74 GREENHOUSES 



carried out below the frost line. This is neces- 

 sary to prevent freezing, as the greatest drip 

 is in the coldest weather. In some forms of 

 construction where pipe side posts are used, 

 they are utilized as conductors of the drip 

 water, but the saving thus accomplished is 

 usually more than counter-balanced by the 

 early rusting out of the posts. Gutters are 

 made of wood, zinc, tin and galvanized iron. 



Purlins. Since sash bars must be small 

 to minimize the amount of shade, it is evident 

 that on wide houses they cannot carry the 

 weight of the glass without support. This is 

 accomplished by means of purlins. They 

 run lengthwise of the house, and are them- 

 selves supported by purlin posts, by purlin 

 braces, by rafters or by some form of truss 

 work to be described later. 



When ordinary wood sash bars are used 

 with glass 16 inches wide, the maximum dis- 

 tance for safety between purlins is not more 

 than 7 feet. For example: If the sash bars 

 are more than 7 feet long, one purlin should 

 be used. If they are more than 14 feet long, 

 two purlins should be used, and so on. This 

 distance decreases as the size of the glass in- 

 creases since there are fewer bars to sustain 

 the same weight. 



