How to apply and maintain rendering on walls

Rendering or stucco applied to the outside walls of a house is both decorative and functional: it brightens an otherwise shabby or crumbling surface, but at the same time helps to waterproof and protect the wall.

Many types of finish can be produced, some of which are smooth while others are textured or mixed with pebbles. They can be applied to almost any surface—bricks, blocks, concrete or even timber-frame construction— so repairs are relatively easy provided the base is prepared adequately and the new render matched carefully with the original.

You should carry out regular checks on walls which are rendered so any faults can be noted and cured before they worsen. Cracks or splits in the material are usually easy to spot but you should also test for render which has ‘blown’ or broken loose from the wall. Tap all over the surface with the handle of a hammer: a hollow sound indicates where the render has blown and how extensive the fault is. Bear in mind that if the damage is severe or over a wide area; it would be better to re-render the entire wall. This is often the case on older houses which have the traditional rendered finishes— particularly the type of finish known as stucco.

Minor repairs

Small cracks or splits less than 5mm wide are easily repaired providing the render around them is sound and the wall below is undamaged. Start by clearing away loose material and dust with a brush, making sure that each crack is perfectly clean. Then, before repairs are carried out, douse the inside and around the edges of each gap with water. This will stabilize dusty patches and help provide a good key for the filling.

Repairs to a small number of cracks are best made with a good quality, exterior-grade filler; but where there is extensive splitting, mortar is more suitable and less expensive. Make up a fairly dry mix of one part cement to five of sharp sand, adding proprietary waterproofei to the mixing water.

Load a manageable amount on to a plasterers’ hawk so that it can be carried more easily to where it is needed. Then use a pointing trowel to scrape the mortar from the hawk and press it hard into each crack. Take care to scrape off any excess mortar once you are level with the surrounding wall. Deep splits may have to be filled in two stages, allowing plenty of drying time between applications.

Leave the repair mortar to dry for about one hour then, while it is still fairly wet, run a damp sponge over the top to smooth it completely fiat. Afterwards texture the repair or add pebbles to help it blend with the rest of the wall.

Repairing damaged patches

Where large cracks have appeared or the render has broken loose from the wall, you have no choice but to cut away the defective material with a bolster and hammer until you reach a sound edge. It is essential to wear safety glasses when doing this to protect your eyes from the loose chippings of hardened mortar which are bound to fly around.

It is not necessary to use excessive force or leverage to break away the loose render. If it has already come awaj’ from the wall, a gentle tap with the hammer on the bolster should be enough to bring it down.

Start from the centre of the damaged area and hold the bolster at right-angles to the wall. Carefully break away the render until you reach sound material around the edges and then clean off the masonry underneath by scraping it with the bolster.

Once all of the render has dropped away, work around the outside of the hole undercutting the edge so that it is wider at the back than the front. This will make it easier to refill the hole with new render and help it to adhere well around the edges.

With reasonably rough surfaces— such as bricks, blocks or concrete—no further treatment is required before refilling, but smooth and dense masonry—such as engineering bricks or flattened concrete—should be roughened to provide a good key. Do this with the hammer and bolster by cutting a shallow criss-cross pattern across the face of the wall. Alternatively, use a comb hammer if you have one. Be sure to wear goggles or safety glasses to protect your eyes from flying chips of masonry.

Undercoating

Rendering a wall is rather like plastering—an undercoat or scratchcoat must be applied first, left to harden and then a top or finishing coat added.

In Canada, rendering is often applied in three coats. This is especially so when rendering on timber-framed walls, where the first coat is pushed through a wire mesh netting stapled over the surface of the wall: the wire mesh provides the key which holds the undercoat in place.

Once the wall, whether masonry or timber, is cleaned and prepared, mix up the undercoat mortar—using a 1:6 mix for smooth stucco render, and a 1:4 mix for roughcast or textured render. Do not add too high a proportion of cement, as this will lead to surface cracking during drying.

Measure and mix the mortar carefully, adding waterproofer to the mixing water to stop it cracking. Load a small amount onto the hawk and transfer this carefully to the wall. By holding the hawk near the hole to be repaired it will be easy to scrape a small amount of mortar off it on to the wall with a plastering float. Push the float hard against the wall and move it smoothly upwards in wide sweeps so that the mortar sticks to the surface.

Build up the undercoat in layers until it is about 5mm from the outside surface of the edge of the hole. Then, while the mixture is still wet, stipple the surface so that a good key is created for the top coat. To do this, dab a coarse, hard brush—such as a scrubbing brush—all over the surface and raise it into small peaks.

The mortar should then be left to dry for at least two—and preferably three to five—days. Do not hurry tne process or apply the topcoat before the mortar has completely hardened or splits could occur. In hot weather cover the drying mortar with tarpaulin or polythene sheeting and douse it regularly with water so that it hardens gradually.

Rendered finishes

When repairing defective patches of render the topcoat must be applied with care so that it matches the existing wall. So, before you mix up

Types of rendering

Fashions in rendered finishes come and go like building styles, but it is nevertheless useful to know how each is applied—nothing looks worse than a patch of repaired, but badly mismatched, rendering.

Stucco and ashlar finishes are often to be found on late 18th and early 19th century town houses in the UK. However, the mortar originally used is not noted for its hardwearing properties and as a result, many houses retaining their original finishes are long overdue for complete re-rendering. Pebbledash and tyrolean finishes are both characteristic of more recent property, especially semidetached houses built in the UK between the wars and so may only require minor repairs. the mortar for this stage, check carefully exactly what type of finish you already have on the wall: Stucco; This is a plain, smooth rendering with no gravel or pebbles added to strengthen or waterproof it. Although it is neater in appearance than roughcast or pebbledash it has less resistance to weathering and must be applied carefully to avoid cracking or crazing as it dries.

Prepare a mortar mix of one part cement to six of sharp sand and apply it to the wall with a hawk and plasterers’ float. When you reach the top edge of the wall, level the mixture by placing a long wooden straightedge across the repair. By keeping each end on the existing wall and moving it up and down in a sawing motion, excess mortar will be scraped off and the repair levelled. Use the straightedge horizontally working upwards, or diagonally moving across the work, but never work downwards or the render will fall out.

Finally, smooth the surface with a wooden float and when the mortar is nearly dry, feather the edges into the surrounding render with a damp sponge. Leave the patch to harden for two or three days before repainting the whole area. Textured finishes: These are created by applying a topcoat in the same way as for the stucco finish and then texturing the surface with one of a number of different tools.

Once the topcoat mortar has been levelled it should be textured before it has had a chance to dry. A number of special tools can be used for this but generally a large hacksaw or saw blade is raked across the surface to give it a lined, pitted appearance.

When creating this texture, take care always to scrape the wall in the same direction and to keep the saw in contact with the surface. If pieces of mortar drop off around the teeth of the saw, the surface is too wet and should be left to dry for a short period before retexturing.

Tyrolean finish: This is a type of textured finish which is supplied in a number of standard colours. It is applied to the wall on top of the finishing coat with a special hand-operated machine available from most hire firms. This throws the paint at the wall in a random fashion to create a deep textured pattern.

Apply the topcoat in the same strength using the same techniques as used for the stucco finish, then smooth it level with a wooden float. Mix up the texture paint according to the manufacturer’s instructions; it is usually supplied in powder form and needs to be mixed to the ratio two to three parts clean water to one part powder.

Load the machine with the mixture and carry it carefully to the wall. To cover the wall thoroughly and achieve the correct texture pattern, apply three coats to each patch.

Take care that the mouth of the machine is never directed towards windows, downpipes or doors. Where this is unavoidable, use a dry brush followed by a damp sponge to remove any finish that lands on the woodwork or glass. Better still, mask the areas beforehand using polythene or news- paper and masking tape.

The first coat should be applied working diagonally downwards from right to left at an angle of about 45° to the horizontal; the second coat down-wai-ds from left to right at 45°, and the third coat working from the bottom of the repair vertically upwards. Cottage texture: This is a finish which is textured by hand rather than with tools or by machine. The random, rugged effect which is produced looks purposefully old-fashioned—hence its name.

First, apply a topcoat of render in the normal way, smoothing it off with a wooden float and allowing it to set hard for two or three days. Then mix up more mortar to the same strength and coat this in a random pattern across the repair using a hawk and plasterers’ float.

Try to mimic the existing texture finish on the wall when you do this by holding the hawk close to the wall and pushing a small amount of mortar from it against the patch with the float. Use a scraping action away from your body to deposit a blob of mortar on the surface. Then, with an upward twist of the wrist, feather the blob away to leave a diagonal trail running upwards across the wall.

While you are doing this, try to pick up the same amount of mortar each time and space it evenly across the wall so that you maintain a regular pattern. For a rough, deeply contoured pattern use more mortar and try to spread it over a shorter distance; for a more delicate effect keep the mortar to a minimum and spread it out across the wall with wide sweeps of the float. Pebbledash: This is a finish in which small pebbles or crushed stones— varying in size from about 6mm to 13mm—are thrown at the still wet topcoat. Once the mortar dries, the stones are held tightly and left uncovered to provide a rough weatherproof surface with attractive variations of natural colour.

As an alternative to pebbles, white spar chippings can be used. These provide a durable finish with a fresh, clear appearance in contrast to pebbles which get dirty after a few years and need to be cleaned or painted regularly.

When applying pebbledash, it is most important that the topcoat of render on to which they are thrown is wet and not packed too tightly. Prepare the topcoat mortar with plenty of water and apply it to the repair with the float, leaving it about 10mm below the existing wall surface to accommodate the pebbles. There is no need to use a wooden float to get the surface absolutely smooth as this will pack the mixture too tightly— simply sweep away any loose mortar or dirt from the ground beneath the patch and pebbledash the wall as soon as is practically possible.

Load the stones in a bucket and throw them at the mortar by hand or with a small hand shovel. Continue until all of the patch is covered evenly. Any stones which do not stick to the wall can be picked up and used again. Roughcast: This is a finish similar in appearance to pebbledash, but instead of the pebbles being thrown at the mortar on the wall, they are mixed with mortar before being applied. It is particularly suitable for walls which are slightly uneven or have poor surface adhesion.

Prepare a 1:5 mortar mix with the pebbles spread evenly throughout it and apply this directly to the undercoat. Once this has set load the mixture on to the hawk and push it hard against the wall with a trowel so that it is held firmly. Build up the mixture in a number of layers until it is level with the existing wall surface, feathering it away around the edges with a damp sponge or cloth once the mortar has partially dried.

Other finishes: Some existing walls may have finishes other than those mentioned above and you may have to do a great deal of experimentation so that you can reproduce the exact finish you want. A great number of renders are textured with special brushes or rollers or are scored in a criss-cross fashion with the blade of a trowel. If the wall you want to repair shows any of these unusual finishes, experiment on a small, unobtrusive patch of wall until you get the exact texture you want before attempting more widespread rendering.

Guttering should be inspected twice a year, in late autumn and again in the spring. It will almost certainly be necessary to sweep out any accumulation of leaves and dirt with a hand brush and trowel or, in the case of plastic guttering, with a shaped piece of hardboard.

Keep the debris well away from the downpipe outlet. If the outlet does not have a cage or grille fixed to prevent debris from entering and blocking the downpipe, roll a piece of galvanized wire netting into a ball and insert it in the neck of the pipe. Do make sure that the wire ball is sufficiently large not to fall down the pipe.

With cast-iron or galvanized iron guttering, check carefully for any rust. Use a wire brush to remove loose flakes of paint and rust and treat the surface with a rust inhibitor. The surface should then be given one or two coats of bituminous paint to form a strong protective layer.

On Ogee-section guttering , or galvanized guttering fixed on with through spikes, rust may well be found around the fixings to the fascia—in which case the damaged section may have to be removed for treatment.

Clearing blocked downpipes

Before unblocking a downpipe, put a plastic bowl or large tin under the base of the pipe at the discharge into the drain to prevent any debris entering the drainage system.

When cleaning hopper heads , use rubber gloves to protect your hands against sharp edges.

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