re frequently built with cavity-walls,
consisting of the inside or main walls with an outer skin [Sidenote: Hollow
walls.] usually half a brick thick, separated from the former by a cavity
of 2 or 3 in. (fig. 2). The two walls are tied together at frequent
intervals by iron or stoneware ties, each having a bend or twist in the
centre, which prevents the transmission of water to the inner wall. All
water, therefore, which penetrates the outer wall drops to the base of the
cavity, and trickles out through gratings provided for the purpose a few
inches above the ground level. The base of the cavity should be taken down
a course or two below the level of the damp-proof course. The ties are
placed about 3 ft. apart horizontally, with 12 or 18 in. vertical
intervals; they are about 8 in. long and 3/4 in. wide. It is considered
preferable by some architects and builders to place the thicker wall on the
outside. This course, however, allows the main wall to be attacked by the
weather, whereas the former method provides for its protection by a screen
of brickwork. Where door and window frames occur in hollow walls, it is of
the utmost importance that a proper lead or other flashing be built in,
shaped so as to throw off on each side, clear of the frames and main wall,
the water which may penetrate the outer shell. While building the wall it
is very essential to ensure that the cavity and ties be kept clean and free
from rubbish or mortar, and for this purpose a wisp of straw or a narrow
board, is laid on the ties where the bricklayer is working, to catch any
material that may be inadvertently dropped, this protection being raised as
the work proceeds. A hollow wall tends to keep the building dry internally
and the temperature equable, but it has the disadvantage of harbouring
vermin, unless care be taken to ensure their exclusion. The top of the wall
is usually sealed with brickwork to prevent vermin or rubbish finding its
way into the cavity. Air gratings should be introduced here to allow of air
circulating through the cavity; they also facilitate drying out after rain.

Hollow walls are not much used in London for two reasons, the first being
that, owing to the protection from the weather afforded by surrounding
buildings, one of the main reasons for their use is gone, and the other
that the expense is greatly increased, owing to the authorities ignoring
the outer shell and requiring the main wall to be of the full thickness
st