64 GREENHOUSE CONSTRUCTION AND HEATING. 
Tn large structures, where the roof is very wide and heavy, 
standards are sometimes placed to support the ridge at 
intervals, as this has a tendency to come down a little at 
the joints, but these are seldom necessary if the structure is 
properly tied together in other ways. Where the rafters are 
long, and great depth is inadmissable, the principals may be 
considerably strengthened by an arrangement of tie-rods 
as shown in Fig. 51. 
It will be plainly seen that if the plates are so secured 
that they cannot bulge or give in the least, the ridge cannot 
possibly ‘‘come down,” and consequently requires no extra 
support. Where several span-roofed houses are built side 
by side, yet separately, with side-glass and ventilators, it 
Z ~~ 
i eS | aed 
Pig. 51. 
is a good plan to fix a stout iron bar, with a T or cross- 
head and a couple of screw-holes at each end, at intervals 
between the upper plates of each pair, as shown in Fig. 4, 
This fixes them securely at the right distance, and prevents 
any bulging absolutely. 
Ventilators.—-These, whether for the roof or sides, are 
made in exactly the same manner as an ordinary frame- 
light or sash, each consisting of a pair of styles, rebated 
on the inner edges to take the glass, and a top and bottom 
rail, each with a tenon cut on both ends to fit into 
corresponding mortices in the styles. They may, however, 
be made of any (rectangular) form and size, and are filled 
in with one or more lengths of slight sashbar stuff running 
parallel with the styles, according to the width of the glass 
