet should be the maximum of the bearing. For a bearing of 8 feet the scantling of the rafter should be 4 by 21. 2041. By a study of the roofs which follow as examples, the architect will be led to other expedients and modifications of the forms submitted to his notice, as circumstances may call forth his ingenuity and talents. We have, we trust, already said enough to lead him on. Where economy must be consulted, the roof shown in fig. 690. may be used; it is only fit for a small building, and the span of such a one should not exceed 25 feet. The left end of the collar beam exhibits what is called the carpenter's boast, but it partakes somewhat of the rule joint, being worked out to a centre. Fig. 690. But in roofs above 25 feet span it is not well to omit the king-post and tie beam, though, if particular strains are to be provided against, even in such small spans the struts should not be omitted, and the form shown in fig. 691. should be adopted, which will answer for spans at least up to 35 feet. In this and other cases of larger span, it is often desirable that the common rafters should not stand above the principals, and then the purlines are framed by mortices and tenons into the principals, as shown at A (fig. 692.), wherein the line be shows the underside of the common rafters notched on to the principals. 2042. From 35 to 45 feet, the tie beam should be suspended from at least three points, or it will be unnecessarily heavy; and this suspension of the tie beam, so that it may be really a tie unsusceptible of alteration in form, is the true cause of this introduction of king and queen posts, as we have before explained to the reader. Indeed, as a Fig. 693. 2043. For spans above 60 feet we have not given scantlings of timber in the preceding tables; but such do not greatly increase beyond 60 feet with practicable spans, and enough has been already said to make the reader acquainted with that part of the subject. Fig. 694. is an example of a roof calculated for a span of 70 to 80 feet, and in fig. 695. a passage or other conveniency may be practised between the queen-posts. 2044. In all the cases given, the roof is supposed to receive no support from any but the external walls, and the trusses to be not more than 10 feet apart. 2045. We shall now proceed to offer a few examples of roofs that have been executed, all of which have already appeared in works relating to the subject, as specimens of the most instructive and useful class for the student. The reader who desires to extend his inquiry into this branch of carpentry, and to become acquainted with a multitude of examples, is recommended to the celebrated work of Kraaft, Recueil de Charpente, and also to Rondelet's admirable treatise L'Art de Batir. The space to which we are limited prevents the insertion of many specimens which we would have gladly published here. The principles have, however, been so explained, that we trust the omission will not be felt. In respect of Gothic examples, a reference to the section of Westminster Hall (fig. 196.) will exhibit one of the modes adopted to span the large ancient halls of the country. In them the tie is rarely found connecting the feet of the principal rafters; for such an arrangement would have prevented the ornamental system which results from the substitution of a collar for a tie beam. 2046. Fig. 696. represents a section of the roof of St. Martin's-in-the-Fields, Westminster, C B H 69 Feet. Fig. 696. designed by Gibbs. The breadth of the building between the walls is 69 ft.; from centre to centre of columns the middle aisle is 39 ft. 11 in. The roof is well contrived and framed; but the timbers are stronger than they need have been. The scantlings of them are as follow:-A, principal rafter, 13 in. by 10 at bottom, and 11 in. by 10 at top; B, straining brace, 14 in. by 10 at bottom, and 11 in. by 10 at top; C, king-post, 9 in. by 9; D, strut, 7 in. by 74; E, queen-post, 8 in. by 9; F, strut, 7 in. by 7; G, tie beam, 14 in. by 9; H, post over the column, 14 in. by 9; I, brace, 7 in. by 7; K, brace, 7 in. by 7; L, post, 8 in. by 9; M, hammer beam, 14 in. by 91; N, brace, 8 in. by 8; P, post in the wall; QQQ, purline rafters, 4 in. by 6. 2047. Fig. 697. is a section of the roof to the chapel of Greenwich Hospital, constructed by Samuel Wyatt, about 1785. The span is 51 ft., and as a variety from the general forms of roofs, it is worth the student's attention. The scantlings of the timbers are subjoined; the distance between the trusses is about 7 feet, and the king-posts are of iron. All the joints are well secured with iron straps. AA, tie beam, whose whole length is 57 ft., 51 ft. clear between the walls, 14 in. by 12 in.; B, an iron king-post, 2 in. square; CC, queen-posts, 9 in. by 12; DDDD, struts, 9 in. by 7; E, straining beam, 10 in. by 7; F, straining piece, 6 in. by 7; GG, GG, principal rafters, 10 in. by 7; hhhh, &c. purline rafters for boarding upon instead of rafters; H, a camber beam, supporting the platform. 2048. Fig. 698. exhibits the roof of the old Drury Lane Theatre, which was built in 1793. It possesses great merit, from the simplicity of its composition and the accommodation afforded in the middle space for the carpenters and painters. By dividing the breadth of the building into three parts, the roof was kept low, and the scantlings much reduced in size. The span is 80 ft. 3 in., the trusses were 15 ft. apart, and the whole length of the roof was 200 ft. It was destroyed by fire on the 24th of February, 1809. The scantlings of the timbers were as follow: - A, beams, 12 in. by 7; B, principal rafters, 7 in. thick; C, king-posts, 12 in. by 7; D, struts, 5 in. by 7; E, purlines, 9 in. by 5; F, ridge pieces, in. thick; G, pole plates, 5 in. by 5; H, gutter plates framed into beams, 12 in. by 6; I, common rafters, 5 in. and 4 in. by 21; K, beams, 15 in. by 12; L, posts, 15 in. by 12: M, principal braces, 14 in. by 12 and 12; N, struts, 8 in, by 12; 0, oak trusses to the middle bearing of beams, 51 in. by 41; P, straining beams, 12 in. by 12. B F 2049. The last example we shall present is of the method in which the external dome of St. Paul's is framed (fig. 699.). The internal dome Aa is of brickwork, two bricks thick, having, at every five feet, as it rises, a course consisting of bricks eighteen inches long, which serves to bind the whole thickness together. This dome was turned upon a centre, which rested upon the projection at its springing, without any support from below, and was afterwards left for the use of the painter. It was banded together with iron at the springing. Exterior to the brick dome (which has indeed, nothing immediately to do with the subject) is a cone of brickwork BBb, 1 foot 6 inches in thickness, plastered and painted, part whereof is seen from the pavement under the cupola through the opening a. On this cone BBb is supported the timber work which carries the external dome, whose hammer beams CC, DD, EE, FF are tied into the corbels G, H, I, K with iron cramps, which are well bedded into the corbals with lead, and bolted to the hammer beams. The stairs which lead to the Golden Gallery on the top of the dome are carried between the trusses of the roof. The dome is boarded from the base upwards, hence the ribs are fixed horizontally at near distances to each other. The scantling of the curve rib of the truss is 10 in. by 11 at the bottom, and 6 in. by 6 at the top. The sides of the dome are segments of circles, whose centres are marked in the figure; and which, if continued, would meet at top, and form a pointed arch. Above the dome rises a lantern of Portland stone, about 21 feet in diameter, and 64 feet high, standing on the cone. The whole of this construction is manifest from the figure, which exhibits the inner and outer domes with the cone between them. The combination is altogether an admirable example of the mathematical skill and judgment of Sir C. Wren. سيليا 52 ft. B A 10 20 30 ft. Fig. 699. 2050. The largest roof that was, perhaps, ever executed, was over a riding-house at Moscow, built in 1790, by Paul I. Emperor of Russia, the representation whereof may be seen in Kraaft, Recueil de Charpente. The span is 235 feet, and the slope with the horizon about 19 degrees. The external dimensions of the building were 1920 feet long by 310 feet wide. It was lighted by a lantern at top, and had an interior gallery round the building for spectators. The contrivance is exceedingly ingenious; but, from the great extent of the span, considerable settlement took place, and alterations, or rather strengthening ribs, became necessary. 2051. We shall close this part of the section with a diagram (fig. 700.) of the roof of the basilica of S. Paolo fuorì le murà, executed in the fifteenth century. The trusses are double, each consisting of two similar frames, nearly 15 inches apart, at intervals from each other of about 10 feet 6 inches. The principal rafters abut on a short-king post k. Between the trusses a piece of timber S is placed and sustained by a strong key of wood passing through it and the short king-posts. This piece sustains the beams by means of another strong key at a. The tie beams are in two lengths, and scarfed together, the scarf being held together by three iron straps. The scantlings of the timbers are as follow: beams t, 22 in. full by nearly 15 in.; principal rafters p, 21 in. by nearly 15 in.; auxiliary rafters b, full 133 in. by full 134 in.; straining beam C, near 15 in. by full 12 in.; purlines d, 8 in. square and 5 ft. 7 in. apart; common rafters, full 54 in. by 4 in., and 8 in. apart. The roof, which is constructed of fir, is nearly 78 ft. 6 in. span, and is covered with the Roman tile, the exact dimensions and form whereof will be found, under the head TILE, in the Glossary appended to this work. The roof is ingeniously and well contrived, and, with a different covering, would suit other climates. It was consumed by fire about two or three and twenty years ago. (275.) Philibert Delorme, in his work entitled "Nouvelles Inventions pour bien bâtir a petits Frais," Paris, 1561, gives a mode of constructing domes without horizontal cross ties, when the springing of each rib is well secured at the foot. It is a very simple method, and of great use in domes, even of large diameter, the principle being that of making the several ribs in two or more thicknesses, which are cut to the curve in lengths not so great as to weaken the timber, and securing these well together by bolts or keys, and observing especially to break the joints of the several thicknesses. This method was adopted in the large Halle aux bleds at Paris, which was many years since destroyed by fire, and has been replaced by an iron-ribbed dome. The fig. 701. will explain the construction; and, if necessary, an iron hoop passed round at different heights will add much to the strength. 2052. The scantlings of the ribs, as given by Delorme, are as under :For domes of 24 feet diameter, the ribs to be 8 in. deep, and 1 in. thick. 36 feet diameter, 60 feet diameter, 108 feet diameter, Fig. 701. 10 in. deep, and 1 in. thick. 13 in. deep, and 2 in. thick. The work of the author from which we have given this short and summary account deserves the study of every one that seeks to be an architect, though in these unfortunate days for the art the reward of study and reading is very doubtful; patronage being of much more importance to the professor than a profound knowledge of construction and design. . 2053. The following instructions relative to the lines necessary to be found in the framing of roofs are from Francis Price's British Carpenter; and though published long since, now nearly 100 years, we have not found that any subsequent work on this particular point gives us more information than is to be there found. Let abcd (fig. 702.) |