The most common form of roof for vegetable houses is the even span, with the houses running either east and west or north and south. The three quarter-span houses, with the long slope either to the south or to the north, are also much used. In the former case, the north wall is usually somewhat higher than the south, but if the long slope is to the north the walls are of the same height. It is possible to build a house fifty or more feet in width under a single roof by placing it on a gentle slope. As much as five-sixths of the roof may then be in the south slope.
The framework.
There is considerable variety in the methods of construction used for vegetable-houses, as indi cated in Figs. 179-183. In some cases, posts of cedar, or some other durable timber, are set at intervals of six feet so as to stand five or six feet above ground. They are then covered to the height of two or three feet with sheathing and siding, with a double thickness of building-paper between. A sill is placed on this and the space up to the plate is filled in with sash-bars and glass. Another method is to build a wall of concrete to the height of two feet. In this, two-inch gas-pipes are set at intervals of five feet. These support the plate, and the space between the plate and the concrete is occupied by glass. In other cases, angle or flat bar iron is used for the posts, to the upper ends of which iron rafters are fastened.
When several houses are built with common gut ters between the adjacent houses, if they are used for the same classes of crops, a row of posts to sup port the gutters will be all that is required. (Fig. 181.) Although less commonly used for vegetables than for flowers, what is known as the ridge-and furrow style of construction has much merit, espe cially for tomatoes and cucumbers. As now con structed these establishments are made up of several narrow houses, with a width of sixteen to twenty four feet, and at least six feet in height to the gut ters. As there is nothing but posts under all except the outside walls, it practically makes one wide house. There is less trouble from the shadows of the gutters than in most narrow houses, as the walks are located where the deepest shadows fall.
The roof.
For the construction of the roof of a green house there is no material equal to southern cypress that is free from sap-wood. If soaked in oil and the joints put together in white lead, a cypress greenhouse will last for many years when kept properly painted. Although iron rafters and purlins make possible the use of lighter sash-bars (Fig. 182), a great majority of vegetable-houses are
built without rafters, the framework of the roof being formed of cypress sash-bars that run from the plates to the ridge. These are two to two and a half inches deep and about one and one eighth inches wide, according to the size of the glass and the distance between the supports. The plates may be either of wood, beveled so that the water will run off, or formed into a gutter to carry the water to a drain ; or various forms of iron plates and gutters may be used. The iron gutters are of course more durable, but the houses are harder to heat and with some kinds the ends of the sash-bars decay sooner than with wooden gutters and plates.
Ventilating.
Ample means should be provided for the ventilation of vegetable- houses. This can be secured by means of a row of ventilating sash at the ridge and another row beneath the plates, which should have a width of two to three feet. They should be supplied with some of the modern ventilating machinery that will permit of opening stretches of one hundred feet at a time.
Glass.
The glass most commonly used is sixteen by twenty to twenty-four inches, double strength, and of " B " quality, although " A " glass is better. For small houses it answers fairly well if it has a width of twelve to fourteen inches. The putty used for bedding the glass should be mixed with about ten per cent of white lead. In laying the glass, it should be lapped about one-eighth of an inch. As the lower edge of each pane will be raised from the sash-bars the thickness of the glass, a sufficient amount of putty should be placed on the rabbets to fill this space before the glass is laid. Care should be taken to select the panes so as to make tight joints where they lap, and they should be held in place by zinc shoe-nails, using four to six ac cording to the size of the panes. The double-pointed glazing tacks also answer well for holding the lower corners of the panes in place. No putty should be used on top of the glass and all surplus should be scraped off, care being taken to fill all of the cracks. In addition to soaking the sash-bars in oil, and giving them a coat of paint after they are in place but before the glazing is done, the roof should receive a final painting after the glass is in place, care being taken to "draw" the putty wherever it shows. It is economy to repaint every five years.