STRUCTURAL MATERIAL 75 



Purlins may be of wood, gaspipe or angle 

 iron. Wood purlins, because of their size 

 (1^x3 inches), cast so much shade that they 

 are now little used. Purlins of i /4-inch gas- 

 pipe are very satisfactory. They are fast- 

 ened to each sash bar, and are supported by 

 posts or braces every 8 feet along their 



Fig. 45. Pipe-strap for fastening sash-bars to purlins 



length. A very satisfactory means of fast- 

 ening them to the sash bars is by means of a 

 U-shaped pipe-strap. This is placed under 

 the purlin and fastened to the sash bars by 

 means of screws. 



Ridge. The ridge furnishes a means of 

 fastening the upper ends of the sash bars and 

 also serves as a support for the ventilators. 



