How to apply render to walls

Render, sometimes called stucco, is a sand and cement covering applied to the outside walls of some houses—both for decoration and for protection. Most types of exterior wall can be covered with render—brickwork, blockwork, concrete, even timber-framed constructions. Once prepared the wall then can be given a number of attractive finishes ranging from plain stucco to pebbledash.

Damaged render

Even if it has been applied correctly, render has a habit of working its way loose over a time. Small cracks caused by frost or accidental damage can quickly develop into large ‘blown’ areas where the material has broken away from the backing.

If you have an existing rendered wall in need of repair, your first task is to assess the extent of the damage. Large surface splits and areas of missing render are easily spotted. What is less obvious are the places where the render has ‘blown’ or parted company with brickwork but without falling down or splitting.

Check the whole wall carefully, working systematically from top to bottom. Tap the render with the handle of a hammer: a hollow sound will indicate areas where the material has ‘blown’. You should also make a note of where the surface is pitted or —in the case of decorative finishes such as pebbledash—where the decorative chippings are missing.

If, after extensive checks, you discover that damage is fairly limited, repairs can easily be carried out by hacking away the affected render and then making good. But if practically all the render on the wall is defective, you have no option but to start from scratch.

Removing old render

Before removing the old surface material, make sure you have a safe platform to work from. Except for the very smallest and very largest areas of wall, a scaffold tower is probably the best and safest type of equipment to use.

Damaged rendering will usually hack off easily with a bolster chisel— for quickest results, and to avoid damaging the surface underneath, hold the chisel almost parallel to the wall as you make your blows.

Preparing a timber-framed wall

Whether you are rendering a wall for the first time or carrying out repairs to one that has already been rendered, it is important to prepare the wall thoroughly. If the substrate is unsound, the render cannot be applied properly and all your work will be completely wasted.

On a timber-framed wall, render is held in place by being forced through a fine wire mesh. The mesh is fixed to the framing studs with plenty of large-headed galvanised nails or staples. There should at least be a sheathing of building paper behind the mesh, and preferably a timber sheathing as well.

Check that these components are present, and in good condition, and renew as necessary. Now is also a good time to check the framing itself for decay and dampness. If timbers need to be renewed, make sure they are properly treated with preservative.

Preparing a masonry wall

A sound surface is just as important on a masonry wall. First, clean off any small pieces of render that remain stuck to the wall by raking the blade of the bolster across the surface. If any of the bricks are loose or broken these should be replaced before you continue the work.

Broken pointing does not need to be repaired—unless it is in a very bad state—since the first coat of render will fill all the available holes.

The next stage is to check that the wall is not unduly damp and that the DPCs are in position and working properly and efficiently. If you need to insert new DPCs, leave the wall to dry out for two to three weeks and check again for signs of damp before continuing with the work.

If the brickwork is smooth and flat, roughen it slightly with a hammer and bolster—this will help the render to adhere better. Work across the surface making a series of criss-cross patterns as you go until the whole wall is covered.

A badly flaking wall can also lead to poor adhesion. To prevent this brush the wall thoroughly with a wire brush and then coat the surface with a proprietary stabilising solution suitable for brick and masonry. Leave this to dry according to the manufacturer’s instructions, and apply a second coat if the surface is particularly bad.

Setting out

To guide your rendering work, divide up the wall into a number of bays using 12mm thick softwood battens. These will guarantee a smooth, level finish for the undercoat and can be removed once it has been applied.

Start by examining the face of the wall in close detail. If it bulges badly you should ‘dub’ out the low points with a thin skim of render first and leave it to dry before setting out the wall with battens.

If and when the wall is reasonably flat, lay a long straightedge across the face at various points, and by trial and error find and carefully mark the high spot. The first batten should be fixed vertically to the wall and run over the high spot so that all the other battens can be aligned with it. This ensures that the render is of uniform depth.

Plan the layout of the battens carefully before fixing them in place. Ideally, they should be spaced roughly 1.5m apart—although the distance between battens will obviously vary near corners and around openings. With timber-framing, set the battens on every third or fourth stud.

The battens are secured in three places—top, bottom, and in the middle —using screws and fixing plugs drilled into the wall behind. This method may seem unnecessarily involved, but it does allow you to make minor adjustments to the distance from the wall of each batten so that it is aligned.

Fix one batten at a time, checking carefully that it is vertical with a spirit level. Then, as you work along the wall, check that the front edge of each new batten is level with the others by placing a long straightedge between them. If you find some of the battens are too hard against the wall, loosen the screws slightly and pack small pieces of wood behind them to make up the difference.

Many walls contain doors and window openings as well as awkward corners. These need to be set out with special care to ensure a neat, professional finish.

Where two walls meet to form an internal angle, set them out as described above but make sure’ that the end battens are placed so that they butt against the adjoining wall. If both walls are to be rendered, two battens should be positioned—one at the end of each wall.

Where two walls meet to form an external angle, setting out is more complex. To guarantee a neat, right-angle finish, the end batten should be fixed to the adjacent wall using masonry or galvanized nails. This ensures that the undercoat can be taken right up to the corner and that the batten is easy to remove.

Use a piece of 50mm X 12mm softwood for the corner batten and secure it to the adjacent wall with masonry nails. Adjust its position carefully to ensure that it is proud of the external corner by about 12mm before you start driving in nails. Make sure that it is well secured to the adjacent corner and aligned with the rest of the battens on the wall. -•” ‘I I

Door and window openings are set out with battens fixed around the outside of the reveals. These should overlap the leading edge of a reveal by 12mm so that they can be aligned with the battens on the wall. The outside wall can then be rendered right up to the edge of each opening using the battens around the reveal as an accurate guide.

The inside of the reveal must be tackled when the render on the outside wall has dried. A builder’s square, consisting of three pieces of wood formed into a right-angle, should be used to guide and assist your work. This is held against the door or window frame so that the render can be checked.

Applying the undercoat

Once the wall is correctly set out, the next stage is to mix up the mortar for the undercoat. The amount you need depends on the area to be covered, but as a rough guide 10 litres of mortar should cover about 5m2 of wall. If you have a large wall to render, it may be worth hiring a cement mixer to make the work easier.

Care taken at the mixing stage will ensure that the mortar adheres well and is waterproofed correctly. The standard mix for roughcast or tex- tured render is one part cement to four of sharp, washed sand. For a finer, stucco finish use one part cement to six of sand—.with one part hydrated lime to make the render less brittle. Whatever the mix, add a waterproofing agent to the mixing water in accordance with the manufacturer’s instructions to prevent the intrusion of damp.

To apply the render mortar you need the same tools as those that are necessary to completely plaster a wall. Mix up a manageable amount and transfer all of it to a spot board placed conveniently close to the site. Then scrape about a third of it on to a hawk, using a plasterer’s laying-on trowel.

Move to the wall, tilt the hawk upwards and scrape a small pat of mortar on to the trowel, holding the blade horizontally so that the load is carefully balanced on top. Start near the centre of each bay and work out towards the edges. Use the blade of the trowel to push the plaster on to the brickwork or force it right into the wire mesh. Build up the correct thickness in layers.

Once you have filled each bay, you can level off the material and refill any low spots. To do this, hold a straightedge across the battens on each side of the bay. By sliding it backwards and forwards over the whole area, you should be able to remove excess mortar and spot low areas which need to be refilled.

After the whole wall has been covered, leave the mortar to dry for about one hour and then carefully remove the setting out battens. Take care that no material is pulled away while you are doing this, particularly around corners and door or window openings.

The inside of the reveals should be covered next. Apply a thin coat the same thickness as that on the main wall using the laying-on trowel.

After two or three hours—or when the mortar is nearly dry—fill the gaps left by the battens with fresh material up to the level of the existing undercoat.

With timber-framed walls a second undercoat, applied 24 to 48 hours after the first, is usually necessary. Before applying the second undercoat scratch the underlying surface to improve adhesion.

In hot, dry climates, the undercoats should be kept dampened with a very fine mist of water for a couple of days to ensure proper curing, and left to dry for a further five days before the topcoat is applied.

Never apply rendering during cold weather unless you can keep the mix warm.

Top coat application

Once the surface has been thoroughly roughened, wet it down to help the topcoat to adhere even better. Try to dampen the surface without soaking it—a large paint or distemper brush and a bucket of clean water are ideal for the job.

The topcoat should be applied directly to the undercoat without using any setting out battens as guides. And if the undercoat has been applied correctly, the topcoat need be only a few millimetres thick to give a perfect, level finish.

Mix up enough material—in the same proportions as the undercoat— to give roughly a 5mm overall covering. Make the mix slightly creamier so that it is easier to spread across the surface.

Transfer all the material to the spot board and from there scrape a manageable amount on to the hawk. Apply the mortar with the laying-on trowel using firm, upward sweeps until the whole area—including the reveals— has been covered.

To give a smooth finish, wet the laying-on trowel and run it over the surface to remove any small ridges. Take special care around corners and where reveals meet the outside wall. Special corner tools can be used to guarantee the correct angle on internal and external corners.

Finishing off

Unless you want a stucco—or flat— finish, the wall should be textured or covered with stones before the mortar dries. Try to prepare the tools and materials needed for this before you apply the topcoat; if the wall dries out too quickly, wet it down again with a brush and a bucket of clean water.

Damaged topcoat

Pebbledash and roughcast renderings are very durable but like many building materials they eventually weather and need to be repaired. Wind and driving rain can—over a period of time—work the small chippings loose, creating bald patches.

If the rest of render is sound cut each of the bald patches back to sound brickwork and repair. In cases where nearly all of the chippings on one stretch of wall have worked their way loose, a thin coating of render should be added over the top and the wall given a fresh decorative coating.

Blown render

If weathering is allowed to run its full course—or the wall behind the render is excessively damp—both the topcoat and the undercoat may part company with the wall. ‘Blown’ render can be tackled only after extensive tests have been carried out to determined whether the whole wall is affected or only small parts of it. Tap the wall with the handle of a hammer: a hollow sound will indicate where the render is loose. Each of the affected areas can then be cut back with a hammer and bolster. The DPCs on the wall below should be carefully checked before repairs take place.

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