How to fill cracks in render

Video by Charlie DIYte. This walk-through draws on the video "How to Fill Cracks in Render" from Charlie DIYte, who tests three different products on a rendered cottage wall and is refreshingly honest about which ones worked and which did not. The comparison of flexible masonry filler versus a cartridge-based repair system is worth watching before you head to the merchants.

1. Assess the cracks before you reach for the filler

Not all render cracks are the same. Hairline cracks — the kind you can barely get a fingernail into — are usually the result of normal seasonal movement or an old render mix that was too rich in cement. Wider cracks, especially ones that follow the mortar lines behind the render or run diagonally from window corners, may be telling you something more significant about the building's movement.

Have a look at the pattern of the cracks before you do anything. A handful of isolated hairline cracks on a rendered wall in a property around Sandwich or Deal is almost certainly just age and thermal movement — normal, and fixable. Wide cracks that have appeared recently, or that you can see have been filled before and have re-opened, are worth monitoring with a crack gauge for a few months before committing to a repair. If in doubt, get someone to take a look first.

2. Rake out the crack properly

This is the step most people skip, and it is the reason render repairs fail. If you simply press filler into a hairline crack, there is nothing for it to key into — it sits on the surface, shrinks slightly as it cures, and pops off within a season. You need to open the crack up first.

For fine cracks, a grinder fitted with a diamond raking disc does the job quickly and cleanly. Alternatively, a cold chisel and club hammer will open a crack wide enough to give the filler something to grip. Aim for a slight undercut if you can — wider at the back than at the face — which helps lock the repair in. Blow out the dust with a brush or compressed air; any loose material left inside will weaken the bond.

3. Choose the right filler for the job

For narrow cracks up to about 5 mm wide, a good exterior flexible filler — the kind that comes in a cartridge and tolerates movement — is usually the right choice. Standard interior filler is not suitable outside; it will soak up moisture, swell, and crack out again by the following spring. Look for something labelled as exterior or weatherproof, and ideally one with a little flex to it.

For wider or deeper cracks, a two-part epoxy-based filler or a dedicated exterior masonry repair product gives a more durable result. These tend to be more expensive and need mixing, but they grip well and do not shrink the way cement-based fillers can. Mind you, for most normal hairline repairs on a rendered semi-detached or terrace, a decent exterior flexible filler from a builders' merchant will do the job without overcomplicating it.

4. Prime the crack if needed and apply the filler

Dampen the inside of the crack with a brush and clean water before applying filler — this stops the dry render from pulling the moisture out of the filler too quickly, which causes it to crack before it has fully cured. On very porous or powdery render, a diluted bonding agent brushed into the crack and allowed to go tacky first will improve adhesion significantly.

Apply the filler in one smooth pass using a caulking gun for cartridge products, or a filling knife for a trowelled repair. Work it well into the crack, pressing firmly so it fills the full depth. Overfill very slightly — you can always scrape back once set, but you cannot add back what has already shrunk away.

5. Finish and texture to match the existing render

Once the filler has gone off but before it is fully hard, scrape back any excess with a filling knife and feather the edges so the repair blends into the surrounding render. On smooth render this is straightforward. On textured or roughcast render, which is common on 1930s and postwar properties across East Kent, you will need to replicate the texture while the filler is still pliable.

A stiff brush, a sponge float, or even a crumpled piece of plastic bag pressed lightly into the surface can recreate a stippled or roughcast finish well enough. It takes a bit of experimenting, so it is worth practising on a small section first. The repair will be more visible before painting than after — once you apply a coat of exterior masonry paint over the whole wall, a well-done repair should be invisible.

6. Paint over the repair once fully cured

Allow the filler to cure fully before painting — read the product data sheet, as this can range from a few hours to 24 hours depending on the product and temperature. Do not paint in direct sun on a hot day, as the paint will skin over before it has bonded properly, and avoid painting if rain is forecast within a couple of hours.

Apply a primer or stabilising solution to the filled area first if the surface is powdery or you have used a product that is particularly smooth compared with the surrounding render. Then apply two coats of a good exterior masonry paint, feathering the edges slightly wider than the repair itself. A large brush rather than a roller gives better control on textured render and reduces the chance of the repair standing out as a flat patch.

When to call a handyman

Call Richard if there are a large number of cracks and the whole wall needs attention, if the render is blowing (hollow-sounding when tapped) in sections and needs cutting out and re-rendering, or if the cracks keep re-opening despite being filled — that is usually a sign of ongoing movement that needs understanding before throwing more filler at it. Larger areas of render repair are also hard to paint in a way that blends well without doing the whole wall, and that is easier to plan with someone on site who can take a look at what is actually there.