External Rendering problem

[FozzieBear]

I am looking for some advice on a problem I have with external render. The areas affected are on the plinth below the DPC on two new extensions. The render has cracked along the joints and is very friable and spalling in places as can be seen from the pictures.

The construction below damp is two skins of 4" cavity brickwork filled with concrete to 4 inches below the DPC. The foundations were built in very wet conditions but had dried out thoroughly before the render was applied. when thoroughly cured I painted the render with bitumastic paint, thinning the first coat by 50% so it soaked into the surface.
I dont know why this render is different to the original extensions which are constructed in the same way and are still really solid.

The plasterer that did the work is no cowboy and has been rendering and plastering for years and I have never had any problems with his work before. I helped him knock up and he did not scrimp with cement in the mix but I can remeber the ratios he used.
Is it possible to stabilise the surface and try and patch in or is it a case of hacking it all off and starting again? If I can avoid hacking off obviously I would prefer that as one side my new sandstone patio butts directly agains the plinth
Fozzie
View media item 46604 View media item 46605 View media item 46606

 

[roughcaster]

That looks far too sandy a mix to me Fozzy, especially picture 3. It looks as if there's no cement in the mix at all, although you know there is, but can't remember how much. There's no way you'll stabilise that at all. If it was me, i'd take it back to brick and start again. You said you've never had any problems with the plasterer before, and then went on to say you helped him knock up the mix, but can't remember the ratios he used. It looks far far too sandy to me,, whadya think guys??
Render drying out too quick can also cause similar problems, as can too much waterproofer.

 

[joe-90]

Could be frost too.

 

[roughcaster]

Forgot to mention that Joe, thanks.

 

[dmplastering]

How it the rest of the base? Is it all affected and how ago did it get rendered and at what time of the year?

Reagrdless, you are going to have to hack off and get it redone

 

[FozzieBear]

Thanks for quick response guys.
I had two extensions done at the same time. The rest of the base on this extension is similar but the other extension done exactly the same time and same mix is no where near as bad. I cant remember the exact time of year but belive it was done in the autumn nearly three years ago. I got him to put the drip beads on for the pargetting above and render the plinth so I could backfill the excavation and tidy up before the rest of the shell was pargetted.
Could I get away with hacking back to the scratch coat if that looks sound when I expose it?
Fozzie

 

[dmplastering]

Yes buti would wire brush it aswellmto make sure it isn't the same sandy texture as second coat. If its sound then rush it all down with water the remove the dust from the surface

 

[roy c]

I agree with rc. thats far too sandy a mix. I would hack it back to the brick work (it looks like you could put a scrapper on it and scrape it off). I doubt wether you could just hack it back to the scratch coat. Also there is no need to put bitumous paint on it if it is rendered properly just paint it with a water-based masonary paint and let the render breathe. Are you sure it was sharp sand and not building sand?... Just looked at pics again and it does look like sharp sand was used...Had the sand been lying around around for a long time and was there any lime in the mix?

 

[cotswoldbuilders]

Cut it all of below bellcast bead, clean the brickwork and paint the bricks black.
You will have nothing but trouble with water wicking up the render.