English Church Architecture of the Middle Ages: An Elementary Handbook

by A. Freeman Smith

Plate X

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.

PLATE XI

GOTHIC VAULTS

PLATE XI

PLATE XII

GOTHIC VAULTS

PLATE XII

Plates XI and XII

GOTHIC VAULTS

STONE-VAULTED roofs became necessary in church building early in the Norman period for security against fire. They were made after the Roman manner, semicircular, with similar vaults intersecting at right angles. The lines of their intersections are the groins. When two intersecting vaults are of equal semicircles, each groin is a semi-ellipse. This groin is the weakest part of the vault; in order to strengthen it the Normans built an arch called the groin rib, underneath the groin, to support it. Difficulties were met in forming intersecting vaults of unequal span, clumsy contrivances were resorted to, until, in the Transition period, 1145-1190, the introduction of the pointed arch solved the problem, and led the way to the development of Gothic vaulting. The Roman and Norman vaults were built upon temporary centering. A centre is a timber frame made like a roof-truss shaped to the form of the arch; a series of centres were placed at convenient distances apart and covered with strong boarding upon which the vault was built. After the masonry was completely set, the temporary centering was removed, leaving the vault to carry itself.

As the system of vaulting developed, cross ribs and wall ribs ([Plate XI]., Fig. 3) were added, and much of the temporary centering was dispensed with; ornamental arrangements were designed by introducing more ribs, and the web—the covering surface of masonry—was reduced to small panels in the Decorated and Perpendicular periods. In the Tudor period, 1485-1558, the web became the principal part, the ribs being mere mouldings worked upon its surface in the form of fan-vaulting, a simple example of which can be studied in the south porch of Chester Cathedral, and the most elaborate in the roof of the Chapel of Henry VII., Westminster Abbey.

A bay of a cathedral is one of the spaces into which its length is divided by the supports of the roof as piers, arches, or principals. The bays of the aisles are usually square—those of the nave, choir, or transepts rectangular on account of their greater width.

Vertically each of the bays of the nave, etc., is divided into three stories ([Plate XI.], Fig. 2), the groundstory rising from the floor; the triforium, or blindstory, having no windows, is over the aisles, and the clerestory over the triforium. The prefix clere—bright—indicates the brilliancy of its light.

In the Norman period these three divisions were nearly equal in height. In the succeeding periods the groundstory attained about half of the total height of the bay, the clerestory was extended downwards, and the triforium reduced, until, in the Perpendicular period, it entirely disappeared.

[Plate XI.], Figs. 3 and 4, show a few square bays of Gothic vaulting in skeleton diagrams with the forms of plan indicated by dotted lines upon their base-planes. All the lines represent ribs.

[Plate XI.], Figs. 2 and 3, show quadripartite vaulting—i.e., having four compartments in one bay. This is the simplest form of Gothic vault, and belongs chiefly to the Early English period. The ribs ah, bh, ci, di are wall ribs; bg, cg, ag, dg, are cross ribs; ae, ce, be, de, are the diagonal ribs. The ridge-ribs eg and h, e, i, are horizontal, and intersect the summits of the cross ribs and diagonals. At every intersection there is generally a carved keystone or boss.

In the vaulting of a nave the breadth across is about twice the breadth of the aisles, so that the nave bays are not square, but rectangular.

[Plate XI.], Fig. 4, and [Plate XII.], Fig. 2, show lierne-vaulting, having lierne-ribs, the short ribs joining and supporting all the ascending ribs as h, k, l, m, g.

[Plate XII.], Figs. 2 and 3, are lierne vaults. That shown in Fig. 3 is under the belfry of a church tower at Coventry, with circular opening for hoisting the bells.

The detail at B shows the method of collecting the three ribs into one at the springers in the corners A, B, C, D, by small arches in the tracery of ribs.

[Plate XII.], Fig. 1, shows fan-vaulting having no ribs. The lines shown indicate mouldings on the masonry imitating ribs. The structure is built up of slabs of stone, accurately joined together forming concave half-cones, their vertices being the springers of the vault. The dotted lines show some of the jointing; the other lines represent the imitation ribs. The crown of the vault is the flat surface gh, gi, generally richly ornamented.

INDEX

[A], [B], [C], [D], [E], [F], [H], [I], [J], [L], [M], [N], [O], [P], [Q], [R], [S], [T], [V], [W].

Abacus, [Pl. III.], Fig. 1
Arcade (Norman), [Pl. III.], Fig. 7
Ball-Flower, [Pl. VI.], Fig. 3
Band, [Pl. V.], Figs. 10, 11
Base, [Pl. III.], Figs. 1, 5
Battlements, [Pl. IX.], Figs. 4A, 4B
Bay, [Pl. XI.], Fig. 2
Belfry Windows, [Pl. IX.], Figs. 1, 2
Boss, [Pl. XII.], Figs. 2, 3
Braces, [Pl. X.], Figs. 2, 7
Brattishing, [Pl. VIII.], Fig. 5
Broach-Spire, [Pl. V.], Fig. 7
Buttress, [Pl. V.], Fig. 9; [Pl. VII.], Fig. 10
Buttress (Diagonal), [Pl. IX.], Figs. 1, 2
Byzantine, [Pl. I.], Figs. 3 to 16
Byzantium, p. 17
Camber, [Pl. X.], Figs. 1, 2, p. 67
Catacombs, [Pl. I.], Fig. 1
Cathedral, p. 29
Centering, p. 75
Chamfer-Cusp, [Pl. V.], Fig. 5
Chevron, [Pl. I.], Fig. 4; [Pl. III.], Fig. 6
Chrism, [Pl. I.], Figs. 15, 16
Clerestory, [Pl. XI.], Fig. 2
Collar, [Pl. X.], Figs. 2, 5, 7, 8
Conventional, p. 44
Corbel Blocks, [Pl. II.], Fig. 3
Corbel-Table, [Pl. III.], Fig. 4
Crocket, [Pl. V.], Figs, 1, 2; [Pl. VII.], Figs. 10, 11, 14
Crypt, [Pl. I.], Fig. 2
Cushion Capital, [Pl. III.], Figs. 2, 8, 9
Cusp, [Pl. V.], Fig. 5
Cylindrical Shafts, [Pl. III.], Fig. 2
Detached Shafts, [Pl. III.], Fig. 8
Dripstone, [Pl. III.], Figs. 6, 8
Engaged Shafts, [Pl. III.], Fig. 8
Fan-Vaulting, [Pl. XII.], Fig. 1
Finial, [Pl. V.], Fig. 2; [Pl. VII.], Figs. 6, 10, 11; [Pl. VIII.], Fig. 6
Fish Symbol, [Pl. I.], Fig. 11
Flowing Tracery, [Pl. VII.], Figs. 2, 3
Flying-Buttress, [Pl. VII.], Fig. 10
Font (Norman), [Pl. III.], Fig. 7
Geometrical Tracery, [Pl. V.], Fig. 5; [Pl. VII.], Fig. 1
Gothic Arch, [Pl. IV.], Fig. 2; [Pl. V.], Figs. 4, 5
Groin, [Pl. XI.], Figs. 3, 4, p. 75
Groundstory, [Pl. XI.], Fig. 2
Hammer-Beam, [Pl. X.], Fig. 7
Hood-Moulding, [Pl. III.], Figs. 6, 8
I.H.S., [Pl. I.], Fig. 3A
Jamb-Shaft, [Pl. III.], Fig. 8
Labarum, [Pl. I.], Figs. 15, 16
Label, [Pl. VIII.], Fig. 4
Lancet Windows, [Pl. V.], Fig. 4
Lierne-Vaulting, [Pl. XI.], Fig. 4; [Pl. XII.], Figs. 2, 3
Long-and-Short Work, [Pl. II.]
Mullion, [Pl. IX.], Figs. 1, 3, 5, 6
Nave, [Pl. XI.], Fig. 1
Ogee or Ogival, [Pl. VI.], Fig. 2
Open Timber Roof, [Pl. X.], Figs. 3 to 8, p. 67
Orders of Arches, [Pl. III.], Figs. 6, 8
Parapet, [Pl. VI.], Fig. 6; [Pl. IX.], Figs. 1 to 4
Pier, [Plate III.], Fig. 1
Pinnacle, [Pl. VI.], Fig. 6; [Pl. VII.], Fig. 10; [Pl. IX.], Figs. 1, 2, 3
Pitch of Roof, [Pl. X.]
Pointed Arch, pp. 37, 38
Principal and Principal Rafter, [Pl. X.], all Figs.
Purlin, [Pl. X.], Figs. 1, 2
Quadripartite Vaulting, [Pl. XI.], Fig. 3
Quatrefoil, [Pl. I.], Figs. 9, 10
Queen Post, [Pl. X.], Figs. 2, 7, 8
Quoin, [Pl. II.], Figs. 6 to 9
Rafters, Common, [Pl. X.], Figs. 1, 2, 4, 5, 7, 8
Respond, [Pl. III.], Fig. 3; [Pl. XI.], Fig. 2, p. 31
Reticulated Tracery, [Pl. VII.], Fig. 5
Sacred Monogram, [Pl. I.], Fig. 3A
Scallop Capital, [Pl. III.], Fig. 6
Scroll-Moulding, [Pl. VII.], Figs. 12, 13
Shaft, [Pl. III.], Fig. 2, p. 31
Sill, [Pl. II.], Fig. 3
Soffit-Cusp, [Pl. V.], Fig. 5
Spandril, [Pl. VIII.], Fig. 4, p. 60
Springer, [Pl. XII.], Fig. 3, detail
Spur, [Pl. III.], Fig. 5
Square and Circle, [Pl. III.], p. 30
Squinch Arches, p. 43
Stiff-Leaf, [Pl. V.], Fig. 8
String Courses and Dripstones, [Pl. IV.], Fig. 4; [Pl. V.], Fig. 6; [Pl. VII.], Fig. 12; [Pl. VIII.], Fig. 8
Strut, [Pl. X.], Figs. 1, 2, 7
Studs, [Pl. II.], Fig. 3
Symbols, [Pl. I.]
Tie-Beam, [Pl. X.], Figs, 1, 2, 3, 8
Tooth Ornament, [Pl. V.], Fig. 12
Tracery Development, [Pl. V.], Figs. 4, 5; [Pl. VII.], Figs. 1 to 5
Transitional, [Pl. IV.]
Transom, [Pl. IX.], Figs. 1, 2, p. 58
Trefoil, [Pl. I.], Fig. 7
Triforium, [Pl. XI.], Fig. 2
Triquetra, [Pl. I.], Fig. 8; [Pl. III.], Fig. 9
Truss, [Pl. X.]
Tudor Arch, [Pl. VIII.], Fig. 4; [Pl. IX.], Fig. 6
Tudor Flower, [Pl. VIII.], Fig. 2
Vaulting Ribs, [Pl. XII.], Figs. 2, 3; [Pl. XI.], Figs. 3, 4
Vesica Piscis, [Pl. I.], Fig. 5
Wall Post, [Pl. X.], Figs. 3, 7, p. 68
Wave—Moulding, [Pl. VII.], Fig. 9
Web (Vaulting), [Pl. XI.], Figs. 3, 4, p. 76

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