Skylights are windows, either fixed in roofs, or themselves forming the roof of a staircase or other building lighted from above.
They are very varied in form, according to the position in which they are fixed.
In many cases the skylight is raised upon vertical or slightly inclining frames filled in with sashes which form its sides (Figs. 190, 191); it is then frequently called a Lantern.
An opening is formed in the slope of the roof (by trimming the common rafters CR), of the same size as the proposed skylight ; a lining1 is attached to the inner sides of the trimmers TT, and of the trimming rafters, extending a few inches above them. Upon this the sash rests; its styles and rails project over the frame, and may be rebated to fit it, or a projecting piece may be nailed on, as shown at c in Fig. 186, to cover the joint.
1 Usually called a "curb" or "kerb," and dovetailed at the angles.
Fig. 186. Scale, 1 inch = l foot.
Lead flashings, ff, are also fixed as shown to prevent the wet from getting in ; and any that may penetrate finds itself in a groove g cut in the upper surface of the top and side linings, down which it runs, escaping at the lower end of the latter.
The sash bars run down the slope of the roof like rafters, and should be made strong enough to resist the weight of glass and snow, force of wind, etc. The rebates should be grooved, so as to carry off any moisture that may pass round the edges of the glass.
The lead apron at the lower extremity of the inside of the skylight should be formed into a sort of gutter, as shown in dotted lines at x, to receive and carry away the moisture which condenses on the lower surface of the glass. It is also desirable to form gutters in the sides of the sash bars for the same purpose, as in Fig. 187.
The panes should run continuously through from top to bottom of the skylight, without cross bars to intercept the wet running off the glass.
If it be necessary to have the panes in shorter lengths, they should overlap, as in Fig. 188, and be secured by metal clips, shown in thick black lines which hang the bottom edge of each pane to the top edge of the pane below it.
It is sometimes necessary, for want of space, to obtain more light, or for other reasons, to make the side linings vertical instead of at right angles to the rafters as shown, but the latter is the stronger construction.
It is becoming usual, especially where a skylight is of considerable length, to avoid the gutter by lowering the head H of the skylight 2 or 3 inches below the lower edge of the slates of the roof. The end at x remains at the same level, so that the slope of the skylight is flatter than that of the roof.
If such a skylight as that shown in Fig. 186 be required to open it must be hinged at H; and in some cases the joint is protected by a strip of lead fastened round the sash, which hangs down over the lead flashing on the sides of the frame.
The glass in skylights is sometimes secured by means of a capping fixed to the upper surface of the sash bars, which holds the glass more firmly and prevents the wet from penetrating.
Another kind of skylight consists of a pair of sashes fixed above the apex of a roof and parallel to its sides.
Two varieties, surmounting a queen-post roof, are shown in Figs. 189, 190.
The skylight, of which half is shown at A, consists of a pair of sashes similar in construction to that just described, raised a few inches above the surface of the side slopes of the roof by means of linings fixed to the purlins resting upon the queen posts.
Fig. 189. Fig. 190. Scale, 1/6 inch = l foot.
The inner sides of the queen posts have backing pieces fixed to them, carrying a lining so as to convert the interval immediately under the skylight into a shaft or boxing.
In some cases, for the sake of appearance, the lower extremity of this shaft is filled in with a sash, Sa, called a counter skylight or ceiling light, containing glass, so as to keep the plane of the ceiling almost unbroken.
The skylight or lantern at B is raised two or three feet above the roof by means of framed sides containing sashes, which may either be fixed, or made to open by being hinged at the top, or (as in Fig. 190) hung on centres.
The sill of the framed sides is fixed to a capping or curb, which rests upon a cross bearer supported by the heads of the queen posts.
This form of skylight gives more light and ventilation than that at A, but is of course considerably more expensive.
Fig. 192 shows a skylight or lantern over a room covered by a lead flat.
This example is taken from the lecture-theatre of an hospital near London, but is in many particulars similar to one over the Museum of Economic Geology, and illustrated in Laxton's Examples of Building Construction.
Fig. 192 is a plan showing the arrangement of these girders, and of the binders and joists supporting the lead flat, the larger portion of which is broken away to show the bearers beneath.
Fig. 191 is a sectional elevation of half the lantern, showing the different parts in sufficient detail to render much explanation unnecessary. The moisture condensed upon the inside of the upper portion of the skylight runs down and is caught in a small zinc gutter formed in the upper portion of the moulding at W, and from thence is led through a hole (dotted in the figure) to discharge upon the lead fiat outside.
The details of the side sashes hung on centres are similar to those already given for such sashes in Fig. 315, Part I.
It will be noticed that the inside bead, x, Fig. 191, is so fixed upon the sill that the skylight when closed does not shut up against it, but an interval is left, which forms a gutter to receive the condensed moisture from the sash. A groove cut in the sill enables this water to escape.
When the side sashes of a lantern are fixed, ample provision should be made for carrying off the moisture which condenses on the inside of the glass, and has a tendency to run down into the room below.
This may be prevented by providing a wide oak sill projecting inwards an inch or two, so as to give room for a deep groove formed on the inside, into which the condensed moisture runs, and from whence it is led outwards by holes bored through the sill.
Fig. 191. Scale, 1 inch=l foot.
Or the inside bead on the oak sill may be kept a little back from those on the sides, so as to form a gutter as explained above and shown in Fig. 191.
It is sometimes advisable to construct the slightly inclined sashes of a skylight so as to open by sliding.
Fig. 192. Scale, 1/12 inch=l foot.
In such a case it is important to keep the rain from penetrating between the frame and the sash.
This may be done by arranging as in Fig. 193.
The sash, S, slides down the frame, F, upon a little brass friction roller, r, fixed in the frame, st is a stop on the sash which strikes against the block, U, attached to the frame, and arrests the fall of the sash when it has gone far enough. c is a capping protecting the upper surface of the joint between the sash and frame. As an additional precaution an angle-iron water bar may be inserted as shown, so as to prevent any water running off the sash from penetrating sideways at the point x. If, in spite of this, any water should penetrate, it will find itself in the groove g. which leads it off through the lower end of the frame.
Fig. 193. Scale, 1 inch = l foot.