Covering over pebbledash over cob?

[Doric]

Have been trawling previous posts on this forum, but no-one's had our specific dilemma.

We've bought a late C19th farm cottage in Wiltshire. The thick, original cob walls have been more recently pebbledashed with a cement render and then painted...pink. I understand that a) cob needs to breathe, and b) that removing the pebbledash would risk damage to the cob. We're looking at a suitable cover material which is smooth and can be painted.

If the existing cement-based pebbledash is already sealing in the cob, will it make it any worse to put a smooth cement render over the top? Or can anyone suggest a better idea? :?:

 

[Nemesis]

The best plan in the long term would be removal of cement altogether - very, very carefully, bottom up, a bit at a time, and render in lime.

You could cover in cement - won't make any difference as there is already cement there - but it will crack when the building moves and trap too much moisture behind it, which will not be able to dry out.

Not a great idea! But proceed with great caution.

 

[andrew gayton]

take it off; do not cover the existing p.d as this will impede permeability further and load the surface of the cob.............. personally, i would adhere to gravity and remove p.d from the top down.

 

[Nemesis]

Interesting - the idea of bottom up was supposed to be to allow the water to drain down, but maybe top down will make it more stable do you think?

Certainly I'd remove it, not put more on - not sure reading what I posted I made that clear!

 

[MdB]

First job is to make a small hole through the render at the base of the cob to see what happens. Mike Wye reports doing this and then getting a complete flood of water coming out. If this is the case, proceed very very carefully as it is possible that the render is all that is holding the wall up.

See the other thread on a cob wall falling down after render had been removed. Mike Wye also reports that he has had one fall down on him after removing render as well.

 

[andrew gayton]

if the walling collapsed after removing the render, then i would suggest that it was about to collapse anyway..................

 

[Doric]

Thanks for those thoughts. The thing is, the pebbledash is absolutely rock hard and shows no sign of falling off, coming off, or being easy to budge. I'd sooner not run the risk of pulling the house down, when it's built like Fort Knox. While the removal and re-liming might be the pure solution, is there a lower-risk alternative involving a new cover of something?

 

[MdB]

The thing is, the pebbledash is absolutely rock hard...

That is exactly the problem with it.

The cob doesn't have this strength and will move with time due to changes in moisture levels. The pebbledash is solid and won't be able to allow for this movement and is therefore likely to crack. Once cracked the water can get in but not back out again and the moisture will slowly distroy the cob. Lime on the other hand will move with the cob to a certain extent and more importantly will let the moisture out once it gets in.

You will also probably find that the cement is not very well stuck to the cob and will probably come off in sheets. Tap it. Does it sound hollow?

 

[Gervase]

I would echo the advice to remove the cement but if you are determined to keep it, limewash will adhere to pebble dash. It won't adhere to modern paints, however, so the pink paint would need to be removed with a stripper or sandblaster first.
Limewash would be the coating of choice only because it is the 'least bad' option, however; it would allow water vapour to leave any cracks that are there while deterring direct saturation. The best approach is to remove the cement.
I would be tempted to cut a shallow core through the pebble dash to see how it interfaces with the cob. Some cement renders are applied over chickenwire nailed to the substrate to give them rigidity, while others are simply put on two or even three layers thick, with each layer scratched for adhesion. Underneath, however, you often find that old limewash and traces of original render form a sacrificail layer which allows new render to be removed without wrecking the cob.
If in doubt, bring in a specialist. If nothing else, they will have insurance!

 

[NT]

Cement coatings cause the cob to get gradually wetter and wetter until it loses stuctural strength, leaving nothing but the render holding the house up. Render cracks and down it all comes.

Removing the cement render really is the only safe option. However there are risks involved, and if not expert in this you'll want to involve someone who is to test the cob and advise on the procedure of the work. Not doing it causes risk of collapse, doing it brings a slight risk but less than not doing it.

Lime render is the suitable coating. It wont need painting unless you want it coloured.


NT

 

[BMcD]

I'd advocate taking the cement render off but in un-adjoining areas of a few meters sq. and letting it dry out if necessary then spatterdashing and scratch coating with lime render allowing this to set before moving on to the next section so you gradually remove the cement render whilst building up a scratch coat of lime then when all the cement has gone you can apply a float coat of lime render or the finish of your choice.
But if in doubt seek expert advice before hand