90 GREENHOUSE CONSTRUCTION AND HEATING. 
and though fairly stout roofing-slates may be employed, 
these must be supported by battens or narrow boards 
laid rather closely beneath them, to prevent their giving 
way beneath the weight. 
Houses for the growth of ferns and similar plants are 
sometimes constructed with a slate staging, as shown in 
Fig. 66, where a is the slate bed, B a low 44in. wall run 
up along both sides of the central pathway to carry the 
cross timbers’ (or lengths of T iron), with openings c, 
placed 3ft. or 4ft. apart, to allow some of the heat to 
escape into the house, and pp the pipes. By fitting the 
openings cc (which may be 18in. by 6in. or Qin. each) 
with sliding or hinged wooden flaps or doors, the heat may 
be confined beneath the staging, and a kind of gentle 
hot-bed thus secured, but in this case top heat should be 
provided by means of a few rows of 2in. or 3in. piping, 
as at EE in the sketch. 
The galvanized (or more correctly zinc-coated) corrugated 
iron which is now so extensively used and sold so cheaply, 
in sheets about 2ft. wide and from 5ft. to 10ft. long, fornis 
one of the very best materials for staging, and is now 
largely employed for this purpose. It is at once light, 
strong, lasting, and a good conductor, as well as being very 
inexpensive. It must, of course, be surfaced with Qin. or 
so of fine gravel, beach shingle, or ‘ ballast” (clay burnt in 
the rough), on which to stand the pots. Ashes must not 
be used, as they quickly corrode the metal, and each sheet 
ought, in any case, to have a coat of thick, hot, lime-wash 
before being used. The sheets may be laid on cross- 
bearers of wood, T or angle-iron, or ordinary gas-barrel, 
placed not more than 18in. apart, and a 8in. or 4in. by 
