Mon, 25 May 2026 13:08:56 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=606#Comment_606 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=606#Comment_606 Thu, 22 Feb 2007 07:56:12 +0000 ANGAND
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=609#Comment_609 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=609#Comment_609 Thu, 22 Feb 2007 08:54:55 +0000 nigel Firstly they are not recommended for use on render which has been waterproofed and secondly they will not help at all where you have cracking render as the mositure is finding its way behind the render. It cannot escape outwards because the cement render is essentially waterproof.

Any other additional render added on top of the cement is a risk, it could work or it could make the situation worse. Also many of these render systems are not cheap anyway.

I hear what you say about removing the render not being an option, is that for cost reasons or some other reason.

If you plan to stay in this house for some time it may well be more cost effective to do it once and do it properly however tempting it may be to find a quick fix.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=611#Comment_611 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=611#Comment_611 Thu, 22 Feb 2007 09:02:11 +0000 Mike George https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=624#Comment_624 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=624#Comment_624 Fri, 23 Feb 2007 10:33:48 +0000 fostertom https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=653#Comment_653 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=653#Comment_653 Sun, 25 Feb 2007 10:13:41 +0000 ANGAND https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=656#Comment_656 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=656#Comment_656 Sun, 25 Feb 2007 15:44:22 +0000 nigel
St Astier make a suitable one as do Keim with both you probably want to use a reinforcement mesh.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=662#Comment_662 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=662#Comment_662 Tue, 27 Feb 2007 00:25:19 +0000 fostertom
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=663#Comment_663 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=663#Comment_663 Tue, 27 Feb 2007 08:59:10 +0000 Mike George
I agree with you on the whole, and I have on occasion been asked to render walls with very weak sand/cement mixes, not much fun though as workability becomes an issue as cement content is reduced -not something I would do out of choice. My preference is a sand: cement: lime render but this depends on the building and/or background. I understand why some prefer all lime mixes and I agree that in some situations this is the way to go. Proportions used are dependent on background, but the golden rule is to make subsequent coats ‘lighter’ ie less cement. 5:1:1 is a good all round mix as a scratch coat.

Regarding Rough cast/Harling, I have not considered the science behind this but it is traditionally recommended as the best finish in areas of high exposure simply because it presents an increased surface area for moisture to evaporate from. Depending on the materials used, it can indeed be porus and if done properly requires no painting for an attractive finish. The best way to achieve this is with a ‘harling bat’, or if a coal scuttle is used, flatten the edges to ensure even spread of the mix. Also this is a very messy job and not to be recommended without goggles gloves, full body armour etc Also cover everything you don’t want covered in mortar with temporary sheeting. Visually, I think tyrolean is a poor substitute for true harling.


Regarding the thickness and number of coats, 3 totalling 25-30mm seems about right but this varies depending on the shape of the background. As to when to cover each coat, that is always a tricky call, and depends totally which of the many finishes you are trying to attain. Trying to roughcast a partially dried wall for example is a recipe for disaster, in terms of the quality of the finish. The material will stick to the dry areas and slide down those which are still damp. Continuity of mix proportions is also a critical factor with this.

Getting someone who is well recommended, and well experienced is always the way to go - not a job for an apprentice.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=664#Comment_664 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=664#Comment_664 Tue, 27 Feb 2007 11:19:35 +0000 fostertom 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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=665#Comment_665 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=665#Comment_665 Tue, 27 Feb 2007 11:45:10 +0000 Mike George
The spatter gun is limited however for bespoke roughcast mixes, as it will only effectively 'fling' minimal size aggregate such as stone dust. If you want a 'heavier' finish such as that with quarter chippins then you have to resort to throwing it on by hand.

I have heard of electrically powered roughcast machines, but have no experience of them. Maybe these work, anyone?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1011#Comment_1011 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1011#Comment_1011 Mon, 12 Mar 2007 18:45:28 +0000 Guest
Cheers,

Brian]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1303#Comment_1303 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1303#Comment_1303 Tue, 20 Mar 2007 15:50:41 +0000 Guest https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1309#Comment_1309 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1309#Comment_1309 Tue, 20 Mar 2007 19:36:31 +0000 Guest https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1387#Comment_1387 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1387#Comment_1387 Fri, 23 Mar 2007 06:18:50 +0000 Guest
I have a brick wall that I wanted to render with a lime:sand:cement mixture (1:0.5:4). the cement is purely for a litle ealy strength and water resistance. I plan to apply a single thin coat of approx 5-6 mm. I do not understand why I would need a thick (25 mm) coating as it would make the wall very heavy - I'm taking about a brick-veneer wall of a house.
As a test measure, I applied this thin coating to a concrete block wall and it has stayed on though a whole year of 4 seasons without any problems. This leads me to think that this 5-6 mm coat should do the trick - or am I missing something ?
Any suggestions?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1389#Comment_1389 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1389#Comment_1389 Fri, 23 Mar 2007 08:10:21 +0000 biffvernon
Leave out the cement. Just use sand and lime 3:1]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1481#Comment_1481 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1481#Comment_1481 Mon, 26 Mar 2007 02:11:16 +0100 Guest But what about my idea of a 5-6 mm single coat ?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1484#Comment_1484 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1484#Comment_1484 Mon, 26 Mar 2007 09:50:39 +0100 nigel https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1490#Comment_1490 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1490#Comment_1490 Mon, 26 Mar 2007 11:23:48 +0100 biffvernon
Of course a really thin layer is called limewash - but leave out the sand.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1511#Comment_1511 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&Focus=1511#Comment_1511 Mon, 26 Mar 2007 17:55:46 +0100 Mike George
The BS for rendering suggests three coats of approx 10mm on a severely exposed site for this reason.]]>