[GSBN] Lightweight concrete roofs

Wed Jun 23 00:23:07 UTC 2010

Derek and anyone interested in these light Acrylic/Cement roofs,

The last paragraph in this is how George describes the Nez mix design. We started with a high slump mix (watered down mix) for the initial coating, literally, so, as you suggested, the mix will penetrate the meshing thoroughly, top and bottom, and painted the mix right over our fabric where it was attached to the framing. 

Mixing was accomplished with a paddle mixer and drill in a bucket (not mixing too much so as to infuse air bubbles, which can happen and weaken the solution). We let the mixed material slake (sit still after initial mixing and absorb) for a few minutes and just before applying the mix, we'd whip it up quickly one more time, then apply. For the initial parging we also used a smaller grit sand...as in silica. Using a larger sand is possible but you will end up with a lot of 'balls' of sand that wouldn't penetrate the weave of the fabric and, in fact, would block the mix from penetrating...and, become problematic for future coatings.  At times this happened with the silica coat and we'd paint some material on the bottom side using a paint brush. You may still get some of this balling with the silica and I would suggest maybe trying a 1-2 or 1-1 cement to silica mix if you're having troubles. We did use masonry sand in subsequent coats. 

Application involved one worker throwing the mixed material from the bucket onto the netting while the other spread it around as evenly as possible using a stiffer bristled broom. We worked in one direction to eliminate excessive cold joints (placing a wet material on a hardened/dried edge). We used a broom with a sturdy long extension handle for all applications. Throwing parts of the bucket at a time seemed to work best since the working time once the material left the bucket was short. Working with a wetter material is the only way to go (spoken like a true concrete flatworker). Working in the cooler hours of the day helps the parging efforts with an even spread. There probably is a better method for application so try something else...

Curing sufficiently between coats is important (it must be hard to the touch or cracking will occur if the mix isn't allowed to firm...maybe overnight in high temps and sun, longer if necessary in cooler conditions). I think you would say the mix failed if there is any cracking. Either you used too little latex or you applied a subsequent coating too soon. The beauty of this method is, if done properly it is elastic enough to take a beating and still keep it's waterproofing properties. 

On subsequent coats we adjusted the mix somewhat; first because the cement and acrylic were the more expensive parts of the mix (budget) and second we intended to build up the thickness. So the second coat we increased the ratio of water to acrylic to 2-1 respectively and increased the sand size to a larger, masonry type sand. I don't recall ever putting on more than 3-4 coats. It probably wouldn't hurt anything if you left the mix of water/acrylic to 1-1. I recall going to a 3-1 ratio because we were going to run out of acrylic at the end of the day and it seemed to work OK. We never really had a goal of of 1/4"-3/8" it is just about what we ended up with for thickness after applying 3-4 coats.

Clean tools and self often. It helps to have a tray of water to rinse the broom in between coats, otherwise the broom just builds up too much material and snaps the handle or the back of the guy pushing it. Clean tools are happy tools!

We attempt to parge the roof before it is raised and still close to the ground. This made the bucket application and broom spreading technique easier, as you might imagine. It can be parged once raised into position but it's a trick for a multitude of reasons. At least parge the first layer. Once that is complete it would be easier to do subsequent pargings while standing on the roof parged netting. We have applied the initial coating with a drywall texture tool with a grand openning and lots of pressure but it still plugged a lot and mask and goggles were a must...plus, there's not always electricity much less compressor and sprayers everywhere. 

As for the structure shape...the http://greenbuildingelements.com web site you gave was George standing in the picture...grey haired fellow. I would defer to George for any structural elements to your question. Mr. Nez has all kinds of shapes for his roofs using the hyperbolic shapes. We were able to catch water just fine with the peak at the center. 

Testing was performed on an in place roof, peak in the center. George and Al Knott had every human around standing in the roof until no more would fit and it survived. They also placed bags of cement on the roof, several high and no failures. They never got the roof to fail and tested it to 80psi before calling it good (which is 2x the 40psi for our local codes and snow load climate).

Insulation can be accomplished using your imagination and keeping in mind building science. This roofing system forms a water impermeable shell.

Nez Mix Design: "The mix design for acrylic concrete is one part Portland cement to
two three parts sand, then add acrylic concentrate and water (at a one to one
ratio by volume) until you get a workable mix. This is a craftsman’s specification.
Adding excess water will make a weaker mix, but such a mix may be sufficient if
you are running low on acrylic. Excessive water in the sand will also produce a
weaker mix. If you must use weaker mixes, run tests on the finished product as
described later in this paper to prove the adequacy of your membrane structure.
When possible it is best to use 1 part Portland cement, 3 parts sand and only latex
concentrate cut one to one added until you get the right consistency for troweling."
I might add that there was no troweling involved and maybe George should have used application instead of troweling.

Hope this helps!
Mark Schueneman
303-444-6027 hm.
303-591-9841 cell

> Date: Tue, 22 Jun 2010 12:31:53 -0600
> From: derek at unm.edu
> To: GSBN at greenbuilder.com
> Subject: Re: [GSBN] Lightweight concrete roofs
> 
> Hi, Mark,
> 
> Thanks for your information. I would love to hear you say more about 
> this part of your posting:
> 
> "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..."
> 
> That "as seems effective" instruction sounds like it could require a 
> lot of experimentation to get right.
> 
> I would also like your opinion on inverting the hyperbolic paraboloid 
> roof elements, as shown on the Green Building Elements website. 
> <http://greenbuildingelements.com/2010/06/14/new-habitat-approaches-build-roofs-first/>. 
> The examples in Albert and George's book have the peak at the center of 
> the roof, while the above site has flat ridgelines, which give the roof 
> a look similar to two intersecting gable roofs. I can imagine 
> advantages to each approach, but I think the latter look would be 
> acceptable to more people in more cultures, and it makes water 
> catchment much simpler.
> 
> Thanks,
> 
> Derek
> 
> 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
> 
> 
> --On Monday, June 21, 2010 8:47 AM -0600 MARK SCHUENEMAN 
> <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 or call me with your thoughts or inquires.
> 
> Regards,
> Mark Schueneman
> 303-444-6027 hm.
> 303-591-9841 cell
> 
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