[GSBN] Lightweight concrete roofs

[Derek Roff]

Thanks, Mark,

Your further description is extremely helpful to me.  I have clarifying 
comment and question.  When I said I thought water catchment would be 
easier on the gable-like roof, it was because most of the water would 
drain at four corners, the four low points of the roof.  The center 
peak puts drainage evenly around the entire perimeter.  Easily handled 
with gutters, of course.  How did you approach water catchment?  Do you 
see a water catchment advantage to either shape?

And one typo:  I think you were loading the roof to 80 pounds per 
square FOOT, rather than square INCH.

Thanks again,

Derek

--On Tuesday, June 22, 2010 6:23 PM -0600 MARK SCHUENEMAN 
<huffnpuff at q.com> wrote:

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