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. Finally, I was not able to Limit the extent of cracks in the wall roughcast, which is finally cracking: I have removed the crumbling parts, and protected the wall with a plastic sheet waiting for a dry weather.
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