124 VEGETABLE GARDENING 



An excellent plan is to build the concrete walls 2^ feet 

 above grade level and bank them with earth on the out- 

 side. Walls made in this way are warm and inexpensive 

 to construct; besides they are much better than those 

 built of wooden posts and boards. Although cedar and 

 locust posts last for many years, the lower boards in a 

 wall will soon decay. If the foundation extends 2.^/2 feet 

 above grade level, there will be a height of 4 feet of glass 

 at the sides between the plate on the concrete wall and 

 the gutter or eaves plate, where sufficient height is de- 

 sired for a walk next to the wall. 



FIG. 27. THREE-QUARTER SPAN GREENHOUSE. TYPE 

 USED NEAR BOSTON 



179. Roof construction. The size of the roof bars is 

 determined by the width of the glass and the distances 

 between posts, braces and purlins. The bars should be 

 heavy enough to prevent sagging. Figure 28 shows an 

 approved method of bracing. The pitch of greenhouse 

 roofs varies from 30 to 35 degrees. When the ridge and 

 furrow plan of construction is used, gutters must be pro- 

 vided between the houses. It is important, however, to 

 avoid having gutters if possible, because they decay 

 quickly unless made of metal. Many growers prefer to 



