Help!external concrete render keeps cracking...

11 Nov 2013
Reaction score
United Kingdom
So I thought this would be a relatively straightforward job. It would appear not...

A vertical crack has developed on the face of the house from the windows sill on the first floor down to the porch. It is mirrored on the internal wall in the bedroom. I am pretty positive it is caused by moisture penetration as there is mould on the inside wall and some of the surrounding plaster is blown.

So to fix it I chiseled out the cracked render and mortar. I filled the crack with external sealant left it to dry for approx 15hrs. Then I applied a mix of 4:1 concrete and sand with added water proofer. By the following morning I now have a hairline crack running through the concrete following the same general route as the original crack (which was more than hairline). This has happened twice so clear ly I am doing something is wrong. Is it my mix,or my method or should I just paint over the hairline crack?

Any suggestions or help would be much appreciated!

Sponsored Links
I think your cracking of your 4:1 mix by the following morning is due to shrinkage as the water dries out of the mix and it sets. You can help prevent this by doing the job in smaller stages, dont fill the crack all in one go,use a base coat with a thinner face coat, wet the brick wall faces with plenty of water just before you put in the 4:1 mix. This will prevent the substrate from sucking the water out of your mix before it sets.

But I doubt your moisture penetration is the cause of the crack, no i don't doubt it I know its not. There is or has been movement there at sometime, this sounds like settlement cracking to me, vertical from one opening to another on both inside and outside walls. It may be historic, and nothing to worry about, the render may have cracked and let water in thus the mould and damp inside. You will know if it is an issue to worry about if your repair cracks again, once done properly that is.
Are there drains under the porch? Could there be a drain problem causing water to wash away support to foundations? There can be many causes, some costly, some not so, and I do not want you to worry but you should investigate the initial crack problem, not just fill it. Steve.
The crack is in the wall, and you won't fix it with just render on the surface. The wall is moving.

You'll need to determine the cause (it won't be moisture) and deal with that.

Then stich repair the wall with resin and steel ties and then re-render.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links