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Rendering cob

Posted by Jacksdaddy - Created: 4 years ago
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7 replies (Showing replies: 1 to 7)

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Posted by dagbaruch - 4 years ago

We built a new house 4 years ago using SIporex (beton cellulaire), and have rendered the outside.  At the time I searched for info, and most of it said special render must be used, which was very expensive.  We decided to used white chalk, white cement, and white/rose tinted sand from a local quarry.  We had done some testing on our old property some years before, with excellent results.

Our new house has had no problems with the render; it is as sound as when it was new.

When applying the mix, we used a friends compressor and equipment.  We soaked the walls with a hosepipe, until the walls were really soaked, applied the base coat, and trowelled smooth.  Some hours later, soaked the walls again, and sprayed a rough coat all over and that was it.

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Posted by Jacksdaddy - 4 years ago

Yasminh14, thank you very much for taken the time to post the very helpful advice. It looks like you have saved my wife and I a lot of time, money and hard work as I did think it would be necessary to use chicken wire under the render and also thought that the render would need protecting with an exterior coating!  I have never used Anglo Info before to get advice and it reminds me of one of the reasons why we love loving in France, lovely like minded and helpful people. THANK YOU TO YOU AND ALL WHO HAVE HELPED SO FAR. 

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Posted by Yasminh14 - 4 years ago

There would be no issue with damp if you use lime render (known here as chaux and easily available in all DIY places such as Bricodepot etc for the simple reason that old cob houses commonly found here are always rendered in this, and stone houses need to be repointed with it, so people need it), and I doubt that you would need to seek permission to do it as it is a necessary measure that needs to be taken to prevent the house from falling down!   You may at most need a déclaration préalable (a declaration of works), but not full planning permission.  Lime is a breathable material and therefore the only thing suitable for stone/cob houses.  You won't need to paint it, the colour will vary from pale cream to yellow depending on the ratio of sand and colour of sand that you use.  If you do want a colour, then you can buy tinted chaux in a range of colours.  But I wouldn't paint it as this may also affect the breathability.

This is our own advice, based on experience and also speaking to several artisans and locals who know what's what:

We also had a couple of walls that were not rendered - as they were at the back of the house and accessed partly via our neighbour's land, we ignored them for a while.  They were never in a great state, but after the bad storms of last winter, they were literally crumbling away - this is just mud after all. So last summer my husband rendered them himself, and they look great and are no longer crumbling.

It is hard work, but not difficult if you are a competent DIYer, and will save a fortune over paying someone.  However, if you want dead smooth, flat walls, then you may have to hire an artisan, and it will probably cost you in the thousands.  We were happier to just follow the contours of the wall for a more organic and antique look, which is more in keeping with the age of the house. 

If you do not render them, then eventually they will just crumble away, but whatever you do, don't do them in cement, as this WILL stop the walls from breathing, and then you will have a problem with damp.  Also, don't as some people will recommend, use chicken wire underneath to create a key or a frame.  This may eventually rust and cause you more problems.  It may also create hollow spaces under your render, and it will fall off more easily.  Unfortunately the English cowboy builder who previously owned our house did render the front in cement and also with chicken wire underneath, and it has caused damp problems in places and sounds hollow when you tap it.  But how on earth do you strip it all off and re-do it in lime render once it's done? Sigh....

Anyway, good luck, hope some of the above helps.


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Posted by Jacksdaddy - 4 years ago

Many thanks Anonymous, wise words.  I have been told a lime based render should be ok. i will also have a chat to the Mairie regarding consent.


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Posted by Anonymous - 4 years ago

As it will substantially alter the look of the house it will be best to ask your question at the Mairie.  Even minor changes can be strictly regulated in certain conservation areas.

I would also do careful research about the damp situation.  Your proposed system will seal damp from entering the house.  Good.  But if you use a non-breathable render on the outside the cob cannot breathe as it should.  There will be no dampcourse so it will be drawing moisture from the ground up.  Moisture + earth mortar = mud.  ie., loss of strength.  Try to find out the best method/material for rendering the cob surface.

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Posted by Jacksdaddy - 4 years ago

Thanks NellP for your prompt response. If I go ahead with this project I will be building internal walls throughout from concrete blockwork with a damproof barrier between these and the Cob. The external walls once rendered will be coated with an exterior coating eg. Sandtex.  The property is in and elevated position so don't think damp would be a problem. Thank you very much for your thoughts.

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Posted by NellP-986196 - 4 years ago

Not sure whether it is a planning consent issue but there might be issues with upsetting the balance of the original construction.  If cob was never rendered or painted originally, there must be a reason.  You might find a damp issue after rendering, or perhaps cob needs moisture.  Rendering might stop it acting as it was designed to.  Google it and see what you can find.