Our old Welsh stone farmhouse was ‘modernised’ and cement rough rendered around 16 years ago. Though the render is largely in good condition there are a few hairline cracks and we are now getting patches of penetrating damp showing as mould, flaking paint and salt efflorescence on internal walls.

Chipping off the render and starting again is not an option unfortunately. So we are considering other ways to rectify the situation. We’ve heard about a microporous product called Andura (used by many companies that advertise render repair and restoration) and Beeck’s silicate paints.

Can anyone advise us or share their experience? The walls are 65cm thick stone/rubble fill so inserting aeration bricks may not be possible.

thanks.

I agree with Nigel, The answer is to have an un-painted porus render. What you have now, sounds like a rich sand and cement mix. I would not render over the top, as any new coats will add weight and if the original coat is not sound, it may crack further and fall off.

The reason for not removing the render is cost (mainly) and time. Also, not sure what it would expose as house has been modified so probably not all stone underneath. We want to build a passive solar space (conservatory) on the south frontage this spring, which will take all our savings. One option is to just weatherproof this wall (which has window openings changed in past so may be block work underneath) and consider taking render off walls that haven't been modified, sometime in future. BUT if we do this, what's the best product for the south facing wall - the Andura system used by many companies makes many claims about durabilty and lifetime...just not sure whether to go for this. And, once the solar space is in place, removing the render will NOT be an option in the future. Help!

If you do take the render off, top option is to just properly repoint what's underneath. If that's not presentable, needs re-covering-up, then yes a breatheable render, and all-lime is good for that, but an unfmiliar challenge to most builders.

As far as I know, there's another way to breatheability, which is common in the everyday building industry. Anyone agree/disagree? It's roughcast render. First two coats can be ordinary sand/cement render (with lowest possible cement content, but still impervious). The final roughcast coat is the key to making this cement-based, therefore impervious, system breatheable. Roughcast is small round aggregate e.g pea gravel mixed up in a slurry and flung at the wall. When it sets, the inevitable hairline shrinkage cracks begin, and the point is that the space between every 'pea' of gravel is a stress-riser at approx. 10-12mm c/cs in all directions. So the shrinkage happens evenly all over at close centres; close enough that every bit of water that enters by capillary attraction is close to an escape route (a crack) and can evaporate out when the sun shines on the wall. Whereas smooth render cracks in much larger slabs. Then the water that's entered by capllilary attraction and spread out laterally by soaking through the permeable masonry behind the slabs of impervious render, is too far from any exit to evaporate out when the sun shines on the wall. Smooth render therefore becomes a one-way pump for water.

The important thing is for all coats to go on green-on-green - i.e. each coat goes on before the last has matured. That is, set but not looking dry. That way, all bonds together chemically and the stress-risers properly affect the whole depth of the render. The final slurry coat is easily done with bag-lime rather than cement, no problem. It's traditionally applied by flinging with a small coal-shovel from a bucket, but it's easier to use a Tyrollean spatter-gun. It can be painted (with permeable masonry paint) or it can be made durably self-finished by mixing e.g. fine Bath stone dust into the slurry. An experienced plasterer should know about all this, mix recipes etc. Not sure myself - does anyone know? Total thickness of the 3 coats about 25mm.

That's added a lot to my knowledge - thanks.
True Tyrollean is also good for similar reasons - that's sandy/gritty, without pea-gravel, flung onto the wall and left all jagged. But agreed it doesn't look half as nice as roughcast(daansaaf)/harling(oopnoorth}, which is smoothly, creamily knobbly. I'm not suggesting Tyrollean, just use of the Tyrollean spatter-gun to apply the roughcast. Haven't tried it myself but am told it's the way to go, much less mess. Definitely a skilled job, either way.

We are in the process of buying a fairly run down post-war semi with rendered brick cavity walls. We want to build a single storey extension & have seen in Germany & Luxembourg a simple system of a single block inner leaf, with insulation fitted, mesh & render. Providing sufficient insulation is fitted will this meet building regs? Anyone any experience of this, including the type of insulation?

Cheers,

Brian

Belzona have a UK base in Harrogate.

NO No NO

Leave out the cement. Just use sand and lime 3:1

Give the guys at www.lime-green.co.uk a call they will give you sound advice.

Porus render relies on thickness for weathering performance. It must be sufficiently thick to accomodate water tracking towards the inside of the building. This then evaporates to the outside before it tracks inside causing damage to internal surfaces.

The BS for rendering suggests three coats of approx 10mm on a severely exposed site for this reason.