NHL on top of existing cement render

[PJS]

Hi all,

I have a customer's damp property to deal with and although the cause is obvious (cement-based internal and external render) the customer can't afford a complete rip back to stone and re-plaster.

I've suggested additional air vents, new heating, and a cover on the internal wall, leaving the external wall as pebbledash.

But the customer can't afford the 3-layer NHL. I'm trying to help them out but I'm not sure how if at all the NHL will sit on the already-rendered walls. And if the absorption will be good enough for the new lime plaster to be a long-term solution.

 

[stuart45]

There's no point putting lime on top of cement render.

 

[Lacan07]

As Stuart has said it's a waste of time and money putting NHL on cement render, and of all the limes NHL is the least breathable anyway. You might as well tank the walls or rip the cement off and limewash the stone as a temporary measure until the client can afford the work.

If you do want to plaster NHL on to render you'll need to scratch the surface up with a devil float to provide a key. Some of the modern NHLs have a bit of cement in them anyway and as such give a pretty solid finish.