Defrassing Guidance

[MatthewC]

Has anyone done any defrassing of stonework? I'd like to have some guidance please on techniques, tools and things watch for.

As I understand it, spalling of the stone face is caused when the surface contains water and there is a big freeze, like in Dec 2010; ice expands and so breaks up the surface. I think that the need for defrassing (removal of the loose stuff) is needed to allow the good stone to breathe, or else the loose stuff retains moisture against the good stuff, which makes the problem more likely to occur again and thus the stone would deteriorate rapidly.

Some of the random rubble Cotswold stone on my house needs defrassing (I am told) and Google for once is fairly unhelpful. Here's a couple of photos showing the general issue, although I stress that these are not my house - I cannot take photos of the relevant bits due to their height and the trees in the way.

 

[FamilyWiggs]

I'll be interested in the answers too Matthew. I have similar problems with some of my stonework. Sadly iris some of he oldest carved stone I have - mullions and like.

 

[plasticpigeon]

I always thought frass was the dust left over from woodworm infestation! I most certainly don't know the answer, but I'm convinced most things to do with buildings only require logical thought and common sense to figure out.

I would imagine that any organic matter would hold water well and contribute to the effect you talk of. I'm sure that could be washed off gently with a scrubbing brush perhaps. I can't see why the surface finish of the stone would be an issue though. I don't have much stone on my house, just window sills and fancy lintels or cappings or whatever they are called made of Bathstone I think, and they showed some degradation where paint had trapped water against the stone. However i would be very careful before letting someone loose on your stonework. I let a local mason loose on my window sills and cappings and they got damaged (surprise surprise). A lot of stone is much softer than I thought and can easily be marked and even disintegrated completely with inappropriate tools and unskilled workers. I found I could clean up the stone going gingerly with a cabinet scraper or even sandpaper!!! If someone comes with metal wallpaper scrapers or industrial looking jet washers I would send them home.

 

[MdB]

I can see the problem. Once some damage gets in, the top surface will be full of tiny voids which will hold moisture well. This in turn will hold moisture in the good layer underneath which will then get damaged in the next freeze thaw cycle. It would be a good to remove the damage - but how to do this in the best way I haven't a clue - sorry.

 

[angrybird]

oh! I have some of that on my limestone walls , never knew it was called that ! you learn something new on here every day, look forward to the replys!

 

[88v8]

Mmmm,the surface is blowing off. The next frost will take more of it off, until sound stone is reached and then it will stop fttb.
If it were mine, all I'd do is ensure that no horizontal surfaces are created to hold water, such as where blowing stone meets sound pointing. Not sure there's anything else one can do.
Delamination of stonework is a natural process, especially if the stone wasn't very well laid in the beginning - it should be 'laid in its own bed' so that any laminations are horizontal and not able to be split off by the weather in this way.

Ivor

 

[plasticpigeon]

It there some way the surface can be consolidated if it is very porous, for example with a thin limewash just to fill the holes?

 

[MdB]

It there some way the surface can be consolidated if it is very porous, for example with a thin limewash just to fill the holes?

It has been said that this can make it worse rather than better.

 

[plasticpigeon]

Ah this is why this forum is great - information transfer. Do you know why it might make it worse?

 

[MdB]

Ah this is why this forum is great - information transfer. Do you know why it might make it worse?

Read through some of this: http://www.periodproperty.co.uk/forum/search.php?keywords=limewater
(I don't know anything more than what was said here)
Basically, the consolidating the surface layer can cause a larger chunk to fall off causing more damage than a bit at a time.

 

[FamilyWiggs]

It's a shame, but I guess inevitable. My place is a mixture of sandstone and limestone. The sandstone is for the few more decorative areas, mullions, window surrounds, etc. sadly it is returning to sand...

 

[worms]

...such as where blowing stone meets sound pointing.

Surely one of the functions of lime pointing is that it acts as a softer, sacrificial layer, absorbing the moisture that would be trapped at the interface between stone and pointing if the pointing isn't absorbant, such as when it is cement?

 

[88v8]

Depends on the lime.
The principal is that the pointing should be softer than the stone. If one indiscrimantly points with a 3:1 mix of NHL5, for instance - not saying that's the case here, just for instance - then sometimes that will result in pointing harder than the surrounding stone. Our external pointing so far has been in NHL2.

And, some stone is simply less durable anyway.
Especially if it was wrongly bedded in the first place. Incompetence is not a modern invention. :shock:

Ivor

 

[MatthewC]

Mmmm,the surface is blowing off. The next frost will take more of it off, until sound stone is reached and then it will stop fttb.
If it were mine, all I'd do is ensure that no horizontal surfaces are created to hold water, such as where blowing stone meets sound pointing. Not sure there's anything else one can do.
Delamination of stonework is a natural process, especially if the stone wasn't very well laid in the beginning - it should be 'laid in its own bed' so that any laminations are horizontal and not able to be split off by the weather in this way.

Ivor

I think the issue is that sound stone is not reached as it keeps on going if you have more freezing weather (not just a frost, I think). I have been told that you need to ensure that the pointing at the bottom of the stone is sloped instead of being a horizontal ledge. I don't think it's delamination as I'm sure the stone is laid correctly - the photos above are from a church built several hundred years ago.

The advice to "defrass" is from an architect who specializes in old buildings; I was hoping that someone had some experience of actually doing it!

 

[Schoolmarm]

"Some of the random rubble Cotswold stone on my house needs defrassing "



Are you sure it should even be exposed? Is there any chance it was rendered originally?

 

[MatthewC]

"Some of the random rubble Cotswold stone on my house needs defrassing "

Are you sure it should even be exposed? Is there any chance it was rendered originally?

I think in places it only had a very thin coat of something containing lime - possibly about one-eighth of an inch. Even so, the loose stone would need to be removed before re-rendering, wouldn't it?

 

[Schoolmarm]

Yes but you wouldn't be too worried about future care if it was rendered up.

 

[MatthewC]

Something in me tells me that actually very few stone houses in the Cotswolds were actually rendered. I think they might have slapped a quick coat of thick limewash over some houses but you don't even see that these days. There are whole villages here with unrendered Cotswold stone walls and I certainly have no intention of being the only person in this place who has a rendered house. :shock:

 

[andrew gayton]

I wouldnt advocate a mechanical 'defrass' only careful removal by hand of areas that are likely to fail over the coming winter and/or that may hold water and only then after much thought on each individual section. You should do this work; with the best will in the world, anyone else might feel judged on results and remove far too much and possibly attempt a general 'tidy-up'.

Also, I wouldnt point anything this friable with any grade of NHL. Non hydraulic lime for 99.9% of works as far as I am concerned.

Limewash is a fantastic consolidant.

 

[MatthewC]

My method of the last few days is to brush the stone face with my stiff churn brush, and to use the back of the brush to tap pieces that might be loose (it's usually quite obvious). If that causes it to come off then it must have been loose already.

I'm using a 1:3 mix of lime putty with some local sand which I bought for the purpose.