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Hi there, any help would be greatly appreciated!! I've just taken off some tiles in the bathroom with the intention of dot and dabbing some plasterboard on to the aerated blocks (I think they're Thermalite) then plastering over the top. (It was a fully tiled bathroom but I'm changing it to a half tiled bathroom).

Unfortunately, where I've taken the tiles down around the window it has brought some of the block away with it (I've attached a photo of it). I'd be very grateful for any advice about what to do about it? Please could someone tell me what to do? Should I use some sort of filler or just dot and dab over it or do I need to get a builder in? I'm not confident enough to replace the brick myself.

Also - it's not important at all - but I'm just really curious about what's already on the walls over the blocks, if anyone could please tell me? It's grey in colour, it doesn't look like plasterboard (no paper of any kind), it comes away fairly easily from the blocks in most places in chunks, almost like it's smoothed on rather than stuck on, if that makes sense? I've put up a photo of a spot where the grey stuff can be seen on top of the blocks.

Thank you in advance for any guidance offered!!
 

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JohnD

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You have some quite big gaps. I'd use sand and cement mortar. Scrape out any loose or crumbly material first, brush or spray with water to remove dust and moisten the dry and absorbent blocks. Don't make your mix wet and runny or it will sag and shrink, leading to cracks. Push it in very hard. You can apply in a few layers if you like, once the previous one is stiff.

Gypsum mortar is sometimes grey.

Mortar is easier to use than plaster as it has a longer working period.

It is much stronger and cheaper than filler.

It is is unaffected by damp and will actually set stronger if you keep it damp for at least a few days when new. Spray with water and cover with clingfilm.

If you are going to tile the walls you can use moisture resistant backerboard rather than plasterboard, but ask in the tiling section.

Don't forget to core out a hole for your extractor fan.
 
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You could also consider finishing with a renovating plaster. Remove any loose material and fill large gaps first.
 
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Hi there, any help would be greatly appreciated!! I've just taken off some tiles in the bathroom with the intention of dot and dabbing some plasterboard on to the aerated blocks (I think they're Thermalite) then plastering over the top. (It was a fully tiled bathroom but I'm changing it to a half tiled bathroom).

Unfortunately, where I've taken the tiles down around the window it has brought some of the block away with it (I've attached a photo of it). I'd be very grateful for any advice about what to do about it? Please could someone tell me what to do? Should I use some sort of filler or just dot and dab over it or do I need to get a builder in? I'm not confident enough to replace the brick myself.

Also - it's not important at all - but I'm just really curious about what's already on the walls over the blocks, if anyone could please tell me? It's grey in colour, it doesn't look like plasterboard (no paper of any kind), it comes away fairly easily from the blocks in most places in chunks, almost like it's smoothed on rather than stuck on, if that makes sense? I've put up a photo of a spot where the grey stuff can be seen on top of the blocks.

Thank you in advance for any guidance offered!!
The blocks would stabilize better with large sheets of plasterboard stuck to them - acting like sheathing. It's usually a fight over the damage you do removing the plaster v's the gains you would get from dabbing boards and the stiffening effect it gives. The fractures in the reveal masonry could be better fixed with a good splash with water and some carefully directed ex-foam - then excess cut back and plastered over.
 

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