How To Repair Render

There are few homes that don’t have at least one area of cement based render covering the original brickwork or stonework. More often than not it’s there to fulfil an important protective — as well as decorative — function, so it makes sense to know how to repair render and ensure that what may start as a small crack isn’t allowed to escalate into something far worse.

DEALING WITH SMALL CRACKS

1. If you spot cracks in render. Always start by checking that these aren’t a symptom of more serious damage. Tap the area around each crack with a hammer handle and listen for the hollow sound indicating that the render has parted company with the wall.

How To Repair Render 2. If the area is basically sound, enlarge the cracks by scraping down them hard with a plugging chisel or a large nail to provide a key for the repair material.

Very small cracks can be filled with exterior grade cellulose or vinyl based filler. Press the filler into the crack and smooth level with the surrounding surface.

Larger cracks — over 4mm —are best made good with ready mixed sand and cement repair mortar. Available in small bags from hardware stores.

Before you apply the mortar. Brush inside the crack with a solution of one part PVA adhesive (such as Unibond) to three parts water.

3. Press the mortar hard into the cracks with a pointing trowel. If any of the cracks are more than about 10mm deep you may need to build up the repair in layers.

In order to get a really smooth finish to the repair, leave it for about 15 minutes and then you should smooth it over again with the trowel, having first dipped the blade in a little water.

TEXTURED RENDER

Rendered surfaces which have been painted may have one of a number of finishes, ranging from ordinary masonry paint (found on stucco) through textured finishes (such as Tyrolean, which is ‘thrown’ on the wall by a special machine) to physically textured surfaces in which roughcast pebbles are added to the paint.

Before rerendering a patch which has been painted, it is vital to scrape off any loose flaking paint around the damaged area so that the new finish can be blended in.

If, in the course of stripping off flaking paint, you find that the base render is sound, sand around the edges of the patch and then apply a coat of masonry stabilizing solution.

If there is any evidence of moss or fungal growth on the wall, coat the surface with a solution of one part bleach to ten of water and leave to dry. Then wash down with clean water and apply a coat of stabilizing solution.

Matching the finish: As with textured render, texturepainted surfaces may need quite a bit of experimentation before you can achieve a perfect match.

Physically textured surfaces are easy to spot, and can usually be patched by mixing roughcast pebbles with ordinary masonry paint — but wash the pebbles in clean water first to remove any traces of dirt.

Swirls can be made by twisting a damp rag in the paint just before it is fully dry.

Fine stipples are best made with an old, stiffened paintbrush; trim the bristles for a slightly heavier effect.

Heavy stipples can be made with a rubber or wooden stippling tool available quite cheaply from decorating shops and hardware stores.

Ridges and ripples really do need plenty of experimentation — but you might try texturing the surface of the render, applying some stabilizing solution, and then painting it in the conventional way.

Tyrolean finishes can be patched without having to hire the special machine originally used to apply them: simply load a wide paintbrush heavily with masonry paint, hold it about half a metre away from the wall, then flick it onto the surface. If you’re unsure about the finished result, it may be worth experimenting on an offcut of board first.

RENDERED PLINTHS

Many houses have a rendered plinth along the foot of the walls to protect them from surface water runoff. If a section of the plinth breaks up the result may be penetrating damp — particu larly tarty if surface drainage is poor.

Start by preparing a length of board the width of which matches the height of the plinth minus the shaped top edge.

Hack away the render in the damaged section by forcing the blade of a bolster down between it and the wall.

1. When you reach sound material on either side, undercut the edges to provide a better gri p for the new render.

Brush the wall behind the old render with a wire brush to remove all traces of loose debris and then dampen it with water. Prop the board in front of the gap with bricks so that it acts as a former.

2. Make up a repair mortar of one part masonry cement (ordinary Portland cement with plasticizer added) to four parts extra fine sharp washed sand. Pack this behind the board to fill the gap in the plinth. Leave the mortar to harden.

3. Form the top edge of the plinth by laying a further fillet of mortar along the edge of the board; trim it roughly to size with your trowel and then create the desired triangular profile by sliding a wood offcut along the board as shown above.

ROUGHCAST AND PEBBLEDASH

Roughcasting and pebbledash are two alternative texture treat.

Ments for the topcoat render which require slightly different repair methods to those employed on plain rendered walls.

Faults with these finishes are usually easy to spot: the topcoat containing the pebbles falls off, leaving the plain rendered undercoat exposed.

The secret of successfully applying roughcast and pebbledash is in mixing up mortar of the right consistency; it must be firm enough not to slump from the wall when you apply it, yet sloppy enough to accept the pebbles.

Roughcasting: In this case mix small pebbles — roughcast — in with the topcoat mortar and apply it in the conventional way. The amount of pebbles you mix in with the mortar will depend on the character of the original render — keep adding more until you get a perfect match.

Afterwards, disguise any difference in the consistency of the pebbles between the patch and surrounding wall by pressing in more of the material with your fingers — wear gloves.

Pebbledashing: Proceed as you would for a plain rendered patch up to and including the point where you apply the topcoat.

At this stage, instead of leveling with a straightedge, smooth the top coat level with your trowel and feather it away into the surrounding pebbledash with a brush dipped in water.

As soon as you’ve finished, cover the ground below the wall with a tarpaulin or some thick plastic sheeting.

Then take a handful of the pebbledash material and throw it at the patch — applying it any other way will not achieve the desired effect. After a few throws gather up the material that has dropped on the sheet and repeat the procedure until you get a good match with the surrounding area.

Finally, blend in the edges of the patch by pressing in odd stones with your fingers.

Note: Pebbles for roughcasting and pebbledashing should be available by the bag from your local builder’s merchant. They are usually sieved and graded according to their size — something worth bearing in mind when you buy.

RENEWING A PATCH OF RENDER

Extensive cracking on a rendered finish usually results in the render itself separating from the base surface — a phenomenon known as ‘blowing’. Blown render sounds hollow when tapped with a piece of wood; it should be rectified immediately — before water is allowed to become trapped behind it.

When checking for blown render, you need to decide whether the area is worth patching or whether the damage is so extensive that the wall would be better off being rendered completely — a more extensive job which is covered in another article.

As a rule of thumb, if the blown patch extends over more than one square metre it is likely to be an indication that the entire render coat is on the point of failing — in which case simple repairs are unlikely to be successful.

To repair a patch, start by hacking off the defective material with a hammer and bolster. Once you’ve got the blade of the bolster behind the render, it should flake away from the walls with little effort.

Continue in this way until you reach sound render right around the edges of the patch. Then trim the patch to a more or less regular shape by nibbling away more render, a little at a time.

Finish your preparations by undercutting the edges — by about 10mm if possible — to help the new mortar grip the wall firmly.

If the mortar joints in the brickwork are clogged with old mortar, clear these too — to a depth of 10mm — for the same reason.

The mortar for the repair should be a fairly creamy mix of one part masonry cement to four of extra fine sharp washed sand. If you can’t get masonry cement, you must add some lime or a proprietary plasticizer to the mix so that it’s flexible enough to integrate with the surrounding render. If you use ordinary Portland cement, the chances are that the repair will crack after a few days.

Immediately prior to mixing the new render, dampen the wall thoroughly with water.

1. Apply the render from a hawk, using a plasterer’s trowel. Start by flicking it off the hawk, onto the back of the trowel and against the wall almost in one movement.

Press hard against the wall and then spread the material out over the patch with a broad, upward sweep of the trowel, keeping your arm straight.

At the edges of the patch, scrape the mortar off against the surrounding surface and flick the excess back onto your hawk.

2. If the depth of the patch exceeds about 12mm, don’t attempt to fill it in a single coat or the mortar will slump. Instead, apply a coat about 12mm thick and get it reasonably level with your float. Then, as it starts to go ‘off’, score the surface lightly with the edge of your trowel blade to provide a good key for a further top coat of mortar for the patch.

Levelling the patch: The procedure here is the same whether you’re filling the patch in one coat or two.

Continue floating on mortar until the patch is just proud of the surrounding surface.

3. Take a straight edged wooden batten half as long again as the patch is wide. Starting at the bottom slide it upwards and slightly diagonally across the patch in a sawing motion to scrape off the excess mortar.

Afterwards, fill any low spots and if necessary repeat the level ling procedure.

4. Then dip your trowel blade in water and feather away the edges of the new patch so that they merge with the surrounding render.

Or, if the existing finish has been roughened or textured, do the feathering with an old distemper brush.

Matching finishes: Smooth render, as opposed to pebbledash and roughcast, can be given a variety of textured finishes to add relief to what might otherwise be a rather dull surface. When patching render so treated, you may have to do a bit of experimenting with different implements while the repair mortar is drying before you can get an exact match.

In hot weather, newly rendered patches should be covered with a piece of sacking pinned over them — soak the cover regularly to stop the mortar curing too quickly, or it will crack.

Some finishes, such as the ‘cottage’ are made by being deliberately heavyhanded (but in a controlled, measured way) with your float.

Others, like the stone blocksimulating ‘ashlar’, are achieved by getting a perfectly smooth surface and then scoring in the ‘joint’ lines with the edge of a pointing trowel run along a straightedge.

Most baffling of all are the purely textured effects — which were often made in the first instance by anything that the builder had to hand at the time. Among the tools you might try out are combs, a stiff brush, saw blades, a wooden float (make your own for this job) and a garden hand fork.

Crumbling corners: To form a guide, nail, tape or prop a piece of board against the corner so that the board’s edge is flush with the adjoining surface — if the board is painted, you’ll get a much smoother finish.

Brush the ‘hole’ with dilute PVA .dhesive and pack in the repair mortar. As you fill up the hole, tap the board every so often to consolidate the mortar down.

Smooth off by running the blade of your trowel upwards against the edge of the forming board. Leave the mortar to harden off before removing the forming board.

The next stage is to repeat the entire procedure for the other side of the corner. But this time avoid pinning the forming board into the first part of the repair.

28. November 2010 by admin
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