Lime varieties Re: render

[electricianid]

What is the current thinking on the type of lime mix suitable for moderatley exposed external render. I have scoured the internet for months, spoken to 'lime specialist' contractors, and even consulted a 'Historic buildings/civil/structural engineers' firm (expensive) only to be more confused than before I started. To make matters worse while looking for some literature for a local plasterer/renderer who is reticent to omit cement or metal lath I even came across this http://www.buildingconservation.com/articles/cement/cement.htm
I'd almost settled on the suggestion of the aforementioned consultant engineers that a mix of weak hydraulic lime (perhaps 2.5 NHL) and course sand would be appropriate, but now I read on this very website that a non-hydraulic putty is to be used (http://www.periodpropertyshop.co.uk/acatalog/Plastering.html)
There is a lot of talk about 'lime render' but no apparent agreement on what type. Any opinions would be respectfully appreciated.

 

[Nemesis]

No cement!!! It simply weakens the mix. Not good. That building conservation article is 12 years old and since then the Smeaton project has come up with more research.

Non-hydraulic putty or weak hydraulic where it's a very exposed site. Limewash on top.

Some recommend making your own putty from bagged hydrate, some might suggest a moderately hydraulic... people have their own views and it's worth reading up.

 

[andrew gayton]

whats the substrate??

 

[chuckey]

Sustrate = background on which you do stuff. Silicon substrate = you build your microprocessor on a piece of silicon.
Frank

 

[andrew gayton]

comedian?

 

[electricianid]

House is made of soft reds not particularly exposed but consultants have specified NHL 2.5

 

[electricianid]

Here is the site that I was directed to by consultants
http://www.stastier.co.uk/stucco-harling.htm seems to advocate NHL for all external render.

 

[Nemesis]

That could be because that's what they sell of course!

 

[David Q]

No particular reason to use hydraulic unless your property is very exposed
Weak or non hydraulic will both still set quite happily as a render. However if you are 800' up a welsh hillside like me, then you may prefer hydraulic.

 

[andrew gayton]

soft reds? - use non-hydraulic only; this will carbonate very well over soft reds...... make sure that the walling is well soaked prior to the first coat as soft reds are very pourous. ensure that the first coat does not dry out too quickly by regular wetting down.... this will prevent cracking and provide a stronger base for top coat.... use a cross grain float so as not to close up the pores...................... same with top coat. then limewash. make limewash from dry hydrare - again non hydraulic - with no more than 1% raw linseed oil to hel with longevity. 3 coats should do it.

 

[MdB]

Sorry to highjack a thread but I'm really interested in the drying out comment. I'm currently using non-hydralic 3:1 mortar to point an old stone internal wall and I'm worried it is drying too fast. Before I start I'm spraying the wall with water (sometimes a couple of times) and then I spray it again after I finish. I'm also spraying it a couple of times the day after as well. However, the stone is very porous and the mud between the stones is very dry and seems to absorb water very easily. This means that the surface of the lime is hard to the touch within 12hrs and sometimes a lot quicker than that. Should I be worrying? No signs of any cracks so far.

 

[andrew gayton]

mdb - the longer the carbonation or drying takes, the better the end result..... I would soak the walling - as you have - before pointing, point up, then compress the joint after 4 hours or so, apply more mortar if need be, then strike the joint or stipple brush to expose the aggregate and to open up the pores (a 'rough' texture will provide a larger surface for evaporation) keep damp for as long as you can.... say 5 days..... I would have used a 1:2 mix - the mix of 1:3 historically refers to the use of quicklime and aggregate prior to slaking.... slaking almost doubles the volume of the lime..... 1:3 SHOULD BE OK THOUGH! DONT PANIC.

 

[electricianid]

If I render with non-hydraulic then can I use the bagged hydrate mixed on the day as it were or do I need to order some mature putty?

 

[Gervase]

I've found that for render, the bagged stuff is better when allow to settle to a putty and then knocked up - it seems to make a 'stickier' mix and to crack less and thus need less rubbing down with the wooden float. For preference, however, I use matured lime putty bought in bulk bags (for around £270 a tonne). It's quicker and easier to work with and gives a better finish.
I'd only use hydraulic lime in very exposed conditions on the windward side of a building - I've used 3.5 on the south-western face of a house on to of a hill by the sea, for example. Otherwise, straight fat lime gives a perfectly acceptable finish with the advantages of extra flexibility and breathability.
To keep walls damp, it's a good idea to hang a barrier in front of them. I use hessian sheet sprayed with a mister spray, but you can use plastic tarpaulins - the idea is to keep a layer of damp air over the render - which is why wind is no friend to the renderer! When rendering or repointing, I wind the sheet once around some 2x1 batten and then screw it up into the soffit, allowing about nine inches clearance from the wall.
You end up with something looking like an installation by the sculptor Christo, but it does really help protect the render/mortar, particularly if the weather is dubious.
Here's one I made earlier:

Finding the door can be a problem!

 

[electricianid]

I like the method of hanging the hessian over newly rendered walls Gervase nice pics too. Where can I get matured lime putty for £270 per tonne? That is way cheaper than I imagined it to be, at that price it'll be my first choice.
Also one of my quotes says that he'd need to screw stainless lath onto the walls prior to rendering, aside from difference's in thermal movement, does anyone know of other reasons why it should not be used so I can convince him to render without it?

 

[Gervase]

EML seems a bit over the top for a simple render application. if you're applying stucco and ruling off the corners to immitate quoin stones in ashlar, then some nails in the substrate can be useful to give a better key and stop slumping, but unless your substrate is glassy-smooth and completely impervious, render should stick. The word 'lime' comes from an old english word meaning 'sticky' after all!
I tend to use hair in mine (in the base coats at least), knock it up quite stiff and trowel it on vigorously, then score the first coat quite deeply to provide a good key.
The pictures show an old cottage being prepared for roughcast render, but the principle is the same:

The render is a 3:1 mix of lime putty and sharp sand, with plenty of hair.
I get my lime putty from Ty Mawr who are fairly local to me - it's cheaper in one-tonne bags, and you don't have the problem of what to do with dozens of plastic tubs (believe me, you can have too many plastic buckets!).

 

[electricianid]

Thanks for the tips Gervase, Ty-Mawr are not local to me (I'm in suffolk) so I will have to research outlets in my area. I think the reason that the aforementioned contractor suggested EML is beacause he thinks that without it the render will crack (I'm on quite a busy road used by HGV's) I've got others quoting for the same work though so maybe I'll use one of them instead.

 

[BMcD]

electricianid wrote:

I think the reason that the aforementioned contractor suggested EML is beacause he thinks that without it the render will crack

It's possible that he thought because they are soft reds they may be more friable and therefore the EML will hold it all together. Or perhaps he's got a load left over from a previous job and wants to off load it onto you