ENGLISH GOTHIC ROOFS

NEXT in importance to the construction of substantial walls comes the necessity of weather-proof and storm-proof roofs, such as should preserve the stability of the walls, or their disintegration would be hastened by that which should be their protection.

The nature of the covering and the action of weather are the principal factors affecting their design and construction. The rigours of the English climate require a covering to be such as to prevent the penetration of rain, and their support to be strong enough to resist the pressure of snow and the hurricane. The high-pitched roof (of steep inclination) is common to all periods of English Gothic. The average Norman roof was pitched about 45°, its apex being about a right angle composed agreeably with the semicircular arches in the gables. The higher-pitched roofs came simultaneously with the introduction of the pointed arch, sometimes at a pitch of 60°, ultimately declining to about 20° or less in the late Perpendicular period. The outer covering is generally of boards overlaid with tiles or sheet-lead, the latter being imperative in the low-pitched roofs. The whole of the covering is carried by common rafters or spars rising from the walls to the apex of the roof.

Plate X., Figs. 1 and 2, show two ordinary principals or trusses. These are in no sense Gothic, but are here given to more clearly explain the principles of roof construction. The common rafters (c.r.) bearing the covering are of light timber, tending to bend under the weight; to prevent this purlins (P.), stout beams, are placed at suitable intervals, and these are carried at their ends by the roof trusses. The tie-beam (t.b) is the chief beam of the truss. The principal rafters (p.r.) are framed into it and into the heads of the King and Queen posts. In the Queen-post-truss the collar unites the Queen posts; struts and cross-braces complete the structure. In a properly constructed roof-truss all the stresses are neutralised in the truss itself, and the whole framework rests as a dead weight upon the walls without any lateral thrust to force them out of the vertical. The King-post-truss is suitable to roofs up to 30 feet span, the Queen-post-truss to 40 feet.

Open timber roofs have their construction visible from the interior. Plate X., Fig. 8, shows a crude Queen-post-truss in an old Worcestershire church, in which the tie-beam has been chosen from a bent log so as to prevent its bending under the roof load. In the trusses (Plate X., Figs. 1, 2) the King and Queen posts act like the keystones of an arch, so that by bolting or strapping up with ironwork at points s.s. these posts are put into a state of tension, the tie-beam (t.b.) is pulled up to a camber, or curve, and is also in tension. In the crude Queen-post-truss (Plate X., Fig. 8) these conditions are reversed, for the tie-beam supports the Queen posts. In Plate X., Figs. 1 and 2, all the spaces in these trusses are triangular, a fact which ensures stability where the parts are of proper strength and properly united. Plate X., Fig. 4, is a trussed rafter roof requiring no purlins nor principal—each common rafter is a truss. This kind of roof is suitable only to small spans; its weakest part is from a. to b.

In the roof-truss, or principal, the Gothic architects objected to the tie-beam as an obstruction to the sense of loftiness, so desirable in the church interior, and therefore invented the hammer-beam principal (Plate X., Fig. 7), which resembles a Queen-post-truss, having a collar and King post. The hammer-beams (h.b.) are substitutes for the tie-beam.

In this principal the space between the collar and the apex of the roof is satisfactorily trussed, but from the collar downwards the Queen posts (Q.p.) and wall posts (w.p.) with their braces (b.) become mere brackets supporting the small King-post-truss above, bringing its load as low as possible on to the walls, so that their weight of masonry and buttresses may effectually resist the lateral thrust of the roof upon them.

Plate X., Fig. 5, shows a collar principal with curved braces. The tendency of all roof principals is to spread at the walls; this has been met by modern church-builders by introducing an iron tie-rod in place of a tie-beam, thus forming a triangle, the only form of absolute stability, for the whole of the principal, as indicated by the dotted line connecting the hammer-beams in Plate X., Fig. 7.

Plate X., Fig. 6, shows a low-pitched Perpendicular roof with principals and half-principal. The common rafters are concealed by a panelled ceiling, the panels being formed by principal rafters and purlins. The arched trusses bring part of the thrust low down on to the walls, which are strengthened by heavy buttresses on the outside. The half-principal is placed over a window arch.

Plate X., Fig. 3, shows a low-pitched roof and ceiling supported by a tie-beam only, strengthened at its bearings by wall posts and braces.