78 THE PRINCIPLES OF FLORICULTURE 



As has been stated, best light conditions are obtained when 

 glass is nearly at right angles to the sun's rays. It is 

 important, therefore, that the roof be not too flat. In 

 even-span houses, an angle of from thirty-two to thirty-five 

 degrees is desirable, and in uneven-span houses, the angle 

 of the southern slope is from twenty-eight to thirty-two 

 degrees and that of the northern slope about forty-five 

 degrees. The size of the angle is influenced somewhat by 

 the width of the house. If glass roofs are too flat, there is 

 an increased tendency toward drip in the houses. 



Constructing the frame. In constructing the roof, the 

 ridge is first placed at such a height as will give the desired 

 angle to the roof. If rafters are used, they are first put 

 in place, iron supporting columns are placed and purlins 

 attached. The distance between purlins will vary accord- 

 ing to the width of the span of the roof. They should be 

 placed closely enough so that there will be no sagging of 

 the sash-bars when the glass is placed. The frame is now 

 ready for sash-bars. These are carefully spaced to allow 

 the size of glass selected to fit evenly and easily without 

 pressure. If top ventilators are used, a header should 

 be placed at the proper distance from the ridge, and the 

 upper end of the sash-bars rests on it. Otherwise, they 

 are attached to the ridge. 



Size and quality of glass. For most commercial houses 

 double thick " B " quality glass is used. This quality of 

 glass is also known as " seconds." First quality glass is 

 much more expensive and does not improve the light con- 

 ditions sufficiently to offset the increased cost. The 

 standard size of glass for commercial houses is sixteen by 

 twenty-four inches, although glass of a larger size is some- 

 times used. 



Laying the glass. The frame is now ready for the glass. 



