described the much less costly method of inserting a damp-proof membrane into a narrow slot cut through a bed-joint. New tools have now been developed for this work and new materials have become available; this Digest brings the de- scription of the newer method of up- to-date.
Method of work
The cut may be made either by a hand-saw or by one of several types of power-driven saw; these are de- scribed below. The cut is started in the selected course at a jamb or cor- this ner, preferably the latter as avoids knocking out a brick to start the cut; once the cut reaches the in- side it is more convenient to con- tinue the work from inside the build- ing. With a new chain saw devel- oped at the Station it is possible to mortice into a joint and so work en- tirely from one side of a wall.
The cycle of cutting the slot and inserting the membrane may then proceed without interruption along the wall. The membrane must be in- serted immediately the cut is made, generally in lengths of about 2 ft; in heavily loaded sections of the wall, e.g. at jambs and junctions, work must proceed in shorter lengths.
The sheets forming the membrane should be wide enough to project aboutin. on each side of the wall. The membrane may be of any im- permeable and durable sheet materi- al; the choice will depend in part on the width of the slot, and this in turn depends on the type of mortar and on the saw that is used. A narrow slot is produced in a wet and sticky mortar by the thin blade. of a hand-saw or reciprocating saw; in this case a thin rigid sheet must be driven into the slot. More usual. ly, in a dry and crumbly mortar, a wider slot will be cut; and the new chain saw cuts a slot slightly over in. wide in any mortar. This will require two layers of a
a thicker material to fill the slot; alterna- tively, a thin sheet may be used and the joint packed, by spreading a bed of mortar on the membrane before inserting it and wedging slips of slate into the joint at intervals of about 12 in.
Even when the membrane is a close fit into the slot there is a slight settlement as the cut advances;
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measurements on a test house have indicated that 1/16 in. may be ex- a joint packed as pected, with described above, about half settlement will occur.
this
As a rigid sheet material, for driving into a narrow slot, half-hard copper may be used or, for a limited subsequent life of, say, 10-15 years, sheet zinc will give sufficient protection at lower cost. A guide frame may be used for driving in the sheets, but this is often unneces-
sary.
With a wide slot, two layers of bitumen felt, each of 24-in. lengths butted together, may be inserted; they should be placed so that the joints are staggered, with at least a 3-in. interval between the joints in the two layers, Copper or polythene sheet may be used and the joint packed. Soft copper sheeting can be inserted into the wide slot without fear of damage, and this is available more cheaply than the half-hard sheet required for driving into a narrow slot. High or low-density polythene sheet, 0-02 in. thick, is a cheaper alternative; high-density sheet is the more easily inserted. The polythene should be filled with carbon black, to retard deterioration where it is exposed to sunlight.
The rate of progress depends much more on the state of the wall than on the type of saw used. In re- gularly coursed brickwork, with a dry crumbly mortar, it may exceed 10 ft per hour in a 9-in. wall. Where courses are irregular or obstacles have to be negotiated, the rate may be only 2 ft per hour, but corners and junctions are usually simple to negotiate. The average rate in brick in old houses is about 4 ft per hour walls of the type generally met with in 9-in, walls, and 6 ft per hour in 44-in. walls. These estimates cover the time spent in preparation, inset. ing the membrane and in overcom- ing obstacles, but not making good the plastering or repointing the brickwork externally.
Saws
The saws available for cutting the slot include hand-saws, which are operated by two men, power-operat ed reciprocating saws, circular saw, and the new chain saw.
a
The reciprocating saws, hand and power-operated, are of mild steel,
with teeth tipped with tungsten carbide or stellite. The hand saw has a 42-in. blade; the 3-in. width is preferable to the 4-in. wide blade normally supplied. Sawing so near the ground is exhausting, but for short periods the work can be done as quickly as with a power saw. For a single job, the hand saw provides a cheap and practical method of cutting the slot; in any case, a hand saw is a most useful adjunct to a power saw.
Two types of
power-operated reciprocating saws have been used: a light pneumatic model, weighing 5 lb and designed to be held by the operator, and a more powerful type which requires a supporting frame for more continuous work. Other forms of saw may well be suitable; larger and more powerful tools are one has developed a circular saw used by specialist contractors, and one has developed a circular saw claimed to be highly efficient.
A disadvantage of reciprocating saws is that the swarf from the slot is not removed as the cut advances, and after a time this slows down the rate of working. With a chain saw (as well as with the hand-saws and circular saws), the swarf is removed by the sweep of the blade. but the chain saws ordinarily used for cut- ting and dressing stone cut a slot that is too wide for d.p.c. insertion
-at least į in. wide, that is, wider than a mortar joint.
a
A new chain saw blade has been developed at the Station for use with lightweight petrol-driven chain the blade tips have been replaced by tungsten carbide
cutting blocks brazed into the links and support. ed by mild-steel buttresses; the chain is in. wide and it cuts a slot only slightly wider. The saw, which weighs 6} lb, is fitted with a guide bar 14 in. long and will satisfactorily cut a 9-in. solid wall. It is driven through a flexible shaft by a light- weight 24 h.p. air-cooled petrol en- gine. This unit can be carried and operated by one man. The expected life of a chain and guide bar (which must have a hardened nose) is 150- 200 linear ft of 9-in. wall.
This new development has over- come several earlier limitations of the method of d.p.c. insertion. The chain saw is convenient to use, the rate of work is faster than with other
THE HONG KỘNG & FAR EAST BUILDER-VOLUME 18. NUMBER 3