Rendering and plastering

External rendering consists of two or more coats of a mixture of cement and sand or cement, lime and sand. The rendering can be applied to brick or block walls as well as other structures such as expanded metal lathing fixed to wooden or metal supports. In addition to providing protection against moisture penetration, the material is used for decorative purposes, either by making patterns on the wet surface or by varying the surface texture by the different methods of application, mixing and finishing. Cement colours can be added, but very careful measurement of the colours and all the other ingredients, as well as careful mixing, would be required to give an attractive appearance, that is not patchy.

In general, the undercoat should be weaker in strength than the walling material to which it is applied, but it should also be stronger than the finishing coat which will be applied on the top of it. Rendering failures are often caused by using mixes which contain too much cement. This leads to crazing and cracking. The addition of lime to the mix will overcome this problem as rendering containing lime will harden to form weatherproof and frost resistant surfaces which will allow structural movement to be absorbed. Small cracks are self-healing.

An external rendering must be durable, it must resist moisture penetration and have an attractive finish. Durability depends on a number of factors such as the type of mix, the bond between the wall and the rendering as well as on the standard of workmanship.

Water is less likely to penetrate through rendering of an absorbent type than it is through the cracks in the surface of a hard, dense coating. Rain falling on a smooth surface which has no absorption does not distribute itself evenly, but tends to run in rivulets down the surface. A rough surface breaks up this flow and therefore avoids concentration of water at any one point.

This means that when cracks appear in impermeable rendering, water may get through them and into the wall behind. When this happens it may cause loss of adhesion, further cracking or even complete disintegration of the rendering through frost action. Complete penetration of solid walls gives rise to damp patches on the interior with the consequent ruining of internal decorations. Serious dampness on the interior of the wall could also bring about the failure of the internal plastering.

Porous rendering will absorb some of the water which falls on its surface and this moisture will be retained until the weather conditions change and the sun dries it out again. In this way the water takes much longer to penetrate right through the rendering into the wall itself so that it rarely appears on the inside.

Water which has penetrated cracks in dense impermeable rendering.will be trapped behind it and will only be able to escape to the interior of the building. It will be obvious that the less water that is retained by the surface coating, the greater will be the risk that water will find its way through cracks into the wall. So a rough textured porous rendering will normally be more effective than a hard and impermeable, smooth finish. Appearance can also be a problem with smooth finishes, because they tend to weather unevenly due to stains from sills, copings and eaves. Rough textures help to break up water flow from these places and therefore they weather uniformly. In any case these stains depend to a large degree on the exposure of the site.

Ordinary Portland cement is used for the rendering mix, but white cement can be used if required. In cases where there is severe exposure to the weather or where a low suction is needed, a waterproof cement may be used for the first coat and it can also be used to advantage to provide an even suction on surfaces when white or coloured cement is to be used for the final coat.

Lime is added to the mix to improve the workability. As it reduces the strength and density of the mix it also reduces the danger of cracking which could lead to moisture penetration.

Clean, sharp sand is the third ingredient and it should be noted that sands with a strong colour will effect the colour of the mix and this is especially important when white cement is being used. For general mixes and for smooth finishes, the sand is evenly graded. For roughcast and similar finished a coarse particle is introduced. As with concrete, the small particles fill in any spaces between the larger ones and all particles must be covered with the cement slurry. Poorly graded sand takes up a lot of water and too much water in the mix will lead to cracking in the finished surface.

Different types of wall require different types of rendering mix. Rendering should not be applied to dense strong materials like concrete walls, hard, dense, concrete blocks or hard, impervious, clay bricks and blocks which do not have keyed faces, without the provision of an adequate key which must be strong enough to resist separation.

A splatter dash coat can be given to these materials to provide the key and this consists of a mix of 1 part cement : 2 parts fairly coarse sand, with the addition of enough water to form a thick slurry. This mix must be kept well stirred. The wall must be cleaned and damped then the cement is dashed against it to give a thin coating of an even roughcast appearance. This coating is wetted again after about an hour or so to ensure complete hydration.

Concrete can also be given a key by brushing on to it a mixture of 1 part cement : 1 ½ parts fine sharp sand mixed with water that contains an equal quantity of p.v.a. adhesive. The mixture is vigorously brushed into the surface of the wall and then stippled using a banister or similar brush so that a close textured key is formed. This is then left for at least seven days to harden before the rendering coats are applied.

Special treatment is not necessary for fairly strong porous materials such as bricks made of clay or sand and lime, or most lightweight building blocks, but it is desirable to rake out the joints to a depth of about 12 mm. It is also important when applying the material that no air pockets are left between the wall and the rendering. Fairly weak, porous materials like aerated concrete, building blocks and soft bricks are not suitable for mixes which are rich in cement. This is because, on drying, shrinkage stresses can be set up which will lead to bond failure between the rendering and the wall.

Where expanded metal is being used three coats will be needed, the first being a dense and relatively impervious mix which will protect the metalwork. This type of metal background is applied over wood, which would otherwise provide an unsatisfactory bond, and over unsound surfaces such as perished and disintegrating brickwork or masonry. The metal should be of 6 mm or 9 mm mesh and it should be applied with the long way of the mesh across the supports. These should be preservative treated battens set at not more than 400 mm centres. Metal packing should be used to hold the expanded metal at least 6 mm from the wooden battens so that the first coating can be forced through the mesh to totally encase the metal.

Good bond depends on mechanical key and on suction. If the background is coarse enough for the rendering to cling to the surface there will be sufficient key, but smoother surfaces need a degree of suction, which is the ability of the background to absorb moisture. Spraying the wall will indicate the degree of suction available, but if the water is soaked in too quickly, then the water in the mix will be extracted rapidly, so that the mix becomes stiff and difficult to spread. Spraying the wall with water will overcome this problem.

New brickwork which is to have a rendered finish should have the joints raked out while they are still green and a good brushing down is all that is required to remove loose material. Old walls, of course, need to be cleaned and free from lichen, mould, soot or paint.

Before applying the first coat, the wall is damped to ensure even suction and where possible the work should be started on the shaded side of the building. If a water repellent is used in the first coat it will not be advisable to dampen the surface before subsequent coats are applied, unless the weather is very hot and dry. In winter, exterior rendering must never be carried out during frost, or if frost is likely before the rendering will have cured sufficiently to resist it.

Straight cement and sand rendering mixes give a strong impervious finish with high shrinkage and cracking; lime is introduced to soften the mix and give the property of being able to absorb moisture which is given off rapidly when the rain has ceased. A mix of 1 part cement : ½ part lime : 4 ½ parts sand is suitable as a first coat for expanded metal and walls of dense strong materials. If the lime content is increased to 1 part cement : 1 part lime : 6 parts sand it will make a good average mix for the first coat on most other walling materials. It will also make a final coating for expanded metal. A good average final coating for all but severe conditions of exposure would be 1 part cement : 2 parts lime : 9 parts sand.

The hydrated lime is mixed with sand to the required proportions first and then the required proportion of cement is added. Ideally, the sand and lime should be allowed to stand for about 16 hours before the cement is added, but it should be protected during this time to prevent it drying out. If the sand and lime are allowed to stand the resultant mix will be more workable. Once the cement is added the mix should be used immediately.

Two coats are standard practice, but three coats are used when very irregular surfaces have to be levelled up and three coats are always used on expanded metal. The first coat should not be more than 16 mm thick. This is the ‘floating’ coat and is given ample time to dry and harden before the second or final coat is applied. The floating coat is given wavy horizontal combings when it has started to set. This is to provide a key for the final coat. These lines should be about 19 mm apart and about 6 mm deep. The final coating is quite thin and should not exceed 9 mm and in most cases is not more than 6 mm.

Roughcast needs a first coat of 1 part cement : ½ part lime : 4 ½ parts sand. The second coat which is the actual roughcast is 1 part cement : 1/> part lime : l ½ parts shingle: 3 parts sand. The shingle can vary from 6 mm to 13 mm. Sufficient water is added to make a plastic mix which can be thrown on to the wall from a bucket using a scoop or a laying-on trowel. The finrl appearance depends on the way in which the material is applied and an endeavour should be made to get an even texture with a wide spread of the rendering mortar.

For pebbledash the first coat is 1 : 4 ½ with a waterproofer added. The second coat 1:1:5 cement, lime, sand is applied about 12 mm thick. This coating is brought to an even and regular surface by drawing a straightedge over it and then while the rendering is still soft aggregate is dashed against it and gently pressed with a wooden float, to ensure a good bond. The aggregate is thrown from a bucket using a laying-on trowel or a scoop, in a similar manner to roughcast and again the object is to get an even and uniform appearance.

Another finish, though not often used, is produced by first applying a mix of 1 part cement : 4 ½ parts sand with a waterproofer added, followed by a final coat mixed at 1 : 2 : 9. When the final coat has started to set, the surface is scraped with an old saw blade or similar tool. Then the surface is brushed lightly to remove all the loose dust. This method produces a textured finish showing the larger particles of the aggregate.

Whatever the finish to be applied, the method of application of the two coats of rendering is the same. The rough surface of the wall is levelled up by applying bands of the mortar horizontally across the face of the brickwork. These bands of mortar are positioned as wide apart as can be spanned by an 1800 mm straightedge. They are made the same 16 mm thickness as the finished coating and are straight and smooth. In really first-class work they are plumbed as well. When they have set, they are used as a guide to level off the rendering. A laying-on float is used to spread the mortar mix between these bands and then the straightedge is used with a sawing action to scrape the excess material off the wall and leave a level and even surface.

The second coat which is much thinner and is applied to a level surface does not usually need these bands of mortar as a guide, but there is no reason why a beginner should not use them to help him to get a good surface. In any case when the coating has been spread over the surface to a fairly even thickness it is scraped off level with a straightedge as before.

Scaffolding is, of course, needed for both external rendering and internal plastering, because the wall has to be coated in bands about 1800 mm wide. This is as much as the average man can reach. These scaffolds must be independent; holes where horizontal supports for the planks have been inserted into the wall cannot be made good after the work is finished. The structure should be a minimum of 225 mm from the face of the wall to which the rendering is to be applied.

Tyrolean finish is an interesting texture that can be applied to walls by means of a hand operated splatter machine. The mix is 1 part ordinary or white cement : 2 parts sand and a colouring agent can be included if required. Enough water is added to make a creamy mix that can be splattered in layers on to the background. Make up only enough mix to be used in one hour. An alternative is to use a proprietary material called Cullamix made by the Cement Marketing Co. Ltd. This is a dry mix that only needs water adding. It can be obtained in a small range of colours.

There are various methods of preparing a suitable base for the decorative finish and these depend on the type of wall and its exposure. Application can be made direct to cavity walls and calcium silicate bricks, provided that the surface is clean and dry and free from any algae or lichen. All walls should have damp proof courses and overhanging eaves. The material is not suitable for application to previously painted surfaces.

All window frames and other areas which are not to be coated must be masked. The material is mixed in a clean bucket using 5 measures of Cullamix to 2 measures of clean water. Application is in layers to give a honeycombed texture. The interval between the layers will depend on the suction of the surface. If the wall is sufficiently absorbent the process will be practically continuous. Where an area cannot be completed in one day the edge is feathered out to blend in with the next day’s work.

The splatter machine should not be overloaded with material, it is better to use small quantities at frequent intervals. Turn the handle so that the maximum amount of material is thrown on to the wall and keep the machine moving with its mouth tilted slightly downwards to avoid overloading the flickers. After the first layer the speed of turning the handle is reduced to allow the flickers to pick up more material which when thrown on to the first layer forms the basis of the texture. The first layer is thrown from as far as possible from the wall but the second layer has to be thrown from much nearer to compensate for the reduction in the spread. A new layer must not be applied while the previous layer is still wet. For the application of the first layer it does not matter whether the machine points squarely at the wall or at an angle to it, but for subsequent layers the material is flicked on at an angle to accentuate the texture.

To provide a protective rendering and decorative finish to most types of brickwork and concrete blocks where there is adequate key, a render coat can be followed by the Tyrolean finish. The render coat is applied about 10 mm thick using a mix of 1 part cement : 1 part lime : 6 parts clean sharp sand. A proprietary mortar plasticiser can be used in place of the lime. The rendering is applied in the usual way and finished with a wood float. It is not keyed. For exposed conditions the rendering has a waterproofing agent incorporated. The mix is 1 part cement : 3 parts clean sharp sand. It is applied in the usual way, well keyed and allowed to dry for a minimum of five days. Next, a second coat mixed at 1 : 1 : 6 is applied and then the Tyrolean finish is applied when the rendering is dry.

An alternative finish is to rub down the surface of the decorative finish after it has been left for at least 24 hours to dry. The surface is then rubbed lightly with a carborundum or other suitable stone, using a circular motion to flatten the high points and leave all the valleys and indentations untouched. As the rubbing proceeds, the dust must be brushed out of the textured surface simultaneously, in order to be able to see just how much to rub to obtain an even texture. Mechanical rubbing is too severe and should not be employed. The operation must not be carried out if the surface is wet. Rubbing down can be done to the finish whatever the age, but it will of course take longer, as the finish hardens with time. Another way of obtaining a similar finish is to rub a straight-grain wooden float very lightly over the surface of the Tyrolean finish two or three hours after the application. The effect is to flatten the high points just sufficiently to retain an open texture.

Independent scaffolding must be used for access when carrying out this work and the boards must be turned back during rain or overnight in order to avoid splashing. If the decoration dries out rapidly due to drying winds or high temperature, then it will be necessary to spray the area lightly with water twice a day until the material has fully hardened. The machines for applying the splatter finish can be hired from the suppliers of the Tyrolean finish materials.

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