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The Work of Art and The Art of Work Kiko Denzer on Art |
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Cob: Re: Re: a question about renders...John Hall hallsoi at netvigator.comMon Jul 30 05:44:10 CDT 2001
Mike, What is the purpose of the "pozzolan" and how does it work in protecting the render from frost? I am wanting to re-render a 15th century cob building in Mid-Devon, so this redering topic is particularly pertinent. The original render would have keyed with the straw sticking out of the fresh cob, but after stripping the original render, there may be not much for a new lime render to attach to. How do you ensure it sticks on?! Regards, John Hall. ----- Original Message ----- From: Mike Wye To: Mike Boerema & Margo Kellett ; coblist at deatech.com Sent: Sunday, July 29, 2001 1:12 AM Subject: Cob: Re: a question about renders... 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 -------------- next part -------------- <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; charset=iso-8859-1"> <META content="MSHTML 5.50.4611.1300" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV><FONT face=Arial size=2>Mike,</FONT></DIV> <DIV><FONT face=Arial size=2>What is the purpose of the "pozzolan" and how does it work in protecting the render from frost?</FONT></DIV> <DIV><FONT face=Arial size=2>I am wanting to re-render a 15th century cob building in Mid-Devon, so this redering topic is particularly pertinent.</FONT></DIV> <DIV><FONT face=Arial size=2>The original render would have keyed with the straw sticking out of the fresh cob, but after stripping the original render, there may be not much for a new lime render to attach to. How do you ensure it sticks on?!</FONT></DIV> <DIV><FONT face=Arial size=2>Regards,</FONT></DIV> <DIV><FONT face=Arial size=2>John Hall.</FONT></DIV> <BLOCKQUOTE dir=ltr style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px"> <DIV style="FONT: 10pt arial">----- Original Message ----- </DIV> <DIV style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B> <A title=mike at mikewye.co.uk href="mailto:mike at mikewye.co.uk">Mike Wye</A> </DIV> <DIV style="FONT: 10pt arial"><B>To:</B> <A title=mboerema at bigpond.com href="mailto:mboerema at bigpond.com">Mike Boerema & Margo Kellett</A> ; <A title=coblist at deatech.com href="mailto:coblist at deatech.com">coblist at deatech.com</A> </DIV> <DIV style="FONT: 10pt arial"><B>Sent:</B> Sunday, July 29, 2001 1:12 AM</DIV> <DIV style="FONT: 10pt arial"><B>Subject:</B> Cob: Re: a question about renders...</DIV> <DIV><BR></DIV> <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></BLOCKQUOTE></BODY></HTML>
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