PRINCIPAL RAFTERS. 17 
upper one (4) to carry the ends of the rafters, and the 
usual lower or wall-plate (B), see Fig. 6. 
The old-fashioned system of constructing the roofs of 
greenhouses was with what are termed ‘‘ principal” rafters 
of stout quartering, placed 3ft., 4ft., or 5ft. apart, the 
intervening spaces being filled in with one or more (accord- 
ing to the length of the rafters) sashes, constructed in much 
the same manner as an ordinary frame-light. This method 
is still employed to a considerable extent in the construction 
of conservatories and ‘‘show-houses”’ ; but, though affording 
a very substantial structure, this has a somewhat heavy 
appearance, and does not adinit enough light for the ordinary 
market grower’s requirements. “A maximum of light being 
necessary to the production of the dwarf and stocky growth 
that is so much desired, especially in the culture of plants in 
pots, &c., nurserymen prefer to erect houses with roofs of 
the lightest possible construction, consisting of wide squares 
of glass, supported by few and narrow bars (rafters), and 
offering the least possible obstruction to light and sun. 
Principal rafters are consequently dispensed with altogether 
in the majority of such structures, the rafters consisting of 
plain sash-bar stuff, usually 3in. by 1Jin., or not more than 
3in. by 2in., in section, with squares of glass, 16in., 18in., 
or in some cases 21in. or even 24in. wide between. If the 
rafters are too long to carry the great weight of glass safely, 
they are supported about the middle by means of a purline 
and standards, as shown in Fig. 29 on p. 50, and Fig. 42, p.61. 
When ‘‘ principal” rafters are employed it is not abso- 
lutely necessary to used framed lights or sashes between 
them ; but the spaces may be filled with small sash-bar 
stuff (23in. by 14in., or 2in. by lin. in section), placed 8in. 
) 
