16 GREENHOUSE CONSTRUCTION AND HEATING. 
Greenhouses are 
in most cases built on low walls of 
brick, stone, or concrete (though in some instances boards. 
A 
lights, and in this 
B 
are used for the sides), the glass of 
which the roof consists being carried 
in suitable wooden rafters, which are 
properly grooved or “rebated” for the 
purpose. The timbers which are laid 
on the top of the walls, and to which 
the lower ends of the rafters are 
nailed, are termed ‘ wall-plates,” and 
along the apex of the roof, where the 
two sets of rafters meet (in a span- 
roofed or three-quarter span house) 
runs a piece of timber called the 
ridge plank. This serves at once to 
keep the rafters at their proper 
distances apart and to hang the venti- 
lators to. 
Where any part of the sides of the 
house is composed of glass this is 
usually fitted in sashes or framed 
case two ‘‘ plates” are necessary—an 
