30 



VEGETABLE FORCING 



If too flat there will be danger of leakage, and snow will 

 be likely to collect on the glass. 



Light will thus be obstructed and the increased weight 

 may damage the roof. In deciding upon the proper pitch, 

 not only must snow and rain be taken into account, but 

 the builder must bear in mind that the rays of heat and 

 light admitted depend very largely upon the inclination 

 of the roof. Modern greenhouses usually have a pitch of 

 30 to 32 degrees. A pitch of 30 degrees reflects 8.4 per 

 cent of the sun's rays, and a pitch of 35 degrees reflects 

 5.7 per cent. 



Ventilators. In modern greenhouse management the 

 houses are in use the year around, for the last tomato or 



cucumber is picked 

 from August 1 to 

 August 15, and lettuce 

 is often planted early 

 in September; the in- 

 tervening time is used 

 in cleaning the houses 

 and sterilizing the soil. 

 For the good of the 

 plants and the health 

 and comfort of the 

 workmen, provision 

 should be made for thorough ventilation. 



In houses varying from 12 to 18 feet in width it is 

 customary to place only one line of ventilators at the 

 ridge, and this should not open toward the prevailing 

 direction of the wind. In wider houses there should be a 

 line on each side of the ridge (Fig. 18), and it is usually 

 desirable also to have ventilators along the sides as shown 

 in this illustration, although many ranges of mammoth 

 proportions are operated without side ventilators. The 

 size of the ventilating sash will be determined by the size 

 of the house, but they should be amply large. 



Fig. 17. (A) A typical roof bar 

 (B) Typical end bar. 



