[GSBN] Lightweight concrete roofs

Tue Jun 22 18:01:03 UTC 2010

Hi, Andy,

I think the insulation value of a thin-shell concrete roof would be 
roughly zero.  The R-value 3/8" thick concrete with a little latex 
isn't worth calculating into the insulation equation.  Short of sprayed 
on insulation, I think attaching anything to the underside of the roof 
would be a losing proposition.  I wonder if wet-sprayed cellulose would 
stick reliably.  I'm guessing no, on the "reliably" part.  Otherwise, 
building a ceiling, and insulating above the ceiling, makes most sense 
to me.  Like any impermeable membrane, the underside of this roof will 
be a potential site for condensation.

According to my memory of reading George and Albert's book, they liked 
using fiberglass (window) screen material for a few properties- low 
cost, high strength, fine, but not too fine hole size/spacing, high 
proportion of open area in the mesh, resistance to decay, especially in 
contact with concrete.  If you can duplicate those properties, you 
should have similar success with a different mesh.

The latex increases water resistance, but more critically decreases 
cracking, which, they said, would severely compromise the water barrier 
aspect of a similar roof made without latex.  It's important that the 
final composite have the mesh encased in latex concrete inside and 
outside, and that the latex concrete be an integrated composite, 
connected through the holes in the mesh.  Any tight-weave fabric would 
isolate the concrete into separate inside and outside layers, which 
would risk delamination.

They mentioned that the shape of the roof was critical to its strength 
and stability at small thicknesses.  Domes and vaults might have 
similar properties, but they don't claim this approach is good for flat 
roofs in the thicknesses mentioned.  Their book has quite a few math 
formulas relating to strength and roof shape, for those who like such 
things.

I don't remember if the authors comment on the pH of the latex 
concrete.  I imagine it would need to be similar to that of regular 
concrete.  If the pH drops too far, concrete begins to lose its 
properties.

I've wondered how UV will affect the latex component of the matrix (at 
the surface).  Based on field experience, it doesn't seem to be a 
problem.  Concrete blocks out light pretty thoroughly, and I imagine 
that the 3/16" (~5mm) coating that they recommend on the sun side of 
the mesh would shield out all the UV (total thickness ~3/8" (9.5mm).

I hope this is helpful, but getting a copy of the book would be much 
more enlightening and reliable than my memory of it.

Derek

--On Monday, June 21, 2010 10:45 PM +0200 Andy Horn 
<andy at ecodesignarchitects.co.za> wrote:

> Hi all
>
>
> Anyone have any idea what kind of insulation one could expect out of
> such a roof?
>
> And how might one attach a ceiling for insulation to such a
> structure?
>
>
>
> I am designing a structure at the moment, which is based on a series
> of hyperbolic parabaloids where cost is a major constraint. We are
> using poles with a reed ceiling overlaid with Hessian and then a
> straw paper clay mix followed by an insulative water resistant
> breathable plaster. The premixed top coat product is called
> "Thermosim". It stuck me that the latex cement idea could offer a
> potential alternate cost effective option.
>
>
>
> On another note, I have seen a roof over a twin vault compost toilet
> at an eco village called Thlolego, where some kind of cement slurry
> over a Hessian cover on off-cut boards was used. I am not sure about
> whether or not latex was used in the mix, but it's finish was clearly
> some kind of latex paint. However I noticed the last time I visited a
> couple of years ago that the Hessian had started to rot with a dark
> mould having developed and a green kind of algae/moss growing through
> it. The structure was about 12 years old. I don't have an
> understanding of the effect of latex on cement but in the long run I
> am wondering if natural fabrics are such a good idea with cement for
> roofs - due to differences in the PH and the moisture contents of
> such materials.
>
>
>
> Any one have an  idea of what kind of idea moisture content or what
> PH a latex cement would have?
>
>
>
> I also wonder what the effects of varying levels of humidity would
> have on such structures? The Thlolelgo project is even situated in a
> pretty dry part of the world.
>
>
>
> I would think a geofabric (we call it Bidim) would also make a good
> and cost effective alternative to fibreglass. There is also a product
> that I use for building sand bag structures, called "Spunbond" which
> is a woven polyester and is even more cost effective, which would
> probably be even better. We get it locally made here, but I know that
> one supplier get's their supply from Germany. While not UV stable the
> fabric does not degrade in the presence of high alkalinity products
> like cement.
>
>
>
> On this note however, any idea how thick a coating of latex cement
> would one need over the fabric to protect it from UV degradation?
>
>
>
> How old are the examples of these latex cement roofs?
>
>
>
> Thanks
>
>
>
> Andy Horn
>
>
>
>  [Image: ]
>
>
>
>
> -----Original Message-----
> From: GSBN-bounces at greenbuilder.com
> [mailto:GSBN-bounces at greenbuilder.com] On Behalf Of Jim Carfrae
> Sent: 21 June 2010 06:29 PM
> To: (private, with public archives) Global Straw Building Network
> Subject: Re: [GSBN] Lightweight concrete roofs
>
>
>
> This is an aside on the topic of lightweight concrete roofs - My
> grandfather, Jim Waller, was a civil engineer and innovator in the
> use of concrete (he built the first concrete boats for the admiralty
> during the 2nd world war). The reason why I mention him is that in
> the 1950s he developed a building system that has similarities with
> the Nez roof. His system was called Ctesiphon after the ancient arch
> in Iraq, and basically involved building a temporary structure based
> on a series of inverted catenary arches as ribs and covering them
> with an envelope of Hessian fixed to the ribs. The hessian was
> covered with thin coats of a cement slurry (no latex in his day) This
> was designed to sag between the ribs causing corrugations which gave
> the shell its structural integrity. The trick was that the arched
> forms could now be removed and reused, leaving a self supporting
> lightweight concrete shell.
>
> The system was used for grain silos in Egypt, emergency housing in
> India and a whiskey storage shed in Ireland among other projects, but
> never took off in a big way and has remained an interesting byway. As
> soon as I followed the link to the Nez roof I was reminded of my
> grandfathers work - it seems they would have had a lot in common.
>
>
>
> Thanks
>
> Jim
>
>
>
> Jim Carfrae
>
> PhD Research Student
>
>
>
> Room 119, Reynolds Building
>
> University of Plymouth
>
> Drake Circus
>
> Plymouth
>
> PL4 8AA
>
>
>
> jim.carfrae at plymouth.ac.uk
>
> 07880 551922
>
> 01803 862369
>
> ________________________________________
>
> From: GSBN-bounces at greenbuilder.com [GSBN-bounces at greenbuilder.com]
> On Behalf Of Paul Olivier [paul.olivier at esrint.com]
>
> Sent: 21 June 2010 16:51
>
> To: (private, with public archives) Global Straw Building Network
>
> Subject: Re: [GSBN] Lightweight concrete roofs
>
>
>
> Mark,
>
>
>
> Please forgive me for asking dumb questions about something you have
> written:
>
> It sounds as though there is plenty of resources for the mix so I
> won't address that too much now...it's not rocket science. You
> basically use the latex as you would water for a cement mix with
> sand. You can use as little latex mixed with water as seems
> effective...more latex (50/50) to water for the initial coat. I can
> attest that the system works well and can be low cost. It is easy to
> learn and quick to build.
>
> I am keenly interested in use of latex with cement,
>
> and I would like to find a good summary of the steps involved.
>
> We will soon start building outhouses in a ferro-cement over
> urine-diverting toilets,
>
> and it is critical that moisture does not penetrate the structure
> over time.
>
> This effort is summarized at: http://www.esrla.com/pdf/toilet.pdf
>
> Today we are putting up the first ferro-cement structure as depicted
> in this presentation.
>
> We must get it right, and your advice would be greatly appreciated.
>
>
>
> Thanks.
>
> Paul
>
>
>
>
>
> On Mon, Jun 21, 2010 at 9:47 PM, MARK SCHUENEMAN
> <huffnpuff at q.com<mailto:huffnpuff at q.com>> wrote:
>
> Greetings All interested parties,
>
>
>
> The 'Nez' Roof system has certainly progressed beyond the
> experimental stages. The are many roofs around the world thanks to
> George and Al Knott. The reason George didn't return your email is
> probably he just returned from training folks his roof system in
> Uganda where they did a roof for a performing arts center. I know
> there's Nez roofs in Afghanistan, Kenya, Rwanda as well as many other
> working demo structures in the Denver area and the mid west US. He
> has trained his roof system to many folks , including myself, and
> we've built the system many times around the world. His model of
> 'roof first' makes sense. He was working with a minister in
> Afghanistan after their earthquake when all they had was tarps. Snows
> would collapse the tarps when they could have built his roof with
> mostly salvaged materials and used the tarps and more salvaged
> materials for walls. Recently he's been in conversations with some
> folks in Haiti, so he could get you in touch with them.
>
>
>
> George is a wonderful man with a vision and plan and energy abundant
> for a Ninety year young fellow. Mr. Nez started doing disaster relief
> work for the federal Government in Central America years ago. He has
> always been more than happy to share his knowledge with anyone
> interested. As a matter of fact, you may want to make a pot of tea
> for a conversation with George. He is a most knowledgeable resource
> when it comes to disaster relief. If you have a project going
> somewhere I'd guess George is a suitcase packing away from joining
> you for a demo of his roof system, as would I, or to discus your
> project.
>
>
>
> His system is more about the shape of the structure and the latex
> coating but the 'fabric' for the roof has been an evolution. He used
> to weave a fiberglass netting for the latex parging but has worked
> with and tested regular old cotton fabric (or even tarps) with
> chicken mesh. These materials seem more readily available in most
> parts of the world and cheaper. The frames can be made of salvaged
> materials and spliced effectively. The latex for the concrete parging
> can be old left over (or oops) paints, if you can live with the
> color. It sounds as though there is plenty of resources for the mix
> so I won't address that too much now...it's not rocket science. You
> basically use the latex as you would water for a cement mix with
> sand. You can use as little latex mixed with water as seems
> effective...more latex (50/50) to water for the initial coat. I can
> attest that the system works well and can be low cost. It is easy to
> learn and quick to build.
>
>
>
> The latex parging seems very durable, flexible (elastic), waterproof
> and enduring. The material itself is not that difficult to work with
> other than that it doesn't lend itself to a great esthetic. My guess
> is that if you're homeless, cold or hot and wet, you could care less
> about esthetics. Once you apply the parging with a few strokes of a
> brush or trowel further strokings only drag, ball and/or tear. I got
> some on my vehicle and missed cleaning it until the next day and when
> I removed it some paint came with it...it's sticky. Clean your tools
> soon and often.
>
>
>
> If interested please feel free to write myself or George
> geonez at peoplepc.com<mailto:geonez at peoplepc.com> or call me with your
> thoughts or inquires.
>
>
>
> Regards,
>
> Mark Schueneman
>
> 303-444-6027 hm.
>
> 303-591-9841 cell
>
>
>
>
>
>
>
>
>
>> From: bob at bobtheis.net<mailto:bob at bobtheis.net>
>
>> Date: Sat, 19 Jun 2010 22:57:55 -0700
>
>> To: GSBN at greenbuilder.com<mailto:GSBN at greenbuilder.com>
>
>> Subject: Re: [GSBN] Lightweight concrete roofs
>
>>
>
>> We've been considering this technique for work in Haiti, but I never
>
>> got a reply to my email to George Nez, asking, among other things,
>
>> whether the technique has progressed beyond the experimental roof at
>
>> the University of Colorado.
>
>>
>
>> Does anyone know of further work with it? I'm especially wondering
>> if
>
>> someone has tried making corrugated or folded plate panels.
>
>>
>
>> Bob Theis
>
>>
>
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>
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>
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>
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>
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>
>
>
>
> --
>
> Paul A. Olivier PhD
>
> 27C Pham Hong Thai Street
>
> Dalat
>
> Vietnam
>
>
>
> Louisiana telephone: 1-337-447-4124 (rings Vietnam)
>
> Mobile: 063 399 7256 (in Vietnam)
>
> Skype address: Xpolivier
>
> http://www.esrla.com/
>
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Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu