This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It is almost invariably specified that no one part of a building shall be carried to a greater height than 5 feet above any other part at any one time, except, of course, finished work. This is done to secure equal settlement as the building proceeds, else the portion which is carried up highest may settle before the rest is brought up to it, and a crack eventually appears at the junction. There are occasional circumstances, however, when it is wise to relax this rule, such as, for instance, when an important lintel stone cannot be obtained from the quarry, and the whole work would have to be stopped if no other portion of the building could go on until this stone were obtained. Under such circumstances the walling must be stepped back gradually, so that a vertical straight joint does not occur, and the Architect should be communicated with, as he may prefer to stop the work.
It may happen that stone which is not delivered may be temporarily replaced by courses of brickwork laid in sand. This enables the work to go on, especially if the sand courses are only face deep, without any material damage being done, the stone strings being afterwards introduced and bedded in cement. This is not infrequently done, particularly with shallow stone or moulded brick strings. If window-sills are inserted as the work proceeds, there is considerable risk of unequal settlement, the heavy jambs bearing down, while the space beneath the sill, not being weighed, exerts an upward pressure upon the stone, with the result that the sill will break across. This is entirely obviated by inserting the sills after the whole of the brickwork has settled. In this, as in all cases where stonework or the ends of metal or wood beams are subsequently introduced, it must be seen that the pinning-in is properly done in cement.
Where new work adjoins old it is a common practice to connect the walling by toothings; but these tend to separate as the new work settles down, as it is certain to do, if in ordinary mortar, to a cosiderable extent. Connection by means of a straight joint and a chase is consequently better, allowing of free vertical movement or sliding, the junction being pointed up when the whole has taken its bearing finally.
The points requiring attention in connection with slating and tiling are comparatively few, though they are of no less importance than those connected with the other trades. The principal thing is, very naturally, to see that the roofs are all water-tight, and that the specification is carried out with regard to the method in which the slates or tiles are laid. An endeavour is frequently made upon the part of a certain class of Contractor to substitute an inferior quality of nails for that which is intended to be used, while, of course, no broken slates should be permitted to be introduced. The lap must be kept, and cut slates, such as occur against hips and valleys, must be properly secured. Double courses at eaves are not often missed; but the Slater sometimes needs watching, to ensure that the lower course is laid with what is usually the upper side undermost. In tiling work, tile-and-half-tiles are obtainable, and should always be used against verges.
Any plumber's work in connection with roofs ought to be done, if possible, before the slates are laid. This is difficult in the case of flashings, but is possible if soakers are used, when the two trades can be at work concurrently. Slates are liable to breakage if walked over, and this should be avoided as much as possible, as once a slate is broken it cannot be replaced so as to leave a perfect job without stripping the whole roof. If soakers are used beneath slates, watchfulness must be exercised, else there is risk of the Slaters' nails being driven through the leadwork, and a way thus made for moisture to penetrate. The necessary lathings and battens should be tested for gauge before the actual slating is commenced, and this is best done by counting the number of laths in a certain length of roof, measured along slope, and making a simple arithmetical calculation as to whether this gives the proper gauge. Just as Slaters' nails must not be driven through lead, so must they be kept from penetrating the under-felt, if there be any, though necessarily the batten nails must do so in order to reach the boarding below, should boarding, felt, and battens all be used. Any nails which penetrate the boarding ought to have their ends clenched, especially if the space within the roof is utilised.
Pan-tiling, if used at all, ought to be bedded in hair mortar, while plain tiles are also sometimes torched on the under side. Valleys are difficult to form in tiles, except to certain pitches of roof in which valley-tiles are obtainable. If lead valleys are specified it is much best to use soakers, if the Architect will permit, or to round off the angle with plain tiles carefully laid, as a large open lead valley looks very bad in a red-tiled roof, while a secret valley is liable to choke with snow. If valley-tiles are used, pointing in cement is necessary. In large roofs tile-hooks are valuable in order to prevent the tiles from being stripped by high winds, while projecting verges need protecting in the same way and for the same reason, either by a cement fillet or by a barge-board.
It is necessary, too, to see that all roofs are laid to their proper pitch, and that these pitches are sufficient, in accordance with the material employed. Slate roofs are rarely permissible at less than 30-degree inclination to the horizon, while tile roofs need a pitch of 45 degrees, except certain patent forms, such as Major's and the Broomhall, which can be employed to as low a pitch as slates.
Metal roofs can all be laid almost flat, a fall of 1 in. in 10 feet being generally considered sufficient, though more should be given if it is obtainable, especially with lead, and if the distance between the drips be more than 8 feet. Sometimes, in long gutters, lead is laid with drips even as much as 15 feet apart; but this is an exceedingly bad practice, as the metal is one which expands in hot weather and forms ridges, while it does not regain its flat shape when the temperature falls again. After a few years of this the lead will crack, and moisture will be admitted. The proper formation of drips and rolls needs extremely careful watching, both in lead and zinc, as there is a great tendency upon the part of both workman and Contractor to allow insufficient lap. This should always be enforced, the lead being carried not merely over the roll, but a few inches along the flat besides, the general specification of first-class work being sufficient justification for enforcing it. It is also necessary to keep an eye upon the thickness of metal which is being used, weighing a square foot if there be any suspicion of a light weight being supplied, it being borne in mind that 7-lb. lead is made both "heavy" and "light." In other words, while apparently of the same thickness, there is a quality of 7-lb. lead on the market which weighs little more than 6 1/2 lbs. per square foot.