A compound roof is one where the span is too wide to be bridged by tie-beams at wall plate level. The hammer-beams, in a roof of this kind, carry vertical posts tenoned into principal rafters at their upper ends, and the tie-beams are fixed at about purlin-level and are, therefore, in effect, collars rather than true tie-beams. Examples of compound or double-aisled roofs are illustrated here in Fig. 44, Nos. 21 and 24. Westminster Hall and Needham Market Church, shown later in Figs, go and 83, are examples of these double-aisled roofs.
Fig. 47. Long Melford, Suffolk. - The roof of the Lady Chapel (1496). 62
In view of the above explanation it is unnecessary to enter into a description of single-thrust or lean-to roofs. The principles are the same, and are self-evident.
No better understanding of the details of timber roof construction can be gained than by the study of roofs of barns of the more elaborate type, such as Harmonds-worth Barn, shown here in Fig. 45. Barn roofs are necessarily devoid of much of the decorative character usually found in those of churches or mansions, and there are, in consequence, no unnecessary details or parts to distract the attention. Barn roofs have also another advantage; from their utilitarian, as distinct from decorative, character, they exhibit early details and constructional methods persisting to a later date. Being made for use only, their evolution is necessarily slow, as a perfect principle, once devised, was adhered to, irrespective of changing fashions, or desires for novelty in decorative effects. The supporting posts, which are the barn equivalents of the domestic hammer-posts, have an advantage in reaching to the floor, whereas, in the great hall, they would be an obstruction. The barn roof, such as in Fig. 45, is, therefore, truly double-aisled at floor level, and it is this form of construction which must have inspired the hammer-post and hammer-beam. The stable properties of cantilevering the hammer-beam would follow when the carrying of the posts to the floor was interdicted. Unfortunately, the supporting of hammer-posts on the tenons only of the hammer-beams (the pendentive type such as at Eltham and Earl Stonham), must have originated from the same source.
Fig. 49. Stoke-By-Nayland, Suffolk. - The Nave. Late fifteenth century.
Fig. 48. Long Melford, Suffolk. - The Nave. Late fifteenth century.
Fig. 51. Monks Eleigh, Suffolk. - Roof of North Aisle. 10 ft. 9 ins. span.
Fig. 50. Wetherden, Suffolk. - Roof of South Aisle (c. 1400).
York Guild Hall, shown here in Fig. 46, is a remarkable example of a roof supported by posts from the floor, forming, in effect, a hall with nave and aisles, and is, probably, the only roof of this kind existing in England at the present day. Although unique now, there is no doubt that this form is earlier than the hammer-beam roof; in fact it must have been the prototype. York Guild Hall is late for this kind of roof construction. Begun in 1446, it was not completed until nearly fifty years later, and records exist which state that the merchants of York who were convicted of illegal practices were fined, not in money, but in kind, having to find timber and oak wainscot for the Hall.
The roof is low in pitch, with little outward thrust, the great stresses being almost entirely downwards, carried on the massive octagonal-section oak posts with their stone bases. The nave is of the firred-beam type. The aisles are constructed with simple lean-to roofs. The problem of the entire roof, therefore, is one more of size than constructional difficulties, involving complicated stress calculations. The principles governing roofs, even of gigantic size, where the timbers are supported from wall-head level, were fully understood, and their advantages appreciated at this date. There are many factors, other than inexperience or timidity on the part of the mediaeval carpenter, which may have dictated this aisle-column form of the York Guild Hall roof.
Fig. 53. Tawstock, N. Devon. - Aisle Roof. 48 ft. long by 9 ft. span. Fifteenth century. The western type of panelled roof. K
Fig. 52. Rougham, Suffolk. - Roof of South Aisle. Late fifteenth century.
A careful study, and memorising of the roof sections illustrated in Figs. 43 and 44 is recommended, as in the illustrations which follow, of actual roofs, the essential details cannot be shown so clearly, as in diagram form. Apart from lighting considerations, with concomitant photographic difficulties, the occultation of the one beam or collar, with its superimposed bracing or posts, by the succeeding one, renders the close study of all the points of a root', from the one view-point only, nearly impossible. With a single photograph, therefore, all the details of a roof cannot always be shown distinctly. Space considerations preclude a redundancy of illustration.
The succeeding illustrations have, for convenience only, been arranged in a progressive order, from the simple to the complex. While there is no doubt that the true evolution of the timber roof actually took place somewhat on these lines, it must not be assumed that a simple roof is earlier in date than a more elaborate one. We have no complete record of very early roofs; the greater number have perished, disappeared and been forgotten long since. At one period in the history of English carpentry, examples could have been illustrated to show the development from type to type, each true to the date of its inception, but that time has passed, many centuries ago. Thus the gigantic roof of Westminster Hall, dating from the closing years of the fourteenth century, is an early example when compared with others existing at the present day, but it is late in the history of the English timber roof. An enormous span of 68 feet between walls would have been impossible to bridge at the dawn of timber-roof construction. It is conjectured that the original roof, which the present one replaced in 1395, was constructed with two aisles and with posts to the floor in the same manner as in Harmondsworth Barn or York Guild Hall, already illustrated.
Fig. 54. St. Osyth, Essex. - Roof of North Aisle.