Lime render guidelines

[Keithj]

We recently embarked on the next stage of filling the cracks that have opened up where the green oak used for our repairs (two years ago) has shrunk. The appropriate lime filler was obtained from Anglia Lime, who suggested the book "Lime in Building" as an instruction manual. It's a fascinating booklet, and made for interesting reading.
It even tells how to slake your own quicklime (with all the warnings and "do and don't") - impressive stuff!

So I got stuck in and filled the first few cracks with my chalk-and-lime filler, in an inconspicuous place while I got my hand in again. I dutifully covered the new work with hessian and watered it, and then looked in the book to check how long I should leave the covers on, how often I should water them, and how soon I can start to apply limewash. Nope, nothing in the book about that!

The local builder recommended to cover for at least four days, keep the covers damp (watering four times a day being typical) and not to limewash for at least four weeks.
Does that sound about right to the assembled expert team?

 

[tobydog]

Keith - is this chalk / lime filler made up of a wet mix putty or dry mix NHL? I can't recall what you used on the outside overall. Also, how big and deep are these cracks? Photos? Not doing the limewashing for 4 weeks seems excessive.

 

[Keithj]

Hi Toby,

No - this is a "ready-made" filler, a mixture of chalk and lime putty. As the man at Anglia Lime put it "a pot of that will keep for ever with the lid on".

The cracks vary in width from 1 to 2 mm, with a few places where it's 3mm.

Here's a gap alongside some new oak:

That gap is about 1½mm wide.

Here's the next stud along, which wasn't replaced when the repairs were done:

No gap at all.

 

[tobydog]

Keith - to be honest I was hoping someone more experienced would have come along by now and use your answers and photos to provide advice. Looking at the gaps that you have to fill, which will probably continue to change for some time to come, it's hard to see how you are going to get anything to bind/bond to the plaster and wood in such a small gap without getting the filler well into It. Maybe if you are able to just get some of the filler in the gap, then 3 or 4 coats of thickish lime wash may be a enough to seal it for now. Another alternative might be to try and caulk the gaps traditionally (ship decks) but force it into the gap leaving a depth of 3 or 4mm to plug with your filler and finish with lime wash. This way the caulking will always be there if you need to do further remedial work. Final alternative might be to rake out the gap to make it bigger and then get a much better bond but I would only want to do this if I knew the wood had stopped moving which I doubt.

Usual caveat - this would be my very amateur approach!.

 

[Keithj]

Quite so, Toby!

My inclination (which is what have been doing) is to brush out any loose material (there isn't much, if any) then wet the existing lime and force the lime filler into the gaps. Judging by the amount of filler I'm using, a fair amount is going into the cracks. My guess would be that it's going in at least to the depth of the existing lime render (a scratch coat and a top coat). The areas I've done so far (two weeks ago and a week ago) aren't showing any signs of cracking or falling out again.

The good folks at Anglia Lime reckoned this filler was good for cracks up to 3 mm wide, after which they recommended using haired lime.
On the basis that limewash is the same stuff again only thinner, I'm going to apply a "mist coat" (2 water/1 limewash) to the new bits, leave that for a few days, then wet the whole surface of each panel and limewash the lot.

I don't know how long green oak goes on shrinking, but I'll see how these patches hold up. If necessary, I'll repeat the process in a couple of years.

 

[middi]

Reading books about lime mixing for different applications is the same as reading books about brain surgery, you learn a lot about both disciplines but extremely inefficient about the practical application, in both instances, without practice.
Firstly it is worth to note that " green oak" ( two or three years old) will carry on shrinking for about 8 to 10 years in our central heated homes until it becomes stabilised ( if there is such a thing with oak !!!!). As a rule of thumb a newly installed green oak stud will shrink 4 to 6 mm ( total from both elevation) in the first couple years or so and approximately the same amount again , progressively, for the next4 to 6 years after that.
One has to think 12mm ( approx., half an inch) lateral shrinkage for a 150x100 thickness new oak stud.
Ideally ( and when the gaps are their maximum after shrinkage) these gaps ( as it was traditionally done) need to be filled with sheep's wall strands that have been soaked in putty lime and smoothed ( finished) off with putty lime. This time tested application will guarantee that if ( and when) the infill will crack again it will stay in position due to the wool's binding and not end up in a pile at the base of the wall. Another application of lime putty will then eliminate the cracks and ready for the next lime washing etc..
As we are an impatient lot ie- no able to put up with gaps in new wall studs for a decade or so, there isn't a blue print to follow as to what to do in between waiting.
Still, hopefully the above information may be of use in the long run.

Middi

 

[Zebra]

That's very interesting and useful middi. Hardly worth buying a bag of sheepswool for such a small amount. I wonder Keith, will you take a walk across a farmer's field and pick some bits off the barbed wire fence and wash them, or take apart a woolly jumper?

 

[Keithj]

Actually, I have a large bag of sheep's wool in the shed, left over from the filling in the walls where they did the repairs. We have wool thermal insulation in most of our walls.

Mind you, SWMBO does keep praising the virtues of sheep's wool for filling gaps, and giving people a "little bag full", so I hope there's some left.

I hadn't thought of lime-soaked sheep's wool in the wider gaps, and most of those now have their new lime putty filling. I'm on the 1mm ones in the gable wall at the moment, but when I get round the back to the wider ones, I'll do that. Thank you!