This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol2", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Skylights are sashes fixed in the roof, following the same slope as the roof. Their exposed position necessitates careful construction and fixing in order to avoid the risk of leakage. To form the opening for a skylight in the rafters, it is necessary to insert two trimmers. A "Lining" or "Curb" is then fixed to the rafters and trimmers. This curb may be rough, and be covered on the inside with a thin lining, or the inside face may be wrought and the bottom edge moulded, usually with a large bead, while the joints at corners should be dovetailed. The curb should be wide enough to stand from 5 to 6 inches above the finished surface of the roof, lead being dressed round it, carried over the top edge and fixed by close copper nailing. When the light is hung it is by the top bar, and in this case a small gutter is sometimes fixed inside the curb to catch any water driving through the top joint. The sash should be out of at least 2-inch stuff, and should overhang the curb from 2 to 3 inches all round, and have a "throat" worked on the under side. The method of putting together is shown in Fig. 140. The bottom bar is thinner than the top and sides, the upper side of it being level with the rebates, so that the glass may run over it. Slight dishings or grooves are formed in it under each pane to carry oft any condensation moisture. The top bar is grooved instead of being rebated, for glass as a putty joint at this point would be liable to fail. The glass is secured at the bottom rail by means of copper clips. It should only be lapped over this rail sufficiently to ensure a weathertight joint, as, if it be carried to the bottom as is frequently done, the sun's rays are focused through it on to the rail, causing the latter to warp, with the risk of splitting the glass.
A Lantern Light is an elaboration of the skylight, and consists practically of a glazed roof, supported upon glazed sides, standing above the roof proper. Such lights are built principally in the roofs of billiard rooms and halls, or in positions where an abundance of top light is required. They are made to various designs, and in plan may be square, rectangular, polygonal, or circular. The opening in the roof is formed by cutting and trimming the rafters, and a stout curb is fixed to carry the sides of the lantern. This curb is usually rough, and faced inside with wrought and moulded, or framed and panelled lining, and carries the sill of the side lights. Lead is dressed up the outside of the curb and over the top edge, and may be turned up into a groove made in the sill to receive it, as shown at A in Fig. 141, or a fillet may be nailed along the top of the curb, the lead dressed over this, and the sill grooved to fit over the whole as shown at B. In a rectangular lantern the sides are generally made in the form of solid frames with mullions, the corner posts being double rebated so as to each take two sides. The sill should be wide enough to overhang the curb inside to a distance equal to the thickness of the lining or framing, so that the latter may be tongued into it and finish flush as shown. The sashes in the sides, when required to open, should be hung from the top and open outwards. As a considerable amount of condensation always takes place in a lantern light, sufficient means must be provided for its escape. A method sometimes adopted is shown at C. Here a groove is run in the top part of the sill, which is carried up to form a guard for the bottom rail of the sash, weep holes being bored from this groove to the throating of sill. In this case the water has to run over the bottom bar of the sash to find an outlet, and a better method, avoiding this and providing a direct outlet, is shown at D. Here a small groove is formed in the top of the bottom bar, immediately at the junction with the glass, and is connected with the throating of the bar by means or holes, as shown by the dotted lines. To prevent the possibility of the wood swelling and closing these holes, they should be "bushed" by having a piece of brass tube fixed in each of the full depth of bar. The best form of roof for a rectangular lantern is that known as a hipped roof. The heads of the side frames form the plates to carry the rafters when any are fixed, but usually hip rafters are carried from the corner posts and are mitred into a ridge; and these support the sashes, which lap over and are tongued to the heads of the side lights as shown. These sashes are glazed in the same manner as those of a skylight, the glass being fixed in a groove on the top bar, overlapping the bottom bar, where a dishing is formed for the escape of condensation.
As a difficulty may be sometimes experienced in determining the lengths of hip rafters, the simplest method of obtaining these, together with the lengths of the sash bars, is here given in Fig. 142.
Let ABCD represent the plan of roof and CEFD the elevation. In neither case is the true length of the hip rafter indicated. Draw a plan of one end as shown by AEC. From E draw a line at right angles to EC, and mark off the distance on same to G, equal to the vertical height EG on elevation. CG is then the length required, and the isosceles triangle AHC, with AC as base and the length CG as sides, is the true shape of the end of the roof. Lines parallel to this triangle at the correct thickness will represent the sash framing, and the lengths of the sash bars may be found by setting out their positions along AC and drawing perpendiculars from them.
The method of obtaining the correct bevel of hip, or, as it is technically called, the Backing, may also be given. It is very necessary that these bevels be true, so that they may lie in the same planes as the sides and ends of the roof. Let BFD in Fig. 143 represent the plan of one end as before. Draw FK at right angles to FD, and in length equal to the vertical height between the top and bottom of the hip. Join KD, and from any point L erect a perpendicular to it cutting FD at M. From M draw a line at right angles to FD, cutting BD at N. From M, with ML as radius, mark the point O; join NO, when the angle NOM will be the required bevel. The best method of ensuring water-tightness is to finish with a roll covered with lead and dressed down over the top bars of sashes, as shown in Fig. 141.