30 GREENHOUSE CONSTRUCTION AND HEATING. 
flat-roofed house, and consequently the evaporation is more 
rapid, and the atmosphere is on the whole considerably 
drier. It is also plainly evident that to construct a 
steep-pitched roof, more materials (glass and wood) will be 
required for a house of the same width than if a lower 
pitch were adopted, owing to the greater length of ratters. 
On the other hand, a very low pitched roof, though 
economical as regards material, and very suitable for many 
subjects, not only requires a good deal of support, but is 
much more difficult to tie together securely than one 
constructed with a sharper angle. Again, snow does not 
slide freely from a very flat roof, and when there is a heavy 
fall the excessive weight often causes serious damage, and 
lastly, houses with low-pitched roofs are always more liable 
to ‘‘drip’’ than if the angle is steeper. 
For the usual run of greenhouses intended for ordinary 
purposes, a pitch of about 45 degrees is a fairly suitable one, 
especially where the width is not great. This is, indeed, 
an excellent average pitch for vineries, tomato-houses, 
peach-houses, and so forth, but for the majority of ordinary 
plant-houses, rose and fern-houses and so forth, 40 degrees is 
undoubtedly preferable, and for cucumbers, stoves, houses 
for gardenias, stephanotis, palms, etc., as well as in the 
case of propagating and forcing-houses, generally 35 degrees 
may be taken as a safe and reliable figure. In some cases, 
roofs as low as 30 degrees are constructed, but so flat a pitch 
cannot be recommended for very wide structures, and 
below this it is never advisable to go. 
Structures with roofs at an angle of 50 degrees or more 
from the horizontal, though common many years ago, are 
now seldom erected, or seen, but in the case of necessarily 
