Rendering onto thermalite blocks?

[cornish_crofter]

Never had to do this before.

However that wetroom has thrown up another issue.

Today, whilst removing the old adhesive from the walls I noticed that about 2 ft up from the floor that the render is actually coming away from the walls - not the plaster, the render!

And to boot, though I think this may be the cause the wall seems to be retaining damp at this height.

The customer tool the tiles off a couple of months ago but the walls just haven't dried out.

As the whole lot is going to be tiled I was just going to render the areas that need to be repaired. No need for multi finish etc.

However, a plasterer once told me that Theralite blocks are difficult to render.

Any tips please.

[whitebeam]

Your plasterer was right, there a pain..

The suction from the thermalites is incredible and need a generous water soaking before applying any render or..

Use sbr/cement to slurry the blocks which will kill the suction and will make the rendering far easier.

Thermalites have a bad habit of cracking due to any slight stress in the wall

[cornish_crofter]

Thanks for that.

I suspect the plaster on the exention I'm working in was the only one who knew what he was doing

The walls are pretty straight and level, and the render is staying on firmly where the damp from the badly fitted shower tray hasn't got to it.

Fortunately it seems that the thermalites are only used on the inner course and the extension is single story.

I don't think I have ever seen such bad workmanship on the blockwork or most other aspects in this country. Seen a huge variation when we were in Ghana, but most of it was better than this!

They've used 5 inch thermalite on top of 4 inch blocks below DPC - crazy!

[whitebeam]

Could have been worse, thermalites under the dpc

[cornish_crofter]

Could have been worse, thermalites under the dpc

Actually, with this building, that wouldn't surprise me

[peckers]

remove all old loose render, brush of blocks no sandy particles left then prime with sbr leave to dry give another coat leave to dry then just before you mix your render apply another coat of sbr then render while still damp. if they are smoth with no good key once old render is removed do as white beam has said and give them a slurry coat with cement and sbr mix.

[Scott]

Make sure you give it suficient time to dry between coats though. Shouldnt be a problem as those blocks suck like a leech!

[cornish_crofter]

Thanks

I think that is one of my jobs tomorrow.

There is around 1m of wall to re render and I've got the sand inside to thaw it out.

There are other areas where the multi finish has had to come off, leaving me around 1/4 of an inch to make up. I was just going to use tile andhesive with primer in those areas.