How to fix repeat plaster cracks once and for all?

Question

Hello, I have a concrete slab double brick house in Perth built in 2000 with hard plaster on the walls. In the hallway there is a section of plaster between some steps and a window sill that keeps cracking. I have repaired it several times previously over the years with various filler products but each time it cracks again. Each time I open the crack up to a v-shape with the Kango plaster tool and then build up layers of filling compound and then usually with a skim coat on top. Once I tried embedding fibreglass tape like Gyprock Easy Tape in it but it didn't make a difference. The width of the crack is how much I have opened it up on previous repairs, previously it was narrower.

I don't think it's a serious crack as it never gets worse than this and there is nothing on the outside wall but it is a nuisance. I am looking for advice on which products to use to try the next repair. Some ideas I have are to brush on a plaster bonding agent first to stop the new compound drying too quickly, and trying to embed a fibreglass tape with a finer weave like fibafuse.

Off to the right here is what looks like a previous crack that may have been patched. It's not perfectly flush but it's never progressed more than that in the 10 years I lived there so it seems whatever they did worked.

Thanks in advance.

R4addZ · Answer

Hi @rsmallri

If you don't want to start hacking plaster of in larger quantities, you could try stabilising whatever is causing the issue without opening it all up. You would need to drill diagonally through the cracked area into the brick behind then insert chemical anchors into the holes glueing the bricks either side of the crack together. Its not like you need to worry about subsidance as it is a concrete slab and there is no cracking on the outside so all we are doing is preventing any movement in that area. Drill some right to left and others left to right so you criss cross the flaw and doing this, there is also the possibility that some of the chemical glue will seep into the existing crack further reinforcing the repair. Let it all dry/cure then fill in the usual way. Just remember to drill far enough away from the crack that you pass through brick and not just plaster either side of the crack.

Nailbag · Answer

Hi @rsmallri

I agree with @MitchellMc in investigating deeper into the masonry to find the cause as the crack is obviously substantial for it to keep repeating.

That said, fibreglass tape isn't a great product when cracks are wide. Paper tape on the other hand can be far more successful according to a plasterer mate of mine.

It's also possible the attempted repairs are not thick and wide enough. If you haven't already, you may need to cut a deeper v-channel and fill it with a flexible base coat. Then apply the paper and apply another thicker and wider coat. Sand this back when dry and repeat the process with widening top layer coats.

Then prime coat and paint.

Nailbag

MitchellMc · Answer

Hi @rsmallri,

Thanks for the clarification.

In that case, if the crack keeps reappearing in exactly the same location despite your repairs, there's a good chance the crack extends into the masonry behind the plaster. It may simply be seasonal movement in the brickwork, with the masonry expanding and contracting enough to keep transferring the crack through to the plaster.

Have you ever opened the crack up enough to see what's happening in the masonry behind? It would be interesting to know whether there's a corresponding crack in the brickwork or blockwork.

If there is, I'd be looking at repairing the masonry crack first before reapplying the hard plaster. Otherwise, any filler or tape applied to the surface is only ever treating the symptom rather than the cause.

Mitchell