GREENHOUSE CONSTRUCTION AND 'HEATING 



27 



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 



Fig. 12. Semi-iron construction, showing posts and purlin supports set in concrete. 



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*/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 



