Page images
PDF
EPUB
[blocks in formation]

the sides and ends. Where a hipped roof covers a perfectly square building, the faces all meet in a point, and form a pyramid; but when, as in the diagram, the plan of the roof is oblong, the planes rising from the nearest opposite walls meet in a ridge. Sometimes the inclined faces are not continued upwards till they meet, but the roof is completed by a horizontal plane. Such a roof is called a truncated, terrace, or cut roof, and may have two, three, or four in clined faces. Fig. 3 represents a truncated roof hipped at one end, and terminating at the other in a vertical wall, like the gable-ended roof. Fig. 3.

[blocks in formation]

This arrangement is useful in diminishing the height of a roof, the level platform being covered with lead to compensate for the want of slope. It should be observed however that even this part is not perfectly level, the centre being slightly elevated to throw off water. A similar saving of height is frequently obtained by means of a roof in which each sloping face consists of two planes of different degrees of inclination. This form, which is denominated a curb roof (or, from its inventor, a Mansarde roof), is very common in London, because it affords more space for the formation of bedrooms in the roof than the simpler forms. A curb roof may be hipped or not, according to circumstances. Fig. 4 represents it hipped at one end only, as the last figure, showing, like the previous diagrams, the plan, and side and end Fig. 4.

elevations.

[blocks in formation]

tween roofs with dripping eaves, and those in which the water is collected in gutters. In the former case the roof projects several inches, or even feet, beyond the walls, and the water running from the roof either drops at once on the ground, or is collected in troughs fixed under the margin of the eaves, and conducted by them to descending pipes. This arrangement has a clumsy appearance, and is perhaps unnecessary where a sufficient projection is given to the eaves, though it is essential to the dryness of the walls when they are of the diminutive size often adopted by modern builders. In gutter roofs the timbers do not extend to the outside of the walls, which are carried up as parapets, of a reduced thickness, to such a height as to conceal the roof either wholly or partially. The gutters, which are troughs of wood covered with lead or other metal, are laid at the bottom of the slopes, just within the parapets, and have a gentle inclination (usually about an inch in ten feet), to cause water to run freely towards the pipes. In extensive the elevated end of the gutter may cover as little of the roof roofs it is well to use two or more falls instead of one, that as need be. Similar troughs are often used in the valleys. Gutters are generally made wide enough for a man to walk along them, and should be sufficiently capacious to avoid all risk of overflowing during a sudden heavy fall of rain.

The degree of slope given to the inclined faces of a roof varies according to the covering material employed, as well as to the climate. The antient Grecian temples had very low, or pediment roofs, varying from about 12° to about 16°, the height being from one-ninth to one-seventh of the span. In Roman buildings the inclination is somewhat greater, being usually 23° or 24°, or from one-fifth to two-ninths of the span. The general introduction of the pointed style of architecture led to the use of very high-pitched roofs, a very common proportion being that in which the length of the rafters is the same as the span, so that they formed an equilateral triangle. In comparatively modern domestic architecture in this country, it has been considered desirable for the length of the rafters to be three-fourths that of the span, and an angle of 45° is still considered by some to be the best pitch when plain tiles are used. As builders can, in the present day, obtain excellent covering materials, the pitch may be made of any required degree, down to the low architecture and the taste of the builder; the most common Grecian pediment, and it therefore depends on the style of height being from one-fourth to one-third of the span. High roofs discharge rain the most rapidly, and do not re tain snow so much as those of low pitch; but where they have gutters they are liable to become choked by snow sliding into them, and to overflow from water running into them faster than the pipes can convey it away. Steep roofs may be covered with small slates, and are less likely to be stripped by violent winds. Low roofs, in consequence of their superior lightness, are less expensive, the timbers not only being shorter, but of proportionately smaller scantling, and they press less injuriously on the walls. The following table, extracted from Tredgold's 'Elementary Principles of Carpentry,' shows the proper angle for roofs covered with the materials specified in the first column, the last column indicating the comparative weight of each kind of covering:

[blocks in formation]

Fig. 6.

[merged small][merged small][merged small][merged small][merged small][ocr errors]

copper 100

{lead... 700

1120

900 to 500

2380

1780

650

or the junction of two planes in such a manner as to form hollows the reverse of hips. When two faces of a roof join so as to form an angle similar to a valley, but in an horizontal instead of an inclined position, the term gutter is applied instead of valley.

A further distinction, which it may be well to mention before entering upon the details of construction, is that be

In describing the timber-work of an ordinary roof, each of the planes of which it is composed may be considered to be bounded by a frame, the parts of which have the general name of bordering pieces. Those which join the wall are the wall-plates; that at the meeting of two faces, parallel to the wall-plates, is the ridge-piece; and the inclined bars extend ing from the wall-plates to the ridge-piece are rafters, those which form the salient angles in hipped roofs being distinguished as hip-rufters. The support necessary for the external covering is given by a series of rafters or inclined bars, extending from the wall-plates to the ridge-piece, and placed • A square of roofing contains 100 square feet.

parallel with each other at equal distances. In a hipped roof, the rafters near the ends, being parallel with the others, are necessarily diminished in length, extending from the wall-plate to the hip-rafter instead of the ridge-piece. All such pieces, being shorter than the length between the wallplate and the ridge-piece, are called jack rafters.

It is not usual to vary the scantling, or transverse dimensions of rafters, in any considerable degree, on account of their various lengths; nearly the same scantling being used in all buildings, and the required strength being obtained by introducing intermediate supports between the wallplates and ridge-piece where the size of the roof renders such necessary. This additional support is supplied by horizontal rectangular bars called purlins, placed under the rafters in such a manner as to divide their length into two or more equal parts, the ends of the purlins being fixed to the sides of the bordering frame. Like the rafters, the purlins are not much varied in thickness according to the strain upon them, but they are in turn supported by a series of bars placed equidistant from each other, and parallel with the rafters, but with their upper face in the same plane as the lower face of the purlins. These are called principal rafters, or, for brevity, principals, to distinguish them from the first described, or common rafters. Where it is desirable to save room by reducing the thickness of a

roof, the purlins may, as shown in fig. 15, be notched into the principals and common rafters, but this practice is not to be recommended, as it weakens the timbers. Where principals are used, their lower ends are mortised into the ends of a tie-beam, which stretches across the building, and rests upon the wall-plates. This beam keeps the lower ex| tremities of the principals from separating, and discharges the weight of the roof on the walls in a vertical direction, relieving them entirely from the lateral thrust of the rafters. The triangular frame formed by the two principals and a tie-beam, with any bars it may comprise for additional strength, is called a truss, and such frames being placed at regular intervals, the timber-work between any two of them is called a bay of roofing. The lower extremities of the common rafters, being elevated by this arrangement above the wall-plates, are supported by pole-plates, or pieces of timber parallel to the wall-plates, resting on the ends of the tie-beams. The supporting frame-work altogether is called a carcass-roof.

Fig. 7, which represents a small carcass-roof supported by four trusses, and having one purlin only between the wall-plate and ridge-piece, may assist the reader in comprehending the arrangement of the parts enumerated; and their names will be found more distinctly by referring to the representation of a more complicated truss at fig. 11.

[graphic][subsumed]

In this figure the common rafters are represented on one half of the roof only, that the trusses may be more distinctly seen; and the end walls are omitted for the same

reason.

The proper construction of the trusses of a roof, with reference to the size of the building and the weight of the covering, is a matter requiring much scientific knowledge. For the want of this it is not unusual to encumber trusses with much more timber than is necessary or useful; and the disadvantage of this is not confined to the increased weight and cost of the roof, as superabundant timbers frequently occasion injurious strains, and the increased number of joints adds to the risk of derangement by the shrinking and warping common to all timber constructions. The general principles to be acted upon may be illustrated by a few diagrams; but in the limited space devoted to this article no attempt can be made to describe all the modifications required by the ever-varying forms of buildings; in the design of which it is too common, instead of assigning its due importance to the roof, to treat it as an unsightly feature, to be concealed as much as possible from view.

In a roof formed as shown in fig. 8, consisting simply of two inclined planes abutting on the walls, it is evident that the weight of the rafters ab and bc, as well as that of the covering sustained by them, will have a tendency to thrust out the walls. This tendency ordinary walls have not the

Fig. 8.

Fig. 9.

strength to resist, and therefore it becomes necessary to add the beam ac (fig. 9), which, by receiving the outward thrust

of the rafters, relieves the walls of lateral strain. If the tension of the tie-beam ac be sufficient to resist the extending force of the rafters without sensible elongation, the only effect that such a roof can have upon the walls is a vertical pressure on each, equal to half its weight; and it cannot fall without the tie-beam, which acts the part of a cord or chain, being pulled asunder, or the rafters being crushed. If the materials were perfectly rigid, no additional parts would be required; but as they are not so in practice, it becomes necessary, when the timbers are of considerable length, to provide means for counteracting their tendency to sinking, or sagging. By adding a bar shaped like bd (fig. 10), the centre of the tie-beam may be suspended from the crown of the roof. This piece is called a king-post, but the

Fig. 10.

b

d

name is perhaps not a good one, as, though it appears like a post to support the ridge or crown of the roof, it is in reality a tie, supported by it, and sustaining, instead of resting upon, the centre of the tie-beam. By cutting the king-post out of a piece of wood of larger scantling than the shank of the post itself, projections of the shape indicated in the cut may be formed at its ends. These are called joggles, and those at the upper end form a wedge between the heads of the rafters, like the key-stone of an arch. It is evident that a weight pressing on the projecting joggles at the base of the king-post will be by it transmitted to the

crown of the roof. These therefore form fixed points, from which support may be obtained, by means of struts or braces, e and f, for the centre of each rafter. Where purlins are added, they rest on those points of the principal rafters that are thus supported by struts, as may be seen by reference to fig. 7. It may be observed that this truss consists of two pieces (the tie-beam and king-post) in a state of tension, and four (the two rafters and the two struts) in a state of compression; and that in every well-contrived truss, however the number of its component parts may be increased, every bar is in one or other of these states. Those parts which are in a state of tension, acting merely as cords to bind the truss together, may be and sometimes are

formed of slender rods of wrought-iron; but the others, needing stiffness as well as cohesion, require bars of considerable substance, and are therefore mostly formed of wood or cast-iron. Sometimes the king-post is dispensed with, and its office performed by two similar posts, called queen-posts, at equal distances from the centre of the truss. In order to keep these in their right position, a short horizontal beam, called a collar-beam, is inserted between their upper extremities, and another, termed a straining sill, between their lower ends. This arrangement is explained by fig. 11, which also shows the position of other parts of a truss. One side is represented as a gutter-roof, and the other with eaves.

[graphic]
[ocr errors]

aa, Wall-plates b, Tie-beam.cc, Principal rafters. dddd, Purlins. ee, Pole-plates. ff. Common rafters. g, King post.
h, Collar-beam. ii, Queen-posts. k, Straining-sill. 11, Struts or braces. mm, Auxiliary rafters. n, Ridge-piece.

The auxiliary or cushion rafters, m, m, are pieces occasionally added, in large roofs, to strengthen the principals; and they, with the collar-beam, &c., form a complete truss within them. The trusses of truncated roofs are formed in this manner, the collar-beam forming, as it were, the keystone of the arch, and being surmounted by a camber-beam, the upper edge of which is formed into two slightly inclined planes, to give the necessary slope to the lead covering. In such a roof, pieces of wood resembling ridge-pieces are inserted at the angles formed by the meeting of the rafters with the horizontal bars that support the flat.

The following representation of a very simple truss, from Nicholson's Carpenter and Joiner's Companion,' illustrates the use of slender king-posts and queen-posts of wroughtiron, and shows how the stress of every part of the roof may be brought to bear on the ridge. The lower ends of the struts rest in stirrups attached to the vertical rods, and the weight

Fig. 12.

a

bearing on the strut a is imparted, through b and c, to the king-post. The tie-beam is suspended by bolts from each of the vertical rods, and the ends of the rafters are secured to the tie-beam by iron straps passing round them, and bolted to the beam at d, d. Trusses on the same principle may be made of timber only.

In curb roofs the upper rows of rafters are called curbrafters, and the horizontal bars that receive the upper ends of the lower rafters, and the feet of the curb-rafters, are known as curb-plates. The proper position of equilibrium for the rafters of a curb-roof may be ascertained by very simple means, within the reach of persons not possessed of sufficient mathematical knowledge for determining it by calculation. If the rafters are to be equally loaded, as in a roof entirely covered with one material, this position will be exactly the reverse of that which they would take by gravity, were they suspended in a chain or festoon, the joints being flexible. If they are framed together in this position of equilibrium, they will balance each other like the stones of an arch; and the tie-beams, posts, and braces will have no other office to perform than that of resisting such irregular strains as might tend to alter their arrangement. rafters thus suspended would fall into the position abcde, fg. 13, a line drawn through the angles being a catenarian P. C., No. 1245

The

curve; and a'b'c'd'e', in the same figure, represents the corresponding position in which they should be placed in an equally loaded roof. If the rafters bc and c' d' are to

Fig. 13.

Fig. 14.

a

b

bear a greater weight than a'b' and d'e', they will, if proportionately loaded when suspended in a curve, fall in such

a way as to increase the angles abc and cde, and diminish bcd, thereby indicating their proper position in the roof. When the roof is to be loaded unequally, and more on one side of the ridge than the other, as it would be if b'c' were to be covered with lead, and the other planes with slates, a corresponding weight added to the centre of gravity of be will cause the bars to arrange themselves as abcde, fig. 14, the angles of which, being transferred to the roof, give the position of equilibrium a'b'c'd'e'. This practical method of finding the proper angles of a curb-roof may be applied under all circumstances, the dimensions of the experimental bars being proportionate to those of the rafters, and their centres of gravity being loaded according to the pressure to be sustained by each plane of the roof. The great advantage of curb-roofs consists in the space they afford for chambers in the roof, such chambers being lighted by dormer windows in the lower inclined faces. When the trusses of the roof form partitions between the bed-rooms, their posts and braces are so arranged as to leave one or more doorways for communication between them.

In roofs of very large span it is often desirable, in order to avoid running up to a great height, to form two or more ridges. When intermediate support can be obtained from partition walls, such constructions may be regarded as combinations of two or more distinct roofs placed side by side. Fig. 15 is an example of a roof of large span without any intermediate support, and having a large available space between the tie and collar beams. It represents the form of the trusses, which were placed fifteen feet apart, of a roof of eighty feet span, erected over Drury-Lane Theatre in 1793.

It is sometimes necessary, in order to obtain additional height inside a building, to raise the tie-beam above th VOL. XX.-U

Fig. 15.

ing of two pieces, one on each side of the rib, notched to it and the beam, and fastened by bolts and straps.

Fig. 18.

level of the top of the walls. In small spans this may be done by the simple arrangement called the carpenter's boast (A, Fig. 16), in which a firm union is effected between the beam and the rafters without the use of nails or pins. Such a roof can only press injuriously on the walls by the rafters sinking into a concave form, which however their lower ends are very liable to do. In such a case additional strength may be obtained by inserting a longitudinal truss, as in B, Fig. 16, where c represents the end of the truss,

Fig. 16. B

which should be firmly built into the gables. d and e are side views of two longitudinal trusses suitable for such a situation, the first being stiffened by an arch of iron notched into the short vertical pieces, and the second formed of timber only. Similar trusses are occasionally introduced under the purlins. Roofs without ties may be greatly strengthened by the use of parabolic curves of iron, notched into the rafters of each inclined face, and abutting on the wallplates, which in such a case are firmly bolted together. The timbers of such a roof may be framed together in planes, each having a distinct ridge-piece, and the ridges being screwed or otherwise firmly connected together. The curves may be cast in short segments, as they are compressed when in use, it being merely necessary to provide that the joints should always abut on a rafter. Tredgold, in his Elementary Principles of Carpentry,' recommends the use of similar curves, of either wood or iron, in the trusses of an ordinary roof, by which the derangement often arising from the shrinking of the king-posts and queen-posts may be avoided. In this case the curves take the place of the principal rafters, and, if made of wood, may be constructed of short straight pieces, arranged as shown in Fig. 17, and held together by bolts or wooden keys. When curved tim

Fig. 17.

ber can be obtained it is to be preferred, as it reduces the number of joints. For small roofs timbers may be bent into the required form, as it is found that a piece of wood the thickness of which does not exceed th part of its length, may be bent into a curve rising one-eighth of its span without impairing its elasticity. Two such pieces may be laid together, and bent by twisting a rope attached to their ends, as is done in tightening the frame of a bow or pit saw; and, being bolted together while curved, they will spring back but little when the rope is relaxed. Another mode of forming such a rib is to take a piece of wood whose thickness is about one-sixtieth of its length, and cutting along the middle with a thin saw from each end, leaving about eight feet in the centre solid. The beam may then be bent, and bolted or pinned together as before described. In either case the rib should be bent about onefourth more than it is intended to remain, to allow for springing back. A parabolic curve is the form most recommended; but a circular arc, rising half the height of the roof, will answer the purpose. Fig. 18 represents the truss of a truncated roof strengthened by a curved rib, the suspending pieces being, when the rib is formed in the manner first described, placed at each joint, and each consist

One of the advantages of this mode of construction is that the tie-beams may be suspended from any number of points, which is important in large spans, where the beams have to be formed of several pieces scarfed together. [SCARFING.] Diagonal braces, though unnecessary with parabolic curves, may be added to meet accidental strains, as shown by the dotted lines in the cut. This principle of construction, with an arc composed of several pieces of timber, was followed in one of the largest roofs ever built, that erected in 1791 over a riding-house at Moscow. The span of this roof, which has been said to be the most extensive in the world, is stated by Tredgold at 235 feet, the slope being about 19°, and the external dimensions of the building 1920 by 310 feet. He states that it had sunk so much that it was proposed to add a second curve for additional strength.

A simple and economical roof, invented by Mr. A. H. Holdsworth, and rewarded by the Society of Arts in 1820, is supported by curved ribs of timber applied in a different manner. A detailed description is given in the 38th volume of the Society's 'Transactions,' but Fig. 19 will sufficiently explain the principle of its construction a is a beam

Fig. 19.

a

serving as a tie-beam, and also to support the upper floor of the building; b b are curved ribs, formed in a similar manner to those just described, the lower ends of which are firmly secured to the tie-beam a. The principal rafters rest on these ribs, and their lower ends bear upon short timbers resting on the walls, these pieces being fastened by strong iron straps to the curved ribs, to counteract the outward thrust of the rafters. By this arrangement the whole of the interior of the roof, which is usually encumbered with kingposts, queen-posts, braces, &c., is rendered available for useful purposes, in addition to which it effects a considerable saving of timber.

Wrought-iron straps of various forms are very useful, when judiciously applied, in strengthening the joints of a roof. They should be fixed with regard to the unavoidable tendency of the timbers to shrinking, so that while they may, in some cases, counteract or lessen its effect, they may so far yield to it as to prevent a strain which should come upon a timber, being entirely thrown, by its alteration of form, upon the strap. Tie-beams are often suspended to the trussing-posts by means of straps, so arranged as to allow the beam to be keyed up to its true position in case of the roof sinking. When this is not the case, the ties are sometimes drawn up into a slightly convex or cambered form, to meet the same contingency. Height may be gained inside a building by disposing the timbers as in fig. 20, the want of a continuous tie-beam being compensated

Fig. 20.

a

for by an iron strap to unite the ties to the bottom of the king-post at a; but it is evident that the safety of the plan must depend wholly on the straps, which alone counteract the outward thrust of the rafters.

In 100fing a church with a nave and side aisles, the con- | tinuity of the tie-beams may be dispensed with, intermediate support being obtained from columns. It is however necessary to guard carefully against any lateral strain to

the columns.

Many of the high-pitched roofs of old Gothic churches and halls are very ingeniously contrived, but they often throw great pressure on the walls, owing to the absence or elevated position of the ties; thereby rendering very solid walls and buttresses necessary. The Norman roof is an ingenious contrivance for the construction of roofs of large span with small pieces of wood. Fig. 21 shows this arrangement, in which all the rafters abut on joggled king-posts, of which there are several, their relative position being maintained by diagonal braces. The timbers of this kind

Fig. 21.

of roof are often left visible, being so carved as to have an ornamental effect. Such a roof may be made to exert very little injurious pressure on the walls.

When the space covered in is of an irregular shape, it is best to arrange the inclined planes of the roof in a similar manner to those of a rectangular building, leaving a level platform in the centre, corresponding to the plan of the inclosed space. Where the space covered is circular, elliptical, or polygonal, although the construction of the roof may appear more complicated to the eye, it is, in fact, simpler and easier than that of a quadrangular building, the strain of the roof being more equally distributed. The nearer a roof approaches to a circle in plan, the stronger it will be, the parts deriving that mutual support from each other which forms the distinguishing character of the dome. Domes of wood, of great size, have been made without trussing, simply by forming the timbers into curved ribs abutting on the wall-plates, which then form a circle, and kept in their proper positions by horizontal circles framed with them at intervals. As the ribs approach the upper part of the dome, the intervals between them diminish in width, to allow for which every second or third rib is discontinued at intervals, the ends of the ribs thus discontinued being received by the horizontal circles, which may be compared to purlins, the ribs taking the place of rafters. The wooden dome formerly existing at the Halle aux Blés, at Paris, was a remarkably bold example of this kind, being 200 feet in diameter, and having a large opening in the centre. It was built at the suggestion of M. Moulineau, and, having been destroyed by fire, has been replaced by a similar structure of iron, but of smaller dimensions.

When the roof approaches the circular form, but not sufficiently to have the character of a dome, it may be considered as consisting of several trusses resembling those of an ordinary roof, but so contrived as to intersect each other in the centre; the king-post being common to all the trusses. Fig. 22, representing a design for a polygonal roof, from Nicholson, may illustrate this, and exemplify also some of the applications of iron straps a shows the form of the strap by which the ties are secured to the kingpost; the post having as many faces, and the strap as many arms, as there are trusses in the roof.

Fig. 22

*

Though the number of contrivances for the construction of roofs is very great, as may be seen by reference to various

works on carpentry, allusion can here be made to only one other. It is an admirably simple plan for making a very flat roof, described in the 37th volume of the 'Transactions' of the Society of Arts, in a communication from the inventor, Mr. Smart. The beams or rafters are cut, with a circular saw, as shown at a, fig. 23, while b represents their form when in use, a wedge being inserted between the ends of the parts that are elevated into a sloping position. These may be raised to an angle of 10° or 12°, and will bear a great

Fig. 23.

a

weight, as they cannot be depressed without thrusting off the ends of the beam, or breaking the lower part of it by tension. This is called, by the inventor, the bow and string rafter, and was used by him to support a roof at the Ordnance Wharf, Westminster Bridge. Strong laths were nailed upon the rafters, and on these a platform of bricks was laid in cement, the whole being covered with tiles also bedded and pointed with cement, and twice coated with hot linseed-oil. The cost of this roof is stated to be not more than half that of lead. For a further notice of the experiments of the inventor of this simple truss, see TRUSSING.

In the valuable practical works of Nicholson, Tredgold, &c. the methods of calculating the strength necessary in the various parts of a roof may be found; and in the Prin ciples of Carpentry,' by the latter author, tables are given of the dimensions suitable for different spans. The table here quoted refers to a roof similar to fig. 7; the trusses being not more than ten feet apart, and the pitch at an angle of about 27° with the horizon, for a covering of slates. The scantlings are suited for yellow fir, and must be some what increased for timber of inferior quality.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

For the strength of different materials, under various circumstances, the reader may consult MATERIALS, STRENGTH OF, vol. xv., p. 8. As a general remark, it may be observed that oak, when exposed to tension, is weaker than fir, and is therefore less adapted for ties. Being however less compressible, it is usually preferred for rafters, straining pieces, and struts; but Tredgold observes that its greater tendency to warping in summer renders it less fit for rafters and purlins than foreign fir. Cast-iron is not much used, except in fire-proof roofs, and each piece requires to be well tested. Wrought-iron is very useful for straps and fastenings, and also for ties and trussing-posts; but care is always necessary to guard against imperfections, which are more likely to pass unobserved than in wood. Wherever iron is applied, provision should be made for its expansion and contraction, and it is desirable to protect it from oxidation by painting. Though iron is far stronger for its size than any kind of timber, it is neither so strong nor so cheap as yellow fir, weight for weight.

The joints in the frame-work of a timber roof are of various kinds, according to the nature of the strain they have to resist. They should be formed with great care, and with due regard to such probable changes of form as all constructions of timbers are liable to from shrinking and warping. Cocking or cogging is the name given to that kind of joining in which one piece of timber, in a state of tension, is so attached to another that it cannot be drawn away without one piece breaking. Figs. 24 and 25 represent two methods of cocking the ends of tie-beams on the wall-plates, giving a plan and elevation of each. In both figures a represents the beam, and b the wall-plate. In the first plan, which was formerly much practised, the contraction of the dove-tailed end of the beam would allow it to be

« PreviousContinue »