14 BULLETIN 230, U. S. DEPARTMENT OF AGRICULTURE. 
absorbent, so that even after a hard rain there is no danger from 
damp walls. In a 1:2:4 mixture, 5 per cent of oil is a sufficient 
quantity to waterproof properly against absorption. 
ROOFS. 
Portland cement mortar mixed with mineral oil and reinforced 
with steel-wire mesh may be advantageously used in the construction 
of roof slabs. These slabs could be assembled in place on the roof 
after they have attained sufficient hardness. Reinforced concrete 
tiles may also be advantageously made with Portland cement con- 
crete mixed with a small percentage of mineral-oil residuum. 
STUCCO. 
Portland cement stucco is widely used in the construction of many 
residences. This type of construction is economical, and, moreover, 
with it many beautiful effects are possible. The term " stucco" is 
given to the exterior finish coat, which may be applied to brick, stone, 
concrete, hollow tile, or frame construction. According to the finish 
desired and the kind of surface to be covered, the stucco is applied 
in two or three coats. The first, or scratch coat, should be mixed in 
the proportions of 1 part of Portland cement and 2 parts of clean, 
coarse sand, with enough water to form a good stiff mortar. If 5 
per cent of oil is added to this mixture, the scratch coat will be per- 
manently waterproof. While this coat is still wet, it is roughened 
with a stick or trowel over the entire surface. The second coat, 
which may be of the same proportions, is plastered on after the first 
coat has set sufficiently to support it. The use of oil in this coat may 
be omitted if desired, and it may be given a rough-cast finish by 
using a trowel covered with burlap or carpet. 
The second coat may also be applied by throwing it on with a 
wooden paddle. This produces a rough surface known as a slap- 
dash finish. A pebble-dash surface may be secured by using a wet 
mixture composed of 1 part cement and 3 parts pebbles one-fourth 
inch in diameter. This mixture is thrown on the second coat while 
it is still soft, and the result is a very pleasing surface. When a 
pebble-dash finish is used, the second coat, as well as the scratch 
coat, may be mixed with oil. In most constructions the second 
coat will be found superfluous, because a sufficiently thick coating is 
usually obtained from the first application of oil-mixed mortar. 
When stucco is applied to stone or hollow tile, care should always 
be taken to have the surface well moistened or otherwise a great deal 
of water will be absorbed from the mortar coat, and so greatly 
weaken it and cause contraction cracks to form. 
IRRIGATION DITCHES. 
The results of laboratory tests, previously referred to, which indi- 
cate that the presence of oil tends to retard very materially the 
