Best way to seal cracked render?

Joshuam08

Hi all! We’ve had some cracks developing on the rendered pillars out the front of our house. I’m not too worried about the visual aspect of them but they’re on top of the pillars and worry water is entering when it rains. What’s the best way to seal this?

I also noticed a new fairly straight vertical crack on the inside wall of the balcony. This part is rendered foam (blueboard?) so I’m guessing that’s the join of two pieces? Can I seal it the same as above?

EricL

Hello @Joshuam08

You’re absolutely right to be thinking about water getting in rather than just how it looks. Once cracks open up on horizontal surfaces like the tops of pillars, they can let moisture track in and eventually cause bigger issues, especially if there’s steel inside or foam substrate behind the render.

From the photos, the cracks on top of the pillars look like typical hairline shrinkage or minor movement cracks in the render. The vertical one on the balcony wall does look very much like a sheet joint in blueboard that has opened slightly. That’s common if the joint wasn’t reinforced well enough or there’s been a bit of movement.

For the tops of the pillars, the key is to use something flexible rather than a rigid patch. A paintable exterior polyurethane or high-quality exterior gap filler (Selleys 300g Monument Roof And Gutter Silicone Waterproof Sealant) works well because it stays flexible and moves with the structure. First clean the crack out properly. Use a stiff brush and blow or vacuum out dust. If the crack is very fine, it actually helps to open it slightly with a scraper or corner of a screwdriver so the sealant can penetrate rather than just sitting on the surface. Make sure the area is completely dry before sealing.

Then apply the flexible sealant, pushing it well into the crack. Smooth it off so water will shed rather than sit. Once it cures, you can paint over it with a good exterior acrylic to match the rest of the pillar. If the whole top surface is exposed to weather, another option is to seal the entire top with a clear penetrating masonry sealer after fixing the cracks, which adds an extra layer of protection against water ingress.

For the vertical crack in the blueboard render, you can treat it similarly if it’s just a hairline joint crack. Use a flexible exterior crack filler rather than standard render patch or mortar. If that crack widens over time or is more than a couple of millimetres, it would be better to rake it out slightly, apply a flexible joint compound, and possibly embed some fibreglass mesh tape before recoating and painting. That helps stop it reappearing along the sheet join.

What you don’t want to use is a rigid cement-based filler, because that will likely crack again in the same spot.

If you need further assistance, please let us know.

Eric