All my window openings are like this.  

Would mesh have reduced the risk of this happening, or is it just inevitable?

 

Definitely not normal. If it's plasterboard then the joins should be away from any edges but that's what it looks like for me. 

 

When I first plasterboarded my front room I just stopped the board at the "easiest point" and guess what, cracked at all these places a year later. Maybe that's what's happened?

 

For what is worth, since then I've cut my boards to keep away from any edges and it's been rock solid.

No someone has messed up.  Using mesh internally would be unusual.

Sorry, forgot to say it's wet plaster, on mortar scratch coat.

No plasterboard.

Thermalite blocks by any chance ?

3 hours ago, Russell griffiths said:

Thermalite blocks by any chance ?

 

No, dense block.  Approx 10 mm mortar under the skim coat.  Plastered over the winter, internal temps never went below 10 degrees.  Dried out very slowly, no excess heat or dehumidifiers.  Took a few months for the cracks to develop, but are not along mortar lines, and given the house is on a Kore slab on hard rock underneath then I don't think this is any subsidence issue.

So when you say all the openings are the same, do they all have a crack running down from the bottom left / right edge to the floor?  Nothing at the top?  Ground and first floor?

On 22/06/2026 at 12:32, Mr Punter said:

So when you say all the openings are the same, do they all have a crack running down from the bottom left / right edge to the floor?  Nothing at the top?  Ground and first floor?

 

First floor, underneath windows at front (SW) side are the worst.  Diagonally down from corners. 

Tops of most doors the same. Diagonally upwards.

Bed joint reinforcement (the train track stuff) above and below the openings may have helped.  I have only specified it on rendered walls or brickwork feature panels like stack bond.  Mesh in the render also helps.  You best option now is filler.

2 hours ago, Mr Punter said:

Bed joint reinforcement (the train track stuff) above and below the openings may have helped.  I have only specified it on rendered walls or brickwork feature panels like stack bond.  Mesh in the render also helps.  You best option now is filler.

Do people really mesh internally? Id be interested to know if blockwork or plaster cracked. Can you hack some off below skirting line?

48 minutes ago, Oz07 said:

Do people really mesh internally?

 

Not me.  I would spec it on external render though.

Don't do any filling now, give it a year to let any drying/ movement/expansion/contraction finish off. And when you do, rake out any narrow cracks with edge of a paint scraper before filling. 

I think I may have figured out what has caused the cracks under all the windows.

Bernard the builder assured me that building internal sills with concrete was "what we always do". I did suggest a block of compacfoam may be a better idea, but what do I know?

Concrete went in October last year, so temperature at the time probably 5 to 10 C.

No expansion joints at the end. I must be missing something here. It can't be what I think it is, surely? That would be way too incompetent, right?

Every window the same, the crack width directly proportional to window width.

Edited just now1 min by Mr Blobby