84 GREENHOUSES 



Modern greenhouse walls, for commercial 

 houses, are almost always of concrete and, 

 being low, give little trouble. Concrete 

 blocks and hollow building tile are much 

 used. The chief requisite. is that the founda- 

 tion shall reach below the frost line. The 

 common practice is to dig a trench 12 or 

 15 inches wide and 3 feet deep and fill with 

 coarse concrete to within a few inches of the 

 surface. A form is then built of lumber to 

 the height required and filled with concrete. 

 When the concrete has "set," the form is 

 taken away and the sides of the wall plast- 

 ered with a cement mortar. In wet, springy 

 soil it is often desirable to lay a row of drain 

 tile along the outside of the wall and nearly 

 to the bottom of the trench, to carry off the 

 water. 



Concrete walls are usually much more 

 satisfactory than either brick or stone. They 

 should be from 8 to 12 inches thick, according 

 to their height and the side strain to which 

 they are subjected. Usually 8 inches is suf- 

 ficient. In wet soils when the boiler is placed 

 below the surface, it may be necessary to 

 waterproof the walls. For data on concrete 

 construction see Chapter XV. 



