If cement-lime mortar is to be used it should not have more than 15
per cent by volume of the cement replaced by an equal volume of dry
hydrated lime. The addition of hydrated lime to cement mortar improves
its working qualities, making it slide more readily on the trowel and
also increasing its waterproofness. Its strength is not decreased
within the limits prescribed.
In concrete work it is as important to have good sand and aggregate
as cement. Sand should be sharp, clean, coarse quartz. The sand used
should not, when it is rubbed in the hand, leave the palm stained.
Gravel which is used as an aggregate should be free from clay or loam,
except such as naturally adheres to the particles. If there is too much
clay or loam, it should be washed with water. When bank gravel is used
the best results will be obtained if it is screened from the sand and
remixed in the proper proportions for fine and coarse aggregate. For
ordinary mass concrete the size of aggregate should vary from ¼ inch to
2 inches, and in reinforced work should not exceed 1¼ inches.
[Illustration: STUCCO ON METAL LATH OVER WOOD STUDS]
The best proportion of parts to use must vary according to the
requirements, but for the small house good results will be obtained
by using 1 part of cement, 2 parts of sand, and 4 parts of gravel or
broken stone.
_Stucco Work._—Stucco is really a Portland-cement plaster used on the
exterior, and its success depends a great deal upon the quality of
materials employed and workmanship. All stucco to a greater or less
degree cracks, but the problem is to make the cracks as small as
possible. The government is carrying on an extensive investigation
of the problem of stucco through experiments on fifty-six exterior
panels which have been under observation since 1915. Each one of these
panels has been spread upon a different base or made with different
proportions. So far only two panels have been found to be entirely
free from cracks, although many are practically uninjured by the small
cracks which have developed. It is therefore quite evident that as
a rule it must be assumed that the stucco will crack to a certain
extent, and in order to cover such defects a rough surface is the
best. As to proportions of mixtures, there is a great variation of
opinion. The commonest is 1 part of cement, 2½ parts of sand, to which
is added about ¹/₁₀ part of hydrated lime by weight of cement. For a
more detailed account on stucco, send for the Progress Report issued
by the Bureau of Standards on the Durability of Stucco and Plaster
Construction.
_Plastering._—The qualities of internal plaster depend upon the
construction of the wall, the methods of application of the plaster,
and the quality of the plastering material.
[Illustration: Scratch coat is for bonding; brown coat for plasticity;
finished coat for appearance]
The walls and ceiling to which plaster is to be applied must be so
constructed as to be practically rigid under the loads that they will
carry. Since plaster is not elastic, any slight change in shape of
the surface will cause it to crack. The common backings which are
satisfactory for plastering are wood lath, metal lath, and masonry,
such as concrete, terra-cotta tile, brick, plaster board, etc. Wood
lath makes the least rigid back of all, and for this reason is not
considered the best, although it is the cheapest. Unless the wood laths
are wet before the plaster is applied, they will absorb the moisture
from the plaster and swell, thus cracking the wall. Metal lath for this
reason is superior. Masonry walls should be made rough to give the
necessary key for the plaster to cling to. In brick walls the joints
are raked out, in concrete walls the surface is picked, and the outside
of terra-cotta tile is marked with grooves for this purpose.
The best results in plaster are secured with three coats. The first
coat is called the scratch coat, and is intended to form a bond
between the wall itself and the plaster. It should be pressed into
the apertures between the lath to secure a good bonding key, and its
surface should be scratched with a tool to give the required bond
between it and the next coat, or brown coat. The brown coat forms the
main body of the plaster and averages about ¾ inch to ⅞ inch thick. The
finished coat is then added on top of this and is intended to develop
a plane surface with the desired color. Each coat should be allowed to
dry out and then be wet before the next one is added. If wood lath is
used, this drying and wetting will cause the lath to shrink and swell,
so that cracks will be developed in the scratch and brown coats. These
should be filled in before the finished coat is added.
The materials which should be used in the various coats depend upon the
requirements which are necessary for each one. As the most important
characteristic of the scratch coat is strength, and that of the brown
plasticity, and the final coat appearance, the materials must be
proportioned accordingly.
SCRATCH-COAT PROPORTIONS
Hydrated lime 133 parts by weight
Sand 400 “ “
Hair 1 part “
BROWN COAT
Hydrated lime 100 parts “
Sand 400 “ “
Hair ½ part “
FINISHED COAT
_Smooth Finish_
1 part by volume of calcined gypsum.
3 parts “ lime paste.
_Metals_
The most used metal in the small house is the so-called tin-plate or
roofing tin. It is not a true tin-plate, for it contains 75 per cent
lead and 25 per cent tin, applied to a base of soft steel or wrought
iron. It comes in two grades, IX and IC, the former being No. 28 gauge
and the latter No. 30 gauge. The lighter is used for roofing and the
heavier for valleys and gutters. The tin does not entirely protect the
base metal, so that it is necessary to paint both sides before it is
applied.
Galvanized iron is another form of sheet metal which is extensively
used for work on the small house. It consists of sheet iron or steel,
covered with zinc. This coating should be free from pinholes or bare
spots, and of a thickness to prevent cracking or peeling. If the
coating is sufficient and well done, it is superior in lasting quality
to the ordinary tin-plate.
Copper, since the war, has come back into use again as a sheet metal
for the small house, for its cost has dropped within reason. In
order to meet a certain popular demand a light grade of copper sheet
roofing has been placed on the market, although it has generally been
considered that sheets weighing less than 16 ounces per square foot
were not suitable for roofs.
_Glass_
There are two kinds of window-glass used, double thick and single
thick. The former is ⅛ inch thick or less, and the latter is ¹/₁₂ inch
thick. It is customary to use double thick in all window-panes over 24
inches in size. The grading is AA, A, and B, according to the presence
of defects, such as blisters, sulphur stains, smoke stains, and stringy
marks.
Plate glass is used only where the expense will permit. It is different
from window-glass in that the latter is made from blown glass, while
plate glass is made from grinding and polishing down sheets of rolled
glass.
There are quite a number of other minor materials which enter into the
construction of the small house, but they are more or less identified
with the mechanical equipment and the finishing, and will be considered
under these headings.
Sheet lead weighing 5 to 6 pounds per square foot is often used for
counter-flashing. Leaders and leader heads of cast lead have been made
practical by one company, which has developed a method of hardening the
lead.
Zinc, like copper, is again being urged upon the public by the
manufacturers since the war demand is over. Zinc spouts are usually
made from No. 11 zinc gauge, which is equal in thickness to No. 24
steel gauge.
There is hardly any need to mention the durable qualities of copper,
zinc, or lead. Wherever the cost permits, one cannot deny that
materials of such durable nature are the proper ones to use.