Repair cracked render — HomeGraft

Rake out, clean and fill a cracked render crack, then match the texture and protect it while it cures.

RC-01Check the crack and the weatherMeasure the crack. A fine hairline crack under 1mm is usually just shrinkage and safe to fill yourself. A wide, stepped, or growing crack (several mm, or linked to bulging render) can mean movement in the wall behind it - get a builder or surveyor to check this before you patch it, as filling won't fix a structural problem. Plan the job for a dry spell with no rain or frost forecast for at least 48 hours, and avoid strong direct sun which dries the repair too fast and cracks it again.

RULES

1 each · Tape measure

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KNOW THE RULES

If your house is listed or in a conservation area, changes to external render (colour, texture or type) can need planning permission or listed building consent - check with your council's conservation officer before you start if in doubt.

RC-02Gather tools and PPEYou'll be chiselling and mixing, so wear safety goggles and a dust mask - render dust and old mortar chips fly off when you rake out a crack. Wear gloves for handling cement-based mixes, which irritate skin. Lay a dust sheet under your work area to catch debris and make clean-up quicker.

1 each · Safety goggles

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1 pack · Dust mask (FFP2)

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1 pair · Work gloves

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1 each · Dust sheet

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RC-03Rake out the crackUse a cold chisel and club hammer, or a purpose-made raking tool, to open the crack to roughly 6-10mm wide. Undercut the edges slightly so the back of the groove is wider than the front - this locks the new patch in and stops it falling out. Brush away all loose and crumbling render with a stiff wire brush until you reach solid material.

1 each · Cold chisel

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1 each · Club hammer

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1 each · Stiff wire brush

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RC-04Clean and dampen the areaBrush out every bit of dust and debris from the groove - dust stops the new mix from bonding. Dampen the crack and the render around it with clean water, using a brush or a garden spray bottle. This stops the old render pulling moisture out of your repair too fast, which is a common cause of new cracks forming. Don't leave puddles or running water.

1 each · Spray bottle

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RC-05Mix the repair materialFor hairline cracks under about 2mm, a ready-mixed exterior masonry filler straight from the tub is easiest. For wider cracks, mix a bagged sand-and-cement render repair mix (or premixed exterior render) to a firm, non-slumping consistency - think stiff peanut butter, not porridge. Only mix as much as you can use in 20-30 minutes, as it starts to go off.

1 tub · Ready-mixed exterior masonry filler

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1 5kg bag · Bagged sand-and-cement render repair mix

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1 each · Mixing bucket

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RC-06Fill the crackPush the mix firmly into the groove with a pointing trowel or filling knife, working from the bottom upwards so it doesn't slump out. Press hard to force out trapped air pockets. Leave the repair slightly proud (raised) of the surrounding render, as it shrinks a little as it dries.

1 each · Pointing trowel

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1 each · Filling knife

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RC-07Match the surrounding textureWork the texture while the repair is still soft, usually 20-40 minutes after filling depending on the weather. For a smooth render, rub gently with a damp sponge float in small circles. For a textured or pebbledash finish, dab the wet mix with a stiff brush, or press small stones into it, to match what's already there. If you're not sure what render type you have, take a photo to a builders' merchant and ask.

1 each · Sponge float

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RC-08Protect the repair while it curesCover the patch with plastic sheeting or a damp hessian sack if rain or frost is forecast within 24 hours - both will ruin an uncured repair. Keep it out of strong direct sun for the first day too. Leave the render to cure fully for at least 3-7 days, or longer in cold weather, before painting.

1 each · Plastic sheeting

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RC-09Paint to finish, if neededOnce the repair is fully cured, brush off any dust and apply masonry paint if you want it to blend in. Small painted patches often show up as a slightly different shade, so painting the whole wall or elevation usually gives a better match than just the repair.

RULES

2.5 litre · Masonry paint

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KNOW THE RULES

If your property is listed or in a conservation area, check with your council before changing the render colour or finish on an outside wall - this can need permission even for a small patch.

RC-10Clean up and check back laterWash all tools and buckets before the mix sets hard - dried render is very difficult to remove. Sweep up dust and debris from the ground below. Check the repair again after a few weeks of normal weather to make sure it's held and no new cracking has appeared.

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