Your Cart

Render Crack Repair

This article tells you how to repair cracks In rendered walls permanently. The repair is to the top surface and retains enough flexibility to accomodate movement in the structure. Note, If the structure is failing you need a crack stitching kit to hold it together.

Render Crack Repair Kit

If you need to affect a render crack repair, we have assembled a kit for you at a discounted price, just click this link: render crack repair kit. This kit is perfect for buildings that shift a little without structural damage, timber frames etc. Please read on to find out why it works so well.

Why Does Render (Stucco) Crack When Used On Walls

Cracks appear in rendered walls of buildings because the ground actually moves, and the building actually moves, and with time and the wet/dry/wet/dry annual seasonal patterns, the ground swells-and-shrinks and the building settles somewhat unpredictably over the decades.  The rendered walls don’t stretch, not worth-a-hoot.  When the building moves, then one part of the foundation goes a bit up, down or sideways relative to another part, and then the render cracks. Render crack repair is then required to keep the building watertight, this article shows you how.

render crack repair

What Is Render (Stucco)

Known as Stucco in America, and Render in the UK, it is basically cement. When painted, it is very weather-resistant and does not have the rot-issues of wood; that’s why it is used on so many buildings.

Why Do Repairs To Rendered / Stucco Walls Fail?

Render crack repair is often done with various caulks or cement, and usually the crack opens up again.  There are two reasons:  The cement patch can’t stretch, and or the caulk did not stick.  These have underlying them one common denominator:  the building is still moving.  Patching the crack did nothing to stabilize the foundation, or change the seasonal swelling-and-shrinkage of the ground.  The crack is going to continue to open-and-close, and usually widens, on-the-average, as time goes by.

Furthermore, the crack movement usually exceeds the elastic limits of whatever-was-used to fill the crack, even if it was not cement.  Here’s why:

Let’s say that we have a reasonably stable building, and the settling factor happened over many years.  However, the ground swelling-and-shrinkage happens seasonally.  So, eventually a crack starts, and over many years it slowly widens.  But, from winter-to-summer-to-winter-to-summer, it is also opening-and-closing-and-opening-and-closing, due to the seasonal ground movement.  So, the crack might be 1/16” one year, and 1/8” five years later.  But, from winter-to-summer-to-winter-to-summer, it might open-and-close by 1/16” more-or-less.  Early in the cycle we have a crack that goes from zero to 1/8”.  Later in the cycle it might go from 1/16” to 3/16”, and then 1/8”-to-1/4”, and so-on.  There just is not anything that will stretch that much and still stay stuck to the render, which has not got great tensile strength anyway. If you even used an epoxy glue in the crack, it would just pull off some render when the building asked the crack to open-up more.

Render Crack Repair, How Do I Permanently Repair A Crack In A Rendered Wall?

This is a Mechanical problem, and it needs a Mechanical solution.

Open the Crack to start the repair – some suitable tools for that job

The simplest and least expensive one I know of is to parallel groove the crack with an angle grinder and a disc called a mortar rake, so it is much wider than the width of the original crack, such as a half-inch for a 1/16” crack. Mortar rakes are available in a variety of different widths, and a cut of say 8mm wide (which will result from using a 7mm blade) will happily take 10mm backer rod. This is many times easier than trying to cut two parallel grooves to the same depth with a diamond cutting disc.

Mortar rakes (sometimes called mortar raking blades) look just like diamond cutting discs for mortar, but wider, and a little more expensive but still around £10 – £20 or so dependant upon quality:

We can sell you a good quality one as part of our render crack repair kit.

makewoodgood mortar rake render crack repair

This type is also available, but are rather harder to use in my experience as they love to wander, and make new cracks to fill whilst doing so:

Mortar Rake 10mm render crack repair

The photo’s below show a grooved crack which we used to recommend, the principles are the same.

I personally find the parallel crack easier to cut, easier to fit backer rod to and it uses less sealant as well.

render crack repair cut groove.

Now there can be enough sealant to accomodate movement in the render crack

Now, when that crack opens-and-closes by 1/16” more-or-less, the surface stretches-or-shrinks by a sixteenth out of ½”, which is one part in eight, and that’s a very mild elongation; one that even a low-quality (inexpensive) latex-acrylic caulk can tolerate. However, if you want a really long-lasting repair, I’d recommend something that does not shrink when curing and sticks by real chemical bonding, and that’s either a sealant called 5200, made by 3M, or one called Stixall, made by Everbuild.

The idea is not to fill the entire groove with the caulk, but to fill the upper part of the groove only. You don’t want it in the base of the groove as if you glue the sides to the substrate, you will stop the crack moving, and another one will open up somewhere else! Incidentally, that’s another reason why filling cracks with cement does not work in the long term, any movement to close the gap cannot be accomodated. A piece of twine or soft rope can be pressed into the bottom of the groove, so the sealant does not go into the crack itself, but I recommend you use backer rod, which is made for the purpose. Whichever solution is used, something compressible should be at the bottom of the groove, to allow the sides to move together, and stop the caulk from adhering to the back of the crack. Backer rod is closed cell foam tube, available in different sizes, and is provided in our render crack repair kit. Backer rod makes the job much easier and saves money as it is cheaper than sealant.

Make sure the sealant sticks by priming the render properly first

Render is similar to wood in that it does not have a strongly attached surface; old render is often somewhat crumbly, due at least in part to its having been made with a lot of air mixed in with the cement-and-sand-and-gravel.  Some render is even sprayed, and that can easily be aerated.  Whipping-in air minimizes shrinkage-and-cracking while curing, and also air costs less than render.  What this means is that we need some kind of primer to glue and strengthen the render surface, and also something to chemically bond to whatever-kind-of-caulk you are going to use. Smith & Company makes a really good one, and it’s called Damp Concrete Primer.

The cement surface must be reasonably dry before continuing. Moving air is the most efficient means of evaporating water.

Now, we are ready to begin product application.  There are two products needed to perform the repair, a liquid and the adhesive sealant. The liquid is the Smith & Co. Damp Concrete Primer (DCP). It won’t take much…an ounce or so of the mix is enough to prime at least ten feet of typical crack.  It is prepared by mixing one part of a liquid-concentrate with two parts of water. A tan emulsion will immediately form.  You must use this within about 30-45 minutes.  The microscopic droplets of the resin-in-suspension will soak into the large, medium and small render/concrete pores, but will not get into the really microscopic porosity that concrete usually develops.  As you apply it to the bare stucco, you will see it soak into the porosity of the bare render at the edges of the crack, and as well as any nearby surface that was cleaned of paint in order to do this repair. If a little gets on the adjacent paint (usually latex) don’t worry;  It can be immediately wiped up with paper towels, and a little bit of the oily film left will stick to the old latex paint and you can paint new latex paint over it when you finish the repair.

Start in the morning of a day.

Apply the Damp Concrete Primer with a very narrow roller or (preferably) a brush.

Hold the roller only horizontally and roll only upwards.  The liquid will wet the surface and flow into the open porosity of the render. The really important thing is that it wets the exposed render surface of the walls of the groove in the render where you widened the crack.

When brushing, hold the handle well-above the bristle-pack and brush upwards, to wet all the stucco in the groove. If you brush with the bristles above the handle, the Damp Concrete Primer will run out of the bristle-pack, down the handle, down your wrist, perhaps past the elbow and possibly make it into your armpit. You really don’t want to let any of that that happen.

render crack repair prime crack surface

Choice of Sealants

The different attributes of both 3M 5200 and Everbuild Stixall are discussed in our Render Crack Repair Kit page.

Using 5200 Sealant

The water-component of the DCP will evaporate over the next hour or so, and the surface will have a clear brownish oily film. Over the next few hours it will go from slippery-wet to sticky-wet to dry-to-the-touch but still soft-to-the-fingernail. When it is no-longer-sticky, that’s the time to apply the caulk: in the afternoon of the same day.

Areas primed with the Damp Concrete Primer but not-caulked-that-afternoon will need to be primed again the morning of the next work-day.  If you don’t reprime, most caulks (not Stixall, see below) will not adhere all-that-well.  We want chemical bonds; to have a result that really lasts.

Using Everbuild Stixall Sealant

Stixall bonds best, with better adhesive than cohesive forces (which means you can’t get it off the primed surface) to completely dry DCP primed surfaces. We recommend that you leave the DCP to dry fully overnight, and use the Stixall on the paler yellow fully cured DCP the next day. Stixall shrinks by a small amount on curing (under 2%) so to minimise any visible concave surface on the repair we suggest using the backer rod to restrict the depth of sealant that is applied to the crack.

Proper Use of Caulks or Sealants

Since caulks are pastes or gels, they don’t really WET the surface unless you smear them against the surface;  feeding a bead out of a caulking-gun and laying-the-bead in the groove does not get actual wetting and thus not real adhesion, and this is easily a reason for a third if not half of all caulk-failures.  The stuff needs to be actually smeared against the surface.

The way to apply caulk is to fill the groove about a half-full, and then with a suitable tool shaped somewhat like the groove (or even a gloved finger-tip), run down the groove so as to smear the caulk against the groove sidewalls. Alternatively a scraper blade can be used to force the sealant into the walls of the groove.

render crack repair work caulk

Only THEN do you fill the groove with caulk and squeegee flush.

render crack repair tool flush

Matching the Surface Texture of the Repaired Render

Now, there’s one final step to have an invisible render-repair.  You must do this while the caulk is still sticky. For Stixall that might be possible immediately. For 3M5200, that will be a day or two.

The last step is to press into the wet caulk some coarse sand or fine gravel: Not to fill-the-crack, but only enough that a layer of the sand/gravel adheres in one surface layer on the sticky-caulk surface. The choice of aggregate should match the finish of your render of course. Now go away and leave it until the caulk has cured. When you paint the area, the paint will drape over the sand/gravel stuck to the caulk surface, and it will have the same texture as the surrounding render, and no one passing-by will know there was ever a repair, if you match the shade of paint.

render crack repair add sand

render crack repair finish sand

Buy a Render Crack Repair Kit

We sell a render crack repair kit containing Smiths Damp Concrete Primer, Backer Rod and Stixall or 3M 5200 Sealant, all you need in one pack to repair cracks. Buy additional sealant and backer rod if you have a lot of cracks, the DCP will go a long way.

We sell a broader range of Crack Repair Kits Here.

Safety Whilst Repairing Your Cracked Rendered Surface

Please do not get chemical products on your bare skin, and do not to breathe the smell of any chemical product.  Have a fan at your back if you are working in a confined area.  Use these or any chemical products with adequate ventilation, and use gloves when working.  Even if the labels on the containers don’t say, many resins are chemical irritants at best, meaning are hazardous if you get them on your skin a lot (or sometimes for some people, only a little). They are quite safe when used wisely, in accordance with proper safety procedures and following the information on the container labels.

Timber joints with problems?

If you have timber to timber or timber to render joints, cracks and other sealing problems, you can read more about those in our article on Timber Joint Sealing.

Copyright Steve Smith and Stephen Dakin 1993 – 2022, All rights reserved