Cob: Re: a question about renders...

Sat Jul 28 12:12:35 CDT 2001

Inger Boerema wrote;
> I am aware that cement renders should not be used on earth constructions, so I have been looking at different alternatives that
> (preferably) do not require constant maintenance. I originally thought that I would use a lime based render, but have been told by some > that it is unsuitable for out door and unprotected walls, and by others that it is achievable. I am writing to you on the hope that you may > have some suggestions in regards to a suitable render, for as I am running out of time, I am anxious to find something that will sufficiently > protect my wall! 

1. Nearly all the cob buildings in South West England that were rendered had lime mortar applied. If the site is exposed and render can freeze in winter, add a little pozzolan to your lime mortar ( in the old days this was volcanic ash or brick dust or earth containing burnt clays - these days you can get a purpose made burnt clay in most countries) Mix 3 parts coarse sharp sand with lime putty, leave for a week if you have the time and patience and then when using remix adding 10 -15% by volume of a pozzolan. You can harl the first coat if your cob is too smooth for a key but otherwise trowel it on.
2. If you're worried about maintenance you can self colour your top coat of lime render with earth pigments -you'll get a shading effect with the change of moisture content in the wall.
3. Or you can use a limewash coloured with an earth pigment - limewash is relatively cheap and long lasting if applied correctly.
4. If your environment is mild and frost free you could leave the cob bare - if the walls are thick ( over 2 feet 6 inches/750mm) its unlikely that severe rain will penetrate. We have many bare cob structures in our relatively wet part of the world and the rate of erosion is very slow. If you're worried about the alkalinity of lime and hence the safety issues if you want the children to be involved/ have access you can use an earth render if you have a reasonable clay content in your subsoil - sieve the dry soil and add chopped straw and water when remixing to a soft consistency. 
Your cob wall needs good protection for the top of the wall to prevent rain penetrating its centre and the cob wall should have been raised off the ground by building on a stone plinth ideally 3 feet/900mm - this will reduce the risk of rising dampness.
hope this helps
Mike

Mike Wye & Associates,
Traditional & Ecological Building Products
www.mikewye.co.uk
01409-281644

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<DIV><FONT face=Arial size=2>Inger Boerema wrote;</FONT></DIV>
<DIV>> I am aware that cement renders should not be used on earth 
constructions, so I have been looking at different alternatives that</DIV>
<DIV>> (preferably) do not require <I>constant</I> maintenance. I originally 
thought that I would use a lime based render, but have been told by some > 
that it is unsuitable for out door and unprotected walls, and by others that it 
is achievable. I am writing to you on the hope that you may > have some 
suggestions in regards to a suitable render, for as I am running out of time, I 
am anxious to find something that will sufficiently > protect my wall! </DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>1. Nearly all the cob buildings in South West 
England that were rendered had lime mortar applied. If the site is exposed 
and render can freeze in winter, add a little pozzolan to your lime mortar 
( in the old days this was volcanic ash or brick dust or earth containing burnt 
clays - these days you can get a purpose made burnt clay in most countries) Mix 
3 parts coarse sharp sand with lime putty, leave for a week if you have the time 
and patience and then when using remix adding 10 -15% by volume of a pozzolan. 
You can harl the first coat if your cob is too smooth for a key but otherwise 
trowel it on.</FONT></DIV>
<DIV><FONT face=Arial size=2>2. If you're worried about maintenance you can self 
colour your top coat of lime render with earth pigments -you'll get a shading 
effect with the change of moisture content in the wall.</FONT></DIV>
<DIV><FONT face=Arial size=2>3. Or you can use a limewash coloured with an earth 
pigment - limewash is relatively cheap and long lasting if applied 
correctly.</FONT></DIV>
<DIV><FONT face=Arial size=2>4. If your environment is mild and frost free you 
could leave the cob bare - if the walls are thick ( over 2 feet 6 inches/750mm) 
its unlikely that severe rain will penetrate. We have many bare cob structures 
in our relatively wet part of the world and the rate of erosion is very slow. If 
you're worried about the alkalinity of lime and hence the safety issues if you 
want the children to be involved/ have access you can use an earth render if you 
have a reasonable clay content in your subsoil - sieve the dry soil and add 
chopped straw and water when remixing to a soft consistency. </FONT></DIV>
<DIV><FONT face=Arial size=2>Your cob wall needs good protection for the 
top of the wall to prevent rain penetrating its centre and the cob wall 
should have been raised off the ground by building on a stone plinth ideally 3 
feet/900mm - this will reduce the risk of rising dampness.</FONT></DIV>
<DIV><FONT face=Arial size=2>hope this helps</FONT></DIV>
<DIV><FONT face=Arial size=2>Mike</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV>Mike Wye & Associates,<BR>Traditional & Ecological Building 
Products<BR><A 
href="http://www.mikewye.co.uk">www.mikewye.co.uk</A><BR>01409-281644<BR></DIV></BODY></HTML>