30 VEGETABLE FORCING 
If too flat there will be danger of leakage, and snow will 
be likely to colléct 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 degrces 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 
Fig. 17.—(A) A typical roof bar and comfort of the 
ci ai ar a 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. 
