on by any craftsman of repute,
for a skin of brickwork 41/2 in. thick is thus carried up with a straight
mortar joint behind it, the proper bonding with the back of the wall by
means of headers being destroyed.

American building acts describe the kind of bond to be used for ordinary
walls, and the kind for faced walls. Tie courses also require an extra
thickness where walls are perforated with over 30% of flues.

The importance for sanitary and other reasons of keeping walls dry is
admitted by all who have observed the deleterious action of damp upon a
building.

Walls are liable to become damp, (1) by wet rising up the wall from the
earth; (2) by water soaking down from the top of the [Sidenote: Prevention
of damp.] wall; (3) by rain being driven on to the face by wind. Dampness
from the first cause may be prevented by the introduction of damp-proof
courses or the construction of dry areas; from the second by means of a
coping of stone, cement or other non-porous material; and from the third by
covering the exterior with impervious materials or by the adoption of
hollow walls.

[Illustration: FIG. 9.]

After the footings have been laid and the wall has been brought up to not
less than 6 in. above the finished surface of the ground, and previous to
fixing the plate carrying the ground floor, there should always be
introduced a course of some damp-proof material to prevent the rise of
moisture from the soil. There are several forms of damp-proof course. A
very usual one is a double layer of roofing slates laid in neat Portland
cement (fig. 8), the joints being well lapped. A course or two of
Staffordshire blue bricks in cement is excellent where heavy weights have
to be considered. Glazed stoneware perforated slabs about 2 in. thick are
specially made for use as damp-proof courses. Asphalt (fig. 9) recently has
come into great favour with architects; a layer 1/2 or 3/4 in. thick is a good
protection against damp, and not likely to crack should a settlement occur,
but in hot weather it is liable to squeeze out at the joints under heavy
weights. Felt covered with bitumen is an excellent substitute for asphalt,
and is not liable to crack or squeeze out. Sheet lead is efficient, but
very costly and also somewhat liable to squeezing. A damp-proof course has
been introduced consisting of a thin sheet of lead sandwiched between
layers of asphalt. Basement storeys to be kept dry require, besides the
damp-proof course horiz