Lime mortar and rendering

[bernardgreen]

I am renovating an old cottage and need to render several stone and rubble walls with lime mortar. The first attempt went well, the mortar between 1/4 and 1/2 inch thick stayed in place but 24 hours later the dried mortar has cracked. Still secure to the wall but with a lot of cracks

Mix was 2 of 3.5 hydraulic lime to 5 of washed sand.

What do I need to do to prevent the top layer from cracking. Small cracks can be tolerated as the wall will be hidden by oak panelling but I do not want it falling off the wall

There is also a need to replace the lime mortar on the window's internal ledge. The existing mortar is about an inch thick and was intentionally criss crossed with deep scoring. Was this scoring intended to allow for shrinkage ? This ledge will also be concealled under a timber shelf so if scoring is necessary it can be done.

 

[Micilin]

Did you build up the background over time so that the coat of render is even? ie do you have some parts much thicker than others ?

Is it just one coat? was the wall dry before applying?

If it is sound then I would not worry but you can work the surface again with a wooden float to close in the cracks, depending on how dry it is

 

[dpt]

You still have time to "rub up" the wall. Get a hose on it and give it a good soak and "rub up" with a plastic or wood float.

If you do any more, clean the wall down and hose it down well until the wall stops "drinking" the water and apply render in 10mm max thickness.

You need to keep an eye on it for at least a week or so depending on the RH (relative humidity) periodically wetting down the wall and "rubbing it up" until the lime cures.