power saws and, when commencing a cut, it is possible to mortice into a joint this avoids knocking out a header brick. The possibility of working entirely on one side of wall brings with it several other ad- vantages which are discussed below. Practical details
a
The level at which the cut is made must be decided after careful in- spection of the site. taking acconut of ground and floor levels.
as
For solid floors, the cut should be made in the first bed joint in which the it is possible to work, above level of the slab and at least 6 in. above the outside ground level. If an impermeable flooring such
the pitchmastic is to be laid over concrete base, it must be linked with the new d.p.c. If a permeable floor- ing that is not sensitive to moisture is used. the finished level of the floor should preferably be at the same level as the d.p.c. or. if this is not practicable and the floor level is below the new d.p.c.. the brickwork between the d.p.c. and the top of the floor should be treated with two coats of bitumen paint and the skirt- ing should be protected against rot. If a moisture-sensitive floor. for in- stance of timber. is to be laid on the concrete. a damp-proof membrane should be provided and linked with the new d.p.c.
up
For suspended timber floors. for proper protection the d.p.c. should be inserted below plate level. Pro- vided that the outside ground level permits it, this can be done with the chain saw working from the outside only, without the need to take up and relay the floor. Where this is not practicable. inserting a d.p.c. in the first bed joint above floor level is very much a second-best solution: if it is adopted. it would be prudent to ensure that the wooden skirting will not be affected by damp and to increase the ventilation in the under. foor space.
An effective barrier against rising damp will be afforded only when the damp-proof course is taken around
all internal and external walls. in- cluding chimney breasts if possible. With the chain saw. chimney breasts can be cut without removing the fire- backs. except where fireplaces are back-to-back: in that case, or if other tools are used. it is possible to
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insert narrow widths of membrane around the chimney breast with the fireback in position, and this pro- tection combined with the heat from the fire may suffice to keep the chim- ney breast fairly dry.
The technique of inserting damp- proof course has been applied both by contractors and by the Building Research Station to a wide variety of dwellings, some of which were in poor repair. It has been shown that if care and common sense is exercis- ed, the method is safe to employ pro- viding: (1) Vertical joints are filled with mortar; otherwise. the brick- (2) work may not bridge the slot. Walls are reasonably plumb and are stiffened by returns or by partitions; this implies that a damp-proof course should not be inserted in, for ex- ample. long parapet walls which sometimes occur in buildings such as factories.
Of course all brickwork should be inspected before the work begins, especially as many old walls were built in two leaves. with the outer half much more carefully laid than the backing and with a broken bed joint. When the brickwork is in a very decayed condition, or there is little mortar in the vertical joints. many bricks in the course immedi ately above the saw may become loose and drop as the saw passes un- der them. Sometimes one or more courses of bricks must be completely removed in sections as the cut ad- vances and. after the membrane is inserted, replaced as headers bedded in mortar and wedged with slate. Despite the very poor conditions, set- tlements after inserting the damp. proof courses in such walls did not exceed 1/30 in. It is convenient for the bricklayers to have several sand- bags filled with dry cement-sand mix which can be prepared quickly to meet emergencies.
A problem that may be encounter ed in this work is that of removing cement mortar. Frequently old brick- work has been repaired
quite mistakenly by raking out the joints and repointing with a cement- sand mortar; elsewhere cement-sand plinths have been added in an tempt to combat rising damp. In both instances the cement must be removed before cutting slot, otherwise progress is slow and the saw blades are quickly damaged.
at.
mortar
a
cold-
Usually this is done with a chisel, a slow process which often damages the brickwork. Both jobs may be tackled more easily with grinding disc. either fitted in an att gle grinder or as a separate
unit driven by the portable engine, which would in any case be required to drive the chain saw. A batten align- ed with the bed joint provides a con- venient method of supporting and guiding the tool; a neat cut without ragged edges. rather more than wide, is produced when two carbor- undum-faced grinding discs are clamped to the same spindle. Slots up to 1 in. deep are cut at a rate of about 1 ft. per min. Incidental- ly, when a wall prepared in this way is subsequently cut, the slot is cut more rapidly and the cost of making good the plinth or rendering is avoided.
Injecting damp-proof courses
in.
There are several proprietary tech- niques which are claimed to produce a moisture barrier in a wall. These generally involve injecting water-re- pellent substances into holes drilled at regular intervals along the base of the wall. Silicone solutions, usually with water-displacing fluids, or an aqueous siliconate/latex mixture, or hot wax or setting resins, are exam. ples of such substances Certain dif- aculties arise in connection with most of these, particularly that of inject- ing a liquid into a wall that is al- ready saturated with water: it is not to ensure that a continuous membrane is formed through the full thickness of the wall. The presence of hygroscopic salts or detergents can also impair the effectiveness of the silicone treatment. Work at the Building Research Station on the in- jection of setting resins is not yet sufficiently advanced for any conclu sions to be drawn.
easy
Of the methods described above, that using a siliconate/latex mix- ture in aqueous media is probably the most satisfactory and one which is much less affected than the others by the presence of aqueous salts and detergents. It may work better when the walls are damp but the durabili- ty of the membrane is not known with certainty.
Electro-osmosis
In this method, which aims at producing a moisture barrier in the wall, electrodes are placed in the
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 3
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