GREENHOUSE CONSTRUCTION AND HEATING 2 
give adequate support to the superstructure. They 
should also interfere as little as possible -with the admis- 
sion of light at the sides and ends of the houses. While 
wood, stone and brick are sometimes used for the walls, 
concrete is now almost universally employed because of 
its economy and durability. The wall must have a 
foundation starting below the frost line—there should be 
no uncertainty about this matter. For large houses it 
should be not less than a foot thick at the bottom and 8 
to 10 inches at the top, except in types of construction 
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Fig. 12.—Semi-iron construction, showing posts and purlin supports set in concrete. 
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where practically no weight rests directly upon the wall. 
The walls in some of the largest houses are only 4 or 5 
inches thick, and this may be ample if the structures are 
well braced and supported in the interior. It is a mistake 
to build the concrete walls very much above the surface 
of the ground. A foot is ample in some instances, and it 
is doubtful if more than 2% feet should ever be allowed, 
because the extra height simply adds to the cost of con- 
struction, and shades the plants near the sides and ends 
of the house. In semi-iron construction (Fig. 12) the side 
