The Modern Bricklayer - A Practical Work on Bricklaying in all its Branches - Volume III: With Special Selections on Tiling and Slating, Specifications Estimating, Etc

By William Frost

Brickwork is a form of masonry that utilises bricks and mortar. Rows of bricks—or, 'courses'—are placed on top of each other in order to create a structure such as a wall. This is volume I of William Frost's “The Modern Bricklayer”, a detailed guide to all aspects of bricklaying, including slating, tiling, planning, materials, tools, and more. Contents include: “House Drains”, “Egg-Shaped and Circular Sewers”, “Sand Courses”, “Retaining Walls”, “Reinforced Brickwork”, “Arches”, “Cornices”, “Gauged Brickwork: Introduction”, “Gauged Work—Various Forms of Arches”, “Gauges Work—Arches”, “Gauged Work: Ninches, Panels, and Mouldings”, “Terra-cotta and Glazed Ware”, etc. Many vintage books such as this are becoming increasingly scarce and expensive. It is with this in mind that we are republishing this volume now in a modern, high-quality edition complete with a specially-commissioned new introduction on DIY.

• Language: English
• Publisher: Old Hand Books
• Release date: Jun 28, 2021
• ISBN: 9781528769044

Book preview

THE

MODERN BRICKLAYER

A PRACTICAL WORK ON BRICKLAYING

IN ALL ITS BRANCHES

WITH

SPECIAL SECTIONS ON

TILING AND SLATING, SPECIFICATIONS

ESTIMATING, ETC. ETC.

BY

WILLIAM FROST

Hon. F.F.B., M.R.Soc.T., Cert.R.S.I.

LATE HEAD OF THE BRICKWORK DEPARTMENT AT THE L.C.C. BRIXTON SCHOOL OF BUILDING. LATE

SENIOR LECTURER AND INSTRUCTOR IN BRICKWORK AT THE BOROUGH POLYTECHNIC INSTITUTE,

AND LATE BRICKWORK INSTRUCTOR AT WIMBLEDON TECHNICAL COLLEGE, CHATHAM TECHNICAL

COLLEGE, AND THE BUILDING CRAFTS TRAINING SCHOOL, GREAT TITCHFIELD STREET,

LONDON. LATE DEMONSTRATOR IN BRICKWORK, UNIVERSITY OF LONDON, UNIVERSITY COLLEGE

(ARCHITECTURAL SECTION), AND MEMBER OF THE ADVISORY COMMITTEE ON BRICKWORK, CITY

AND GUILDS OF LONDON INSTITUTE, 1930 TO 1937. AUTHOR OF "CONSTRUCTIVE SANITARY

WORK; BRICK ARCHES—THEIR SETTING OUT AND CONSTRUCTION; THE BONDING OF

BRICKWORK; BRICKLAYING FOR BEGINNERS; ASSOCIATE EDITOR OF THE BUILDING

ENCYCLOPÆDIA 1937; BRICKLAYERS REPAIR WORK; QUESTIONS AND ANSWERS ON

BRICKWORK" (ELEMENTARY AND ADVANCED)

VOLUME III

Copyright © 2018 Read Books Ltd.

This book is copyright and may not be

reproduced or copied in any way without

the express permission of the publisher in writing

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

CONTENTS

VOL. III

CHAPTER I

PREVENTION OF DAMP

CHAPTER II

CLEANING BRICKWORK

CHAPTER III

MEASUREMENT OF BRICKWORK

CHAPTER IV

SCOTTISH MODE FOR THE MEASUREMENT OF BRICKWORK

CHAPTER V

TALL CHIMNEY SHAFTS

CHAPTER VI

DRAWING

CHAPTER VII

SUPERVISION

CHAPTER VIII

NOTES ON SPECIFICATIONS

CHAPTER IX

NOTES ON ESTIMATING

CHAPTER X

PRACTICAL AND THEORETICAL TRAINING

CHAPTER XI

LONDON COUNTY COUNCIL BY-LAWS

CHAPTER XII

AMERICAN AND CANADIAN PRACTICE

CHAPTER XIII

WINTER BRICKLAYING

CHAPTER XIV

GLOSSARY OF TERMS USED IN BRICKLAYING

CHAPTER XV

TABLES AND GENERAL DATA

INDEX

LIST OF PLATES

VOL. III

INTERESTING EXAMPLE OF BRICKWORK, CARRIED OUT WITH S6 PURPLE AND RED BRICKS FOR RECESSED PANEL FILLING, BRIGHT REDS FOR PIERS AND DARK REDS FOR PLINTH

FOOTINGS OF A CIRCULAR CHIMNEY SHAFT

A CHIMNEY SHAFT IN COURSE OF CONSTRUCTION

CHIMNEY SHAFT WITH SQUARE BASE, PANELLED AT THE SUMMIT

TWIN CHIMNEY SHAFTS WITH ORNAMENTAL BASE AND CAPS

A RANGE OF FOUR CHIMNEY SHAFTS

INTERESTING EXAMPLE OF BRICKWORK, CARRIED OUT WITH S6 PURPLE AND RED BRICKS FOR RECESSED PANEL FILLING, BRIGHT REDS FOR PIERS AND DARK REDS FOR PLINTH.

Berkshire bricks by Thomas Lawrence &Sons.

THE MODERN BRICKLAYER

VOL. III

CHAPTER I

PREVENTION OF DAMP

CAUSES OF DAMPNESS. SITE PROTECTION. VERTICAL DAMP-PROOF COURSE. PARAPET WALLS. FLAT ROOFS. LEAKY PIPES. SPECIAL TYPES OF DAMP-PROOF COURSES: Blue Staffordshire Bricks—Ruberoid —Sheet Lead—Asphalt—Slates—Glazed Earthenware—Cement and Sand—Sand and Pitch. INSERTING A DAMP-PROOF COURSE IN AN EXISTING WALL. KNAPEN SYSTEM OF DAMP-PROOFING. POINTING. CONDENSATION. CAVITY WALLS: Ties—Building Cavity Walls—Examples of Cavity Wall Construction.

ONE of the main objects of modern sanitation is to prevent or mitigate dampness in buildings of every description. Dampness not only deteriorates house property, hastening its falling into decay, but is considered a primary cause of many diseases and general unhealthy conditions. It is largely preventable, for dampness in buildings can generally be traced to three causes: bad workmanship, faulty design, or inferior materials and fittings. Dampness can only be prevented or cured when sound materials are used by good craftsmen with a special knowledge or under expert guidance.

CAUSES OF DAMPNESS

The causes of dampness are many, though most of them can be grouped under four divisions:

(1) Bad drainage of the ground under and surrounding the building; the omission of, or bad construction of, horizontal and vertical damp-proof courses; faulty construction, or faulty materials, used in laying the site concrete; faulty gullies and drains attached to buildings. All these lead to dampness rising from the ground upwards.

(2) Rain beating against the faces of walls; defective or broken water pipes, waste pipes, or water heads; the omission or bad construction of vertical damp-proof courses—all of which lead to dampness entering the building in an horizontal direction.

(3) Defective or badly constructed damp-proof courses attached to parapet walls, etc.; leaky or blocked gutters; badly constructed roofs, or roofs out of repair—all of which cause the dampness to creep downwards.

(4) Faulty fittings, defective or burst pipes, inadequate supply or size of waste and overflows, which distribute dampness in awkward and unforeseen directions.

In all these cases dampness is induced or accentuated by the law of capillary attraction, which for our present purpose we may define as the porosity of materials.

Another very unpleasant but less dangerous form of dampness is due to condensation, caused by the deposit of moisture in the atmosphere on a colder, hard, dense and usually polished or smooth surface. Thus, under certain weather conditions, walls, either inside or out, may be dripping with water. Though difficult to cope with, even this can be modified.

FIG. 1.

FIG. 2.

The horizontal progression of dampness through a wall is shown in Fig. 1. In this case it is due to the absence of a vertical damp-proof course between the external face of the wall and the wet earth at the back, whence the dampness, meeting with no obstruction, penetrates and spreads. Similar cases are often met with in connection with boundary walls, garden walls and house walls, where the level of the ground is much higher than the level of the house, as in basements, houses backed against hills or banks, etc. The section of a 9-inch wall, with footings and concrete seal, where the dampness is spreading upwards, is shown in Fig. 2. In this case there is no horizontal damp-proof course to obstruct the rise of moisture, probably drawn from the earth at the sides of the footings, and even through the concrete. If the concrete is spongy, open in texture, even though made with the best of materials and waterproofed, moisture will rise through, attracted by the warmth and dampness above it. On the other hand, if the concrete were absolutely impermeable, dampness would be drawn from the wet ground by the footings. In Fig. 3 we have the section of a wall where dampness is travelling downwards from the top brick on edge course through the wall, which would happen if inferior mortar or porous bricks were used in building the course. It would also occur if the joints were not solidly filled, or if the fillets on the projecting courses decayed owing to atmospheric conditions or the use of bad materials, when a ledge would be left for the rain to settle on, and find its way into the brickwork, as indicated by the arrow. In Fig. 4 we have the section and portion of the elevation of a 9-inch boundary wall, with a brick on edge and tile-creasing course. This particular wall, which the author had occasion to examine, was showing on its face dampness of a peculiar nature. It was due to moisture penetrating through the wall from the earth in the garden, which was on a much higher level than the pavement on the external face of the wall. There was no vertical damp-proof course on the inside of this boundary wall to protect the internal face from the very moist bank of earth behind it. It was obvious that the dampness came from the earth at the back, but the peculiar point was that this only showed on the faces of the majority of headers, whilst the stretcher bricks between them were in a particularly dry state. The reason for this can be better understood by referring to the detail in the left-hand diagram Fig. 4. This shows a small section of the wall with three courses of the brickwork. The thick lines represent the horizontal bed joints and the vertical wall joints. The centre arrow represents the header, and it will be seen that the dampness could pass through this brick (especially if of a porous character) from the internal to the external face of the wall. The arrows placed horizontally above and below the centre one indicate the stretchers on the back face of the wall, which were wet, so the dampness had passed through them. This dampness penetrated up to a certain point, the wall joint between the back and front stretchers; and not being able to proceed farther on account of this joint, composed of cement and sand, it had to take a different direction. What happened was that this vertical wall joint and the top and bottom horizontal joints formed a watertight cell or pocket, protecting the external stretchers from the moisture, and so they are marked as dry on the diagram. But having to find a way out, the moisture penetrated the headers, or through bricks. This, of course, might occur in the wall of the building, and be aggravated by the difference of temperature inside and out. A suitable cure in such a case would be to construct a vertical damp-proof course attached to the back face of the wall, and so prevent the dampness penetrating the brickwork from the earth.

FIG. 3.

FIG. 4.

A weak place in many small houses is the bay window on the ground floor, where considerable dampness frequently shows at the base, and may spread widely from there. In the majority of cases it will be found that these bays have no horizontal damp-proof courses. At other times the defect may be due to bad paving round the base of the bay, or occasionally to earth being piled up to a level above the damp-proof course. A too-common attempt to cure this nuisance is to render the face of the wall with cement mortar. This may be perfectly watertight as regards the face of the wall, but does not touch the real source of evil, the dampness at the foot of the wall, which penetrates below or at ground level and spreads upwards through the wall.

The precise form of cure for this manifestation of dampness can only be found by thoroughly examining the wall and its surroundings, with a view to ascertaining (1) whether the wall has an horizontal damp-proof course or not, and if not, to insert one; (2) the position of the damp-proof course if existent. Such course should not be less than 6 inches above the level of the ground.

If lower than, or below, heaped-up soil, the soil should either be removed to a lower level, or a vertical damp-proof course built on the external face of the wall. Such a vertical course should extend 3 inches below the level of the horizontal damp-proof course and 6 inches above the level of the surrounding ground. This would prevent dampness from rising in a vertical direction in the wall, while at the same time dampness would be prevented from gaining access in an horizontal direction from the surrounding ground. This is a good illustration of the effective combination of the two systems to meet a threat from two directions.

FIG. 5.

SITE PROTECTION

Fig. 5 is the section of a wall showing the various positions of the damp-proof courses. Starting from the base, the site bed of concrete is shown. This must be of good materials, properly gauged and well laid. This is important for all buildings, but particularly as regards dwellings. The site concrete should be composed of a good cement, with the addition of suitably graded aggregate, such as Thames or other river ballast, or clean broken bricks, and clean sand, waterproofing material being added either to the dry cement or in the gauging water. It should be well mixed dry, and thoroughly worked, but not overworked, when gauged. This should be solidly laid all over the whole site, so as to form an impervious seal, which will prevent dampness from the soil rising through it into the space beneath the floor. The ground is almost always damp, at some times markedly so, not always from rain in the immediate neighbourhood, but often from the fluctuations of the level of the ground water, which may be fed from some distance. If the site is not properly sealed, it will be more or less damp continually, and at certain periods dangerously so. And this inconvenience will also be felt if the sealing is incomplete or inadequate. Then, if the concrete is too thin, or of a light, spongy nature, dampness will rise, as indicated by the arrow heads. But that is not all, for dampness would find access at B if the concrete was of this unsatisfactory character.

The space between the site concrete and wood floors should be well ventilated and kept dry, and for this purpose a current of air should be induced from back to front by means of air bricks placed in correct positions near the base of the front and back walls. Such pierced air bricks must be well above ground level, so that they cannot become blocked up by the soil. Sleeper walls are placed in position on the site concrete in order to support the floor plates, which in turn support the floor joists and boards. A section of the sleeper walls is shown by the arrow D, this wall having an horizontal damp-proof course with the floor placed in position above it. An elevation of part of these walls is shown at E, provided with a series of openings. These are for the purpose of ventilation, a current of air passing through them from front to back of the house. If the site concrete were of a light, spongy nature, and the sleeper walls were not provided with damp-proof courses, the moisture would soon find its way upward from the ground to the floor plate, which would in time become rotten and cause damage to the joists and floor boards, and so make the house damp. A is the horizontal damp-proof course in the main wall, and is extended in length so as to take the floor plate, while C is the cement plinth on the face of the wall, the internal angle at the base of which is given a round finish with a view to keeping dampness from entering at this point. Very often, when such angles are finished square, the cement shrinks here, and so forms a more or less deep and wide crack through which surface water can easily find entrance and then dampness spreads in the wall. If there is a good through ventilation between the site concrete and floor, the current of air will tend to dry any moisture which may appear on the internal face of the wall. In some cases the site concrete is finished off with a layer of cement and sand, or of asphalt, to ensure imperviousness.

FIG. 6.

VERTICAL DAMP-PROOF COURSE

In many houses the basement of cellar floor is below the level of the surrounding ground outside the building (Fig. 6). In such a case a vertical damp-proof course is necessary to prevent dampness from percolating through the wall in an horizontal direction from the ground outside. For this purpose an appropriate form of damp-proof course would be a layer of slates, properly lapped and bedded in cement and sand mortar. But other materials are used for a damp-proof course in this position, such as asphalt, coal tar and sand, cement and sand mortar, etc. The horizontal damp-proof course prevents dampness from entering the wall from points indicated by the arrows A and B. The floor of the cellar is finished with a layer of asphalt, is rounded at the internal corners, and finished with a plinth 6 inches high. These are a few of the causes of dampness and the positions in which they occur at the base of a house. In the cases cited above, the dampness will rise by capillary attraction, either in a vertical or horizontal direction within the wall, the effect always being greater where porous bricks or other materials, or faulty joints, are used in their construction.

PARAPET WALLS

Fig. 7 shows a parapet wall built with good hard stock bricks, two courses of Broseley tiles well lapped and bedded in well-made cement mortar, and finished off with a course of bricks laid on their edges. All joints of the bricks on edge course should be solid and well pointed. This prevents dampness from percolating into the top course of the wall. The two courses of tiles when solidly bedded act as an horizontal damp-proof course, and protect the lower position. They should project beyond the two vertical faces of the wall for at least 1 1/4 inches, and should be finished on their upper projecting surface with a weathered fillet of cement, constructed on its outer surface at an angle of 45°. This weathering of the fillet throws off the rainwater, so that it does not fall on the wall below. The two external faces of the wall should be well pointed and solidly jointed, as this portion of the wall is usually subjected to severe weather effects, such as wind and wind-driven rain or snow. If this portion is composed of porous bricks (as shown by the arrow A), and badly constructed joints, the rain, etc., will penetrate into the brickwork and the dampness will probably take a downward course. If this occurs, the damp-proof course, formed with the tiles and cement, would be useless in preventing the downward flow of the moisture or spread of dampness. On the inside of the parapet the section of a lead gutter is shown, the side of which is turned up at right angles to the wall for a height of 6 inches, and covered with a lead flashing, the top of which is inserted into the bed joint of the wall for a depth of 1 inch, and well pointed after the lead has been fixed in position. The other side of the gutter is turned up at an obtuse angle and extends well under the eaves course of the slate roof. This forms a sound waterproof construction if the proper material is used. If poor material is used, however, only one thing is likely to happen later on—that is, the quick deterioration of the material through a variety of causes. For instance, a slate sliding down the roof, such slate having become unfixed owing to bad nailing, has often been known to cut through lead material of a poor quality, or too light a gauge, when it strikes the gutter, thus forming an opening into which the rain will penetrate, and so cause moisture to penetrate into the house.

FIG. 7.

Another cause of a leaky gutter arises at times when a workman has to walk along the gutter to repair it. In doing so he may tread upon particles of grit or small stones, which, on being pressed into poor quality or light-gauge lead, may possibly pierce through the whole thickness of the lead, again causing a leaky roof. Damage is occasionally caused by the repairer wearing heavy hobnailed boots. It is a fact that very poor quality slates are easily damaged, even by the pecking of pigeons with their beaks, the slates becoming riddled with small, rain-admitting holes. A cheap slate will also crack across its whole width either from intense heat of the sun or extreme cold of a severe frost.

FIG. 8.

FIG. 9.

Two forms of construction used for parapet walls are shown in Figs 8 and 9. The horizontal damp-proof course in the first instance consists of two courses of tiles (these are drip tiles, turned up at the ends) in a downward direction. This takes the place of the cement fillet shown in Fig. 7, and like it, throws the rain off away from the face of the wall. In the second instance the construction is of natural stone, or of cement and sand mortar moulded in a box. The upper surfaces are in the form of two inclined planes, which throw off the rain downwards from the centre apex. The under surface is extended on either side of the wall for a distance of 2 1/4 inches, with an inverted curve or throating, which forms a drip for the rain which collects at this point from the upper surfaces of the coping. This throating, formed by running a groove in the under surface (which may be done with a round-tipped trowel, a length of piping, or other round object, pressed into the wet cement mortar), keeps the drip from accumulated moisture from the wall, thus protecting it from a wetting.

FIG. 10.

FLAT ROOFS

Fig. 10 shows a flat roof constructed of reinforced concrete and a finished coat of asphalt. In the illustration can be seen the internal angle with a fillet of cement, usually run in this position in good work, and against which the asphalt is placed, finished off with a concave curve, continued upwards for 6 inches, and then worked carefully into the joint horizontally for a depth of at least 3/4 inch, or more if possible. When finished, it should be pointed neatly with cement. This form of construction makes a really sound, watertight job at the weakest spot, the connection between the roof and the wall.

In Fig. 11A we have a detail of the connection between the highest point of the roof and the parapet wall. In this case the lead soaker is placed at the top against the wall and underneath the last course of slates, and over the soakers are placed a wide flashing of sheet lead, which is secured by hardwood wedges, and entered into the joint as shown. When the lead is in its position, it is pointed with a neat weathered joint of cement and sand. This method gives a double protection at this particular point. Sometimes small pieces of sheet lead folded together are used to wedge the lead flashing into the horizontal joint of the brickwork. Yet another example of a similar connection is seen in Fig. 11B. In this case the soakers are fixed in position under the last course of slates, and then finished off with a cement fillet rounded off at an angle of 45°. This is not such a good connection as the former one, as there is a liability to shrinkage in cement; especially if the wall has not been roughed and wetted sufficiently to enable a good key for the cement. If the wall is smooth there will be poor adherence, and if dry and porous the cement will be deprived of its moisture before it has thoroughly set, and so will be weakened. The cement should be mixed to a stiff paste to counteract the tendency for it to slip down slightly on the surface of the roof, which provides an inferior key to the brick wall; but it should be sufficiently wet to ensure proper hydration, which alone enables the cement to set firmly. It follows that with a view to protecting such a weak spot great care must be taken not to leave an opening, be it merely a crack, and that the double sealing before described offers the better security.

FIG. 11.

LEAKY PIPES

So far dampness occurring at the lowest and highest parts of houses has been dealt with. These are the most common and usually the most serious. Others, however, have their importance. One of these is the leaky or cracked pipe (Fig. 12), a frequent cause of dampness on the external wall, which generally, especially in brickwork, penetrates right through. In bad cases of leaky joints and cracked pipes the water will wash out the joints of the brickwork, leaving it exposed to further infiltration and the effects of weather. Such faults, and the exact defective point, are soon detected. It is worse when the crack or the defective part of the joint is at the back of the pipe, because then a small leak may travel down the wall for some distance before finding a weak place in the brickwork and then striking inwards. It will sometimes happen that the inside face of an external wall will be reeking with dampness in one or more rooms without aprent cause, and is only with difficulty traced to a small leak at the back of an upper section of a soil or a rainwater pipe. When once located the defect can be easily remedied by removing the faulty section and replacing it with a new pipe. In doing this, however, great care must be taken in cutting out the old joint and easing the top and lower sections to admit of the new pipe being inserted. When an old pipe is in question, this cutting out, easing and making good again may possibly fracture another pipe or a socket. Occasionally a quite serviceable pipe will be found very fragile, or partly perished (this is frequently the case with light-weight galvanised-iron pipes from which the spelter has worn away), which will be unable to withstand handling and the extra weight of the new section.

FIG. 12.

Leaky joints are sometimes the result of a blocked pipe. This is more liable to happen where there are bad connections with branch pipes, wastes, etc.

SPECIAL TYPES OF DAMP-PROOF COURSES

Blue Staffordshire Bricks.—A damp-proof course, composed of two courses of Blue Staffordshire bricks bedded and jointed in cement and sand, makes an excellent job. These bricks are practically impervious, and in conjunction with the bedding material form a durable structure, well able to withstand the pressure from the walls above it.

FIG. 13.

Ruberoid.—Fig. 13 shows a damp-proof course of Ruberoid, a softish substance compounded of rubber. It is often reinforced with lead, a thin sheet of lead being sandwiched between two sheets of Ruberoid. This is effective, but great care must be taken, when laying the rolls, to ensure that both materials are laid flat on an even surface. No small stones or projections of any kind must be left on the bedding, because such irregularities would pierce the protective course, rendering it practically useless.

FIG. 14.

Sheet Lead (Fig. 14). —Equal caution in laying must be taken with this very excellent material.

FIG. 15.

Asphalt (Fig. 15).—Asphalt is usually spread hot on dry walls to a thickness of 3/4 inch. Externally asphalt is usually finished off 3/4 inch back from the face of the wall, the remaining part being pointed with cement and sand. If finished vertically on the external face of the wall it would, if constructed in hot weather, be affected by the pressure from the wall above it, probably being forced out. Asphalt is particularly useful, being plastic, so that it can be carried upwards or downwards, over or round projections, etc.

FIG. 16.

Slates.—Two courses of good-quality slates, bedded in cement mortar, are very generally used for this purpose (Fig. 16). Slates are used both horizontally and vertically. The slates must be of a non-porous character, and care must be taken to have solid, unbroken joints. Slates require care in laying evenly, otherwise they may be fractured, which is the chief drawback to their employment. Fracture not infrequently occurs from even slight settlement of walls.

FIG. 17.

Glazed Earthenware.—Specially made glazed stoneware perforated bricks provide a high-class damp-proof course (Fig. 17). They usually measure 9 inches by 9 inches, but other sizes are procurable. These bricks are impervious, and should be bedded solidly in cement mortar, the cross joints being treated in the same way. The joints in the courses above and below the glazed bricks should not be laid in a vertical line with the grooved and tongued joints, the breaking of the joint being to minimise the possibility of any dampness passing this point. These bricks being perforated provide excellent air inlets for ventilating basements, cavity walls, etc.

Cement and Sand.—A well-made cement mortar makes a capital damp-proof course. The materials should be thoroughly mixed, and the proportions should be i part of Portland cement to 2 parts of clean sand. The thickness of the layer should be 1 1/2 inches, screeded and levelled.

Sand and Pitch.—Sand and pitch are sometimes used, as well as bitumen. The mixtures should be applied warm to dry walls.

Among other substances used for damp-proofing are Pudlo, Ironite and Ledrinda.

INSERTING A DAMP-PROOF COURSE IN AN EXISTING WALL

In many cases of houses having been built without horizontal damp-proof courses, the only cure for dampness is to insert one, a difficult and lengthy operation. It is specially difficult in a stone wall, or a wall built of mixed materials. The operation is carried out in this way. The requisite position for the damp-proof