The Development

Fig. 44. OF THE ENGLISH TIMBI

24. Compound hammer-beam roof with large arch-ribs (Westminster Hall).

BER ROOF.

18. Hammer-beam roof with hammerposts and wall-posts. Both hammer-beams and collar are arch-braced.

21. False single-hammer-beam roof (pendentive) (Eltham Palace type). The hammer-posts bear on the tenons only of the hammer-beams, not on the beams themselves.

to allow of a slight fall to the gutters, the points of stability to be considered are only threefold.

  • 1) The walls must be strong enough to support the dead-weight of the roof.
  • 2) The beams and joists must be of such thickness that they will not sag.
  • 3) The ends of the beams, where they are housed into the wall, or where they rest upon its top, shall be efficiently protected against rot or decay.

It is obvious that 011 these beam-ends the stability of the whole roof depends. With a completely framed roof, the beams are mortised at their ends to reccive the' wall-plates, which arc laid on the wall-head.

With roofs of large span, the liability of the principal beams to sag, and thus to pull away the ends from the supporting walls, dictates the cambered beam, that is, one with either a natural or an artificial upward curve or bend in its length, or one which is deeper in the middle than at the ends. Such a beam, fixed with its concave side downwards (i.e. with its camber upwards), resists any tendency* to sag, in a very efficient manner. Examples of cambering will be noticed in the tie-beams illustrated in this chapter.

The outer covering of a flat roof, whether of lead, zinc or other material, is liable to perish by atmospheric action, or to be injured mechanically. Slates or tiles have been found to be more lasting, but their use necessitates the lean-to or the pitched roof. Tiles or slates, with their overlap, must be 011 a slope, otherwise the rain and snow will percolate, or be driven under the overlappings. Their use, therefore, dictates either the lean-to or the pitched roof, as a logical necessity.

Both these types of roof introduce a new principle, the necessity of resisting the downward and outward pressure, or thrust, which tends to force either the supporting walls out of perpendicular,1 or the roof itself off the walls. With the lean-to roof, the type largely used in the aisles of churches, this outward thrust is exercised on the one wall only ; with the pitched-roof it is thrown 011 both.

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, arc called the common rafters.2 Where, for greater strength, some of these rafters are

1 Brookland Church, near Romnev in Kent (see small illustration on page Go), 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.

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 likety 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.

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 arc 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

1 Also known as main collar-beams.

2 The term is also used to signify a tie-beam roof with either king- or queen-posts above.

HARMONDSWORTH BARN, MIDDLESEX.

Interior showing the roof timbers. Span 37 ft. y ins. Length 191ft. Sins. Width between posts, 18 ft. 1 in. Height 37 ft. 3 ins. 13 trusses.

BROOKLAND CHURCH, KENT

An illustration of the effect of roof-thrust.

HARMONDSWORTH BARN, MIDDLESEX.

Interior showing the roof timbers. Span 37 ft. y ins. Length 191ft. Sins. Width between posts, 18 ft. 1 in. Height 37 ft. 3 ins. 13 trusses.

at the wall-head, usMlly 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.

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, arc 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

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.

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, constitute s a hammer-beam roof. \\ here a single row only is fixed,

BROOKLAND CHURCH, KENT

An illustration of the effect of roof-thrust.

Hammer Beam Bent
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