The open Gothic roofs of the middle ages, differing as they do in essential matters of construction from those at present in use, ought not to be passed over in silence. In many, if not the majority of such roofs, tie-beams were altogether dispensed with ; the only tie between the principals, to pre vent their spreading, consisting of a collar-beam placed high up in the roof. So much care, however, to prevent spread ing, in these roofs, was not required ; for the principals were laid at a high pitch, and therefore had not so much tendency to thrust outwards, as a roof of modern construction ; beside which, the walls were built of very great thicknesses, and were often strengthened by massive buttresses. Sometimes, in lieu of a eollar-beam, two cross-braces were used springing at about half way up the principals, and intersecting each other in their course upward to the opposite principal, and occasionally these and the collar-beam were employed in the same roof ; not unfrequently we find the collar strutted up from the rafters. Sometimes again, the principals rest on arched beams or planks, of which some are carried down the walls, and rest on corbels projecting from them ; by this mean., the thrust is carried down lower, and you gain the advantage of the weight of the walls above, to resist it.
Sometimes tie-beams are used, which are usually of large scantling, and have the upper surface sloped upwards to the centre, so that the depth there is greater than at any other part. Occasionally, upon the centre of the tie-beam is placed a post, from which spring branches to prop up the collar beam, and sometimes the rafters also. This post must not be confounded with the king-post of modern roofs, for it is not used to hang up the tie-beams, but rests upon the tie beams to support the upper part of the roof.
Such were the most usual form of roofs in the earlier Gothic examples ; but at a later time, when the pitch of the roof was lowered, other methods were adopted. Sometimes, merely a horizontal beam was thrown across the upper side, being formed into two inclined planes similar to the tie-beams mentioned above; at others, inclined rafters were used with their ends resting upon upright posts, placed against the walls and resting on projecting corbels at a distance from the top of the walls, the rafters being connected with it by a curved strut, so as to prevent deflection of the rafters near the cen tre. The upright posts are termed pendant posts, and the struts are often cut out of thick planking, so forming solid spandrels.
Frequently a horizontal beam is laid across the roof on top of the pendant posts, supported by curved struts as before, and above this, rafters are introduced, strutted up in the centre from the horizontal beam. Where the pitch is some what higher than usual, inclined struts are added to the cen tral strut, so as to divide the bearings of the rafters. This kind of roof is very similar in appearance to a king-post truss, but this is not the ease; that Nr h eh has the appearance of the king-post, not being employed to suspend the tie-beam, but to support the rafters.
The hammer-beam roofs so common in halls of the 15th and IGth centuries, are of a different description to any of the above. They are usually of a high pitch, and of con siderable span, and often bear evidence of very great skill in construction : a simple roof of the kind, consists of two prin cipals connected by a collar-beam, and resting on the ends of two hammer-beams, which project horizontally from the wall and carry two queen.posts. The queen-posts and the collar beam are usually connected by a curved brace, as are also sometimes the hammer-beams with pendant posts. Of such
roofs, the most remarkable are those of Hampton Court and Westminster Hall.
The roof of the great hall, Hampton Court, consists of principals which fall short of the apex, and are secured to a collar or straining-beam at their upper extremities, which is supported by three vertical posts resting on a lower collar beam, at about halfway down the principals. Under the lower end of the principals, and resting on the top of the walls, is a hammer-beam projecting out about a quarter of the entire space. and supported at its inner extremity by a curved brace, which rests upon a pendant post, and this again upon a corbel projecting from the wall. The hammer-beam carries two uprights, one of which supports the principal rafters, and the other the lower collar, which is still further supported at its centre by a curved brace, which rests upon the extremity of the hammer-beam. A couple of inclined rafters springing from the top of the principals, meet at an apex over the centre of the root', where they are supported by a strut resting on the collar-beam.
The roof of Westminster Hall has always been much admired ; and to its great height and extensive dimensions, must be attributed mueh of the grandeur of the building. It is, indeed, impossible to enter this magnificent room, with out being struck with admiration. An uninterrupted open space, nearly equal to the size of a large cathedral church, is presented in one view, and the scientific spectator, gazing with delight on its lofty roof, admires the elaborate and artist-like arrangement of its timbers. These serving at once the purposes of utility and decoration, and uniting the apparently opposite qualities of massive solidity and airy lightness. • The angle of the roof is formed on what country workmen still term common pitch, the length of the rafters being about three-fourths of the entire span. The cutting oil' the girders, or tie-beams, which, crossing from wall to wall in common roofs, restrain all lateral expansion, was the first circumstance peculiar to this construction. To provide against lateral pressure, we find trusses, or principals, as they arc technically designated, raised at the distances of about eighteen feet throughout the whole length of' the building. These trusses abut against the solid parts of the walls between the win dows, which are strengthened in those parts by arch-butresses on the outside. Every truss comprehends one large arch, springing from corbels of stone, which project from the walls at twenty-one feet below the base-line of the roof, and at nearly the same height from the floor. The ribs forming this arch, are framed at its crown into a collar-beam, which connects the rafters in the middle of their length. A smaller arch is turned within this large one. springing from the hammer-beam which is level with the base-line of the roof, and supported by two brackets or half-arches, issuing from the springers of' the main arch. By this construction of the trusses, each one acts like an arch. and by placing these springers so far below the top of the walls, a more firm abutment is obtained. The main arched rib is constructed of three thicknesses, somewhat after the principle of Philibert de Lome. From the extremity of the hammer-beam rises a vertical post which supports the end of the collar-beam already alluded to, and this again, by means of two other cubical posts, supports a second collar-beam, and a central or king-post.