Rendering and airtightness

[Cpd]

i have spent a large part of my life involved in stone masonry and for ten years i was building and rebuilding historic stone walking tracks in Australia, i always used a 4:1:1 mix and sometimes 5:1:1 (sand cement hydrated lime) and a tiny amount of fairy washing up liquid to make it so much more workable (the use of cements were minimal as it was "historic" ) but this mix was so good in so many applications, i even made a test of rebuilding the front of some stone steps that had five thousand people  Plus people using them a year, it was also in an environment that was often fully exposed to the severe sun during the day and very cold at night, but would also be subjected to freezing conditions in winter. I monitored these sections over a five year period and i was amazed at how well they held up, the repairs were on sandstone that is known to expand and contract in diffrent conditions and this mix reflected this or was able to absorb this movement without cracking at the joining lines. I have used it to rebuild various buildings on my property, sometimes putting in more sand if just using it as a bedding for stones and am still very happy with the mix. I do not know if this would create an airtight layer or if it would be waterproof, but it is a very good robust and flexible  application as well as being relatively  cheep. Totally frowned upon by  purists and i understand why it should not be used in many traditional applications but in a new build or full renovation......   

I used this mix on the front of one of my buildings to rebuild the stones that had eroded away, it was then painted, there has been no cracking or separation from the origanal stonework.  

[ToughButterCup]

Thanks @Cpd, @Barney12 and @JSHarris. Very informative and interesting.

Now the question is .... Will cheap Aldi washing up liquid do the trick too?

 

I think, just for the crack (craik?) I'll try putting a bit of lime in on the South face of the house - the surface that's going to experience the biggest temperature range.  

[Jeremy Harris]

Many years ago, washing up liquid used to be commonly used to make mortar "wet" a bit better and to make it more workable.  It was found that it tends to make the mortar weaker, especially if a bit too much was used, and especially if mixed for a long time, where the foaming tended to cause far too many air bubbles (some air bubbles are what makes the mortar more workable).  As a consequence, it's not recommended now, and admix was formulated to do the same job, but without the problems that some detergents can cause, especially those that use salt as a thickening agent.   FWIW, when I was a small boy, being taught how to lay bricks, the very good bricklayer that was teaching me always used a squirt of washing up liquid in the mix, but that was in the mid sixties, before admix was around.

 

People have been parge coating walls with a wash of strong cement  mix and a brush or roller for decades without problems, so frankly I think you're over-thinking this.  This is just an airtightness layer you're applying, isn't it, not the base for a proper render system?

Edited October 25, 2017 by JSHarris

[ToughButterCup]

Over-thinking it , yes. As usual.

[Construction Channel]

you might as well stick a little lime in it, 2 bags will probably do the whole lot.

 

or you could use this......... which looks more impressive 

 

 

[Calvinmiddle]

On 25/10/2017 at 10:38, recoveringacademic said:

crack (craik?)

 

@recoveringacademic

 

fyi - it’s craic