How To Stop Cracks Forever At The Joint Between New Brick Walls And Old Concrete – The Fibreglass Reinforcing Mesh Solution

About three or four years ago, I took on a job. The owner wanted to turn a large open-plan space into two bedrooms, so we put up a new brick wall in the middle. The joint between that new wall and the existing concrete load‑bearing wall – the renovators just filled it with filler and painted over it. Looked fine at first. Took a photo and you couldn't see a thing.

 

Three months later, the owner called. The joint had cracked – a thin, vertical line right at the corner of the wall. Really obvious.

 

The renovation company came back and fixed it. Scraped it out, refilled, sanded, repainted. Two months later, it cracked again. The owner was getting frustrated and asked me if there was a permanent fix – no more patching.

 

I went to have a look. Checked the crack, felt the walls on both sides. Then I told him – if you don't want to repair this every year, you can't treat it like a surface defect. You have to treat it like a structural joint.

 

A lot of tradespeople don't really understand why new brick walls always crack at the junction with old concrete. The reason isn't complicated – brick and concrete shrink at different rates, and they settle differently too. A new brick wall keeps drying out and shrinking for a while after it's built, while the concrete wall settled years ago. They also expand and contract at different rates with temperature changes. If you just fill the gap with filler, of course it's going to crack.

 

I told the owner that fixing it properly isn't that complicated, but there are a few steps you can't skip.

 

The first thing you can't skip is waiting. You can't plaster a new brick wall straight away. You have to let it dry out and do most of its shrinking first. I usually wait at least one to two weeks – about a week in summer when it's dry, maybe three weeks in winter or damp weather. If you plaster too soon, the wall will keep shrinking and the joint will crack eventually.

 

The second thing you can't skip is the prep area. You need to clear at least 20 centimetres on both sides of the joint – chip off the old coating right back to the concrete substrate. A lot of people just clean a thin strip right along the joint. That's not enough. The mesh needs a decent width to bond to. I usually go 30 to 40 centimetres wide to make sure the mesh has enough surface to grip.

 

Once the area's clean, you get rid of all dust, grease and loose bits. If the old concrete is too smooth, you have to roughen it up with a chisel or an angle grinder so the mortar can key in. On the brick side, you sweep off the dust and give it a light spray of water – damp, not soaking.

 

Then comes the critical bit. Put down a first layer of polymer‑modified mortar, about 3 to 5 millimetres thick. While it's still wet, press a full sheet of fibreglass reinforcing mesh into it. Not just a strip along the crack – a full-width sheet, pressed right into the mortar so it's fully embedded, not floating on top. I use a trowel and work from the centre outward to push out air bubbles and flatten any wrinkles. The mesh should sit about a third of the way from the surface – too shallow and it'll show through, too deep and it won't do its job.

 

Once the first mortar layer has cured, you put on a second layer to level it off and completely cover the mesh. Together they're about 8 to 10 millimetres thick – much more stable than a single thick coat.

 

There's another detail that's easy to miss – the ends of the joint. A lot of people only put mesh along the middle of the wall and stop at the corners or the ceiling. But the ends of the joint are actually where the stress concentrates most. My approach is to leave an extra 10 centimetres of mesh at each end, wrapping it into the corner or up to the ceiling, so the stress spreads over a larger area and doesn't all pile up at the end of the joint.

 

Curing is also important. After the mortar's on, you need to keep it damp for a few days – don't let it dry out too fast. If it dries too quickly, the mortar itself will shrink and crack, and all that work is wasted. I usually spray it with water a couple of times a day for three or four days. In summer, I also shade it from direct sun.

 

Thinking back to the renovator's two attempts – the first time they just used filler and paper tape. The second time they used standard mesh but only a narrow strip along the crack. Both times they missed the two key points: letting the new brick wall dry out first, and using a wide enough reinforced layer. No wonder it kept cracking.

 

I've used this method on quite a few renovation projects since. One time I did a whole second-hand apartment with five new walls, and I used this approach on every joint. Two years later I went back to check – not a single crack. The owner told me his neighbour's walls cracked within six months of their renovation, but his were still perfect. He asked me what material I'd used. I told him it's not about expensive materials – it's about doing it differently.

 

Of course, this method depends on the new wall having a solid foundation. If the brick wall's base isn't sound and the whole thing is settling, no mesh in the world will hold it. So before I start, I check the floor and make sure the new wall has a proper footing.

 

Another common issue: if the joint happens to be exactly where you're going to hang a cabinet or a picture, you need to be extra careful. The weight of a cabinet presses on the wall, and if the mesh hasn't been extended to that area, it might crack from the cabinet edge over time. When I run into this, I talk to the owner and extend the reinforced area to cover wherever there'll be load.

 

The cracking problem at new‑to‑old wall joints is really a stress concentration caused by different materials. If you ignore that stress and just fill it with filler, it'll teach you a lesson sooner or later. If you take it seriously and use a wide mortar layer with mesh to spread the stress, it'll stay put.

 

Converting a large room into bedrooms is becoming more and more common. Whether that new wall looks good long‑term doesn't depend on what colour paint you use on the front – it depends on whether that side joint stays closed. Three months after you move in, the paint colour doesn't matter any more – that crack is what you'll see every day.

 

People in the trade often say that hidden work shows your conscience. The joint between new and old walls is classic hidden work – once you cover it with the finish, you can't photograph it or inspect it. But a few months later, if the crack appears, your workmanship is exposed.

 

I never cut corners on this step. Not because the client will check – most clients don't even know this step exists. It's because I know that if I do a cheap job, that phone call will come in six months. And it's better to do it properly the first time than to go back and fix it later.

 

If you're doing a similar renovation, or if you're a homeowner stuck with a crack like this, give this method a try – scrape back the plaster on both sides, use mortar and fibreglass reinforcing mesh to create a reinforced layer, then replaster and repaint. Fix it once and save yourself the annual patching.