ROOF VENTILATORS. 33 
structure, it is evident that only two rows or sets of 
ventilators can be employed. The span-roofed form (which 
from a constructive point of view practically consists of two 
lean-to's placed back to back) admits of four rows—one on 
each side of the ridge and one along each side—while in the 
three-quarter span three sets are necessary, or at any rate 
desirable. 
The simplest, commonest, and on the whole best 
form of roof ventilator is that of the flap, which may 
either be glazed, or in the case of cheap structures 
consists merely of two or three lengths of match 
boarding joined together. The whole is 
hinged to the ridge-plank, or in some 
eases to a separate ‘‘ hanging-piece,” and 
opened by means of a casement-stay and 
pin, or by a lever or with a rod, eye, pulley 
and cord fixed to its lower edge (see Fig. 
18). 
In the Worthing district, where an 
immense area of glass is devoted to the culture of 
grapes, tomatoes, ‘cucumbers, etc., for market, the roof 
ventilators are almost invariably constructed to contain three 
squares of glass of the same size as that employed in 
the roof itself, the usual size (of the glass) being 20in. by 
16in. Each ventilator consequently covers four rafters 
or sash-bars, the styles, of course, overlapping the two 
outer rafters a little way. Where this class of venti- 
lator is employed, they should be hung alternately on 
each side the ridge (in span-roofed or three-quarter 
span houses), with a space between each, on the same 
side, equal to the length of a ventilator, or three widths 
D 
Fig. 18. 
