The later type of pitched-roof commences, at its apex, with a longitudinal beam known as the ridge-purlin, or ridge, from which sloping battens are carried down to the tops of the outer walls, where they are notched into long timbers fixed thereon, known as wall-plates. These battens, which form the skeleton sides of the roof, are called the common rafters.2 Where, for greater strength, some of these rafters are made thicker than the others, at regular intervals, they are known as principal rafters, or principals. Should the rafters be of such length that they are likely to sag, they are supported, generally at half their length, by longitudinal beams, or purlins, running parallel with the ridge-purlin. A roof without either principals or purlins is termed a single-framed roof; with both principals and purlins, it is known as double-framed.
1 Brookland Church, near Romney in Kent (see small illustration on page 60), is a good instance of where the thrust of the nave roof has pushed both the outer walls and the aisle columns out of the perpendicular.
2 The earliest type of pitched roof has the rafters halved together or " finger-jointed " and pegged at the apex, without ridge-purlin. This type is known as a coupled rafter-roof.
A roof such as the one described above would have two elements of weakness; it would be liable to sag in its length from its ridge and down its outside faces, and excessive wind pressure would tend to push it, together with its wall-plates, either off the supporting walls, or to collapse the two sides together. To correct this tendency to close up, or flatten out, - it is usual to fix beams across the short span. If these ties are fixed at the level of the wall-plates, they are known as tie-beams;1 if between the principals at a short distance from the ridge, they are known as collar-beams or collars. If it be desired to support the ridge-purlin still further, posts are fixed from the top of the tie-beam, or the collar, to the under side of the ridge. When these posts are central with the tie-beams, that is, when they are fixed directly under the ridge-purlin, they are known as king-posts. Where they are fixed one on either side of the centre of the ridge, into the principals, and at the other end into the tie-beam or the collar, they are known as queen-posts.
To minimise the wind-strain on the sides of a high-pitched roof, and to remove the tendency of the entire roof being pushed off the wide walls, vertical posts are tenoned into the tie-beam or principal and carried down to the wall, on to stone brackets or corbels. A roof with straight beams across its shortest span, reinforced by wall-posts, is known as a post-and-beam roof.2 With side walls weakened by the insertion of many windows, these wall-posts are very necessary to carry the thrust of the roof below the wall-plate level.
A pitched roof may be either high or low. One formed entirely of cambered tie-beams, with the top camber increased by "firring-pieces," or long wedge-shaped battens fixed to the top of the tie-beams to increase their slope, is known as a firred-beam roof. Its pitch is, obviously, a low one.
Where a beam or collar is reinforced by a short piece of timber fixed bracket-wise, one end into its under side at an angle of approximately 45 degrees, and the other into a principal or a wall-plate, such reinforcing piece is known as a brace. When this brace is cut in the shape of a segment of a circle or an oval, it is known as an arch-brace.
1 Also known as main collar-beams.
2 The term is also used to signify a tie-beam roof with cither king- or queen-posts above.
A series of beams projecting, horizontally, into the interior of the hall or room, either from the wall-head or from the principal rafter at a higher level, acting as cantilevers in supporting posts or braces, and thereby relieving the wall-plates of some of the thrust, constitutes a hammer-beam roof. Where a single row only is fixed, at the wall-head, usually coinciding with each principal, but sometimes with each alternate one, the roof is known as a single hammer-beam. Where an upper row exists, above the first, tenoned into the principals at about purlin level, the roof is called a double hammer-beam.
Brookland Church, Kent. - An illustration of the effect of roof-thrust.
Fig. 45. Harmondsworth Barn, Middlesex. - Interior showing the roof timbers. Span 37 ft. 9 ins. Length 191ft. 8 ins. Width between posts, 18 ft. 1 in. Height 37 ft. - 3 ins. 13 trusses.
To act as parts of the construction, in their capacity as cantilevers, it is essential that the braces and posts strengthening the principals should be fixed almost at the ends of the projecting hammer-beams, bearing upon their upper surfaces. In some instances, however, the hammer-beams, especially the upper tier, are introduced merely for decorative effect, and the arch-braces bear only at the junction of the hammer-beams with the principals. These roofs are termed false hammer-beams. The hammer-beam 60 itself takes no strain, and fulfils no purpose; it merely projects into the air, uselessly.
Fig. 46. York Guild Hall. - A very rare type of a double-aisled roof with posts to the floor. - Mid-fifteenth century.
93 ft. long by 43 ft. span. About 30 ft. high.
Another variety of false hammer-beam, one which is not constructionally sound, is shown in Fig. 44, No. 21. This is known as the pendentive type. The roof at Eltham Palace is an example. Instead of the hammer-posts bearing on the hammer-beams, they are taken down beyond them, in decorative moulded finials, and the ends of the hammer-beams are tenoned into them. The support to the hammer-post, therefore, is not on the hammer-beam itself, but only on its tenon. It is obvious that this method is constructionally bad, as the Office of Works discovered when the Eltham Palace roof was recently restored and reinforced.