Mud Hutters

Standardized plaster erosion tests

2012-01-29T10:13:00.000-08:00

In the last few days, we carried out some standardized plaster erosion tests with the help of Chris from SunBuild. The tests are based on those used by the New Zealand and Australian Earth Building Code (the only difference being we could only attain 40kPa water pressure instead of 50kPa, and used a shower head as close as possible to that specified, see below). Interesting is the effect of lime - 5% resulted in a harder plaster than 10%, i.e. too much makes the plaster soft. These plaster panels are about 4 weeks old and will continue to harden/carbonate for up to 2 years depending on moisture. The effect of linseed amazed us - made the difference between total erosion of plaster and none on one panel after 60min of spraying!! In all cases the erosion rate was <1mm/min, which according to the Code is 'Satisfactory'. In linseed coated panels there was no erosion - very satisfactory..Page down for a demo and see graphs at end for results.

Earth Lime Plaster tests

2012-01-14T07:57:00.000-08:00

While the roofing structure is being put up by Chris's team, I've been carrying out plaster tests using:

Clay-sand subsoil with about 20% clay (a very sandy clay)
Builders sand (local Phillipi sand of ca. 0.2-1.0mm grain size)
Lime (hydrated lime powder for plastering)*
Newspaper pulp (fibre for strength)
Our own black subsoil (for colour)
Oxides (for colour)

*Lime plastering was considered an artisan trade before cement became used conventionally. It's tricky - too much or too little negates its benefits, its success depends on proper hydration, air moisture while drying and temperature. Indeed I've seen few examples of good lime plastering on modern earth houses. The best lime source for plastering is lime putty. This is not available in SA. Our choices are hydrated lime powder or quicklime powder (must be hydrated on site with much heat production and splattering of the highly alkaline liquid). Local knowledge says the quicklime option does not result in a better plaster than the hydrated lime and only adds to the hazards and inconvenience on site.

The plaster will have two coats, the first one is a 'scratch coat' which should have high enough clay to stick to the cob, as well as some lime to key into the lime in the finish coat. Fibre from straw, manure, newspaper, etc can be added for flexibility and strength. This is less important on cob walls that already have straw sticking out of them. It is not necessary to add lime, but it does make for a stronger plaster if added in the right amount, ususally 5-10%. Lime plaster re-carbonates over time as the CO2 from the atmosphere binds with the calcium in the hydrated lime, essentially returning to its natural state before being mined, i.e limestone! We decided on a lime plaster because the concept of re-carbonation to limestone is rather pleasing in itself and besides, lime plaster affords similar weather protection to a cement plaster without being, well, cement. Also lime plaster remains breathable. Cob houses must be breathable or trapped damp will cause the walls to deteriorate. And cement does not bind well to earth walls. Many old adobe farmhouses in SA have had cement plasters added later, which crack and fall off. The photo below is an example of an 100 year old farmhouse in N Cape with adobe brick and cracked cement plaster, both are clearely old. However, cement plasters can fall off even after a few years.

The tests
We made up plaster mixtures, with varying amounts of clay, sand, lime and newspaper (and coloured subsoil/oxides for the finish coat). Squares of plaster were placed on the cob walls and subjected to various tests:

Eyeballing for cracks (too much clay/lime), dusting (too much sand)
Scratching with hand and metal nail to compare strength
Tapping with metal rod - the higher the 'ring' the more dense/strong the plaster. Note strength is not everything and some high density plasters were cracked
Erosion test: this is based on a New Zealand standardized test for natural plasters. A shower head is attached to a pressure gauge and a stream of water at 1bar pressure is aimed at the plaster for 30min. A certain range of erosion (measured in mm) is acceptable, also plasters can be compared.
Balls of each mixture were also made, and crushed under pressure to compare strength.

Base Coat/Scratch Coat - 0.5-1.0cm or thicker where needed
The base coat is applied to even out the surface and we decided on a mix of 55% clay subsoil, 35% sand, 5% lime, after trying a range of mixtures. The newspaper pulp made the plaster smooth and nice to work with but was not worth the extra effort in terms of improved strength. And it did bugger up my liquidizer! The plaster looks rather white as the realtively small lime particles get wicked to the surface when the layer is polished. The plaster is actually rather earth coloured. This test was not scratched but normally would be so that the next layer can 'key' in.

Finish Coat - 0.3-0.6cm
The finish coat should have two characteristics: weather durability and aesthetic appeal. The lime provides the durability; and oxides or clay slips/subsoils provide colour. Personally we are tired of the reds (even pinks once lime is added) and yellows of many earth coloured plasters, and fancied a stone hue similar to the surrounding Table Mountain Sandstone in the hills. The panel below shows tests of panels with clay and sand as for the scratch coat but with (from right to left)

A: black oxide, 5% lime
B: brown oxide, 5% lime
C: brown/yellow oxide, 5% lime
D: black subsoil, 5% lime
E to H are the same as A-D except with 10% lime

Quite like A at this stage (top right)...but have time to decide or do more tests since the base coat is going on first and will take 2 weeks or so. We have a great team of plasterers - pictured below is Richard from Malawi. I will try my hand (again) at plastering under the instruction of Richard. Apparently people naturally separate into plaster 'mixers' and 'plasterers' and I suspect I fall into the former category. Maybe Andy will fall into the latter..

Ringbeam up close..

2011-11-22T04:19:00.001-08:00

Here's some more ringbeam pics...ooooh very exciting. Well it is if it has taken 2 years to get this far!

Ring beam started...

2011-11-08T06:04:00.000-08:00

My friend from ancient PE days, Chris Herselmann (Sunbuild cc.) and his excellent team are helping us in the final stages by putting up the formwork for the ringbeam and doing the carpentry for the roof. The ringbeam ties the building together and contains the outward forces that the roof would have on the walls. The ringbeam can be anything from wood to reinforced concrete. We are going for the latter which was designed by a structural engineer. Iron frames go into the concrete and iron hoops extend out of the ringbeam to attach to the roof. Where we have wooden poles (which the council insisted we have to support the roof) we have attached thick iron posts to tie the poles into the ringbeam. The guys have had an interesting time designing formwork for the rounded (but not exactly round or exactly anything!) walls. It has taken some headscratching and then some very neat carpentary. See wooden formwork on top of the walls. Meanwhile the cobbing continues but we only have a few cubic m to go!

Loft floor finished

2011-09-22T04:27:00.000-07:00

Here are the loft floor beams up as well as a few pics of the start of the small side room roof, which will be a living roof with an angle of 5 degrees (pitch 1:12). The succulents that folk donated /exchanged last December are growing so well that many of them had to be transplanted into the garden, from which I am taking new cuttings again. The one's in bigger pots are all doing very well. Thanks again.

Loft floor going up & poor mans stained glass

2011-07-11T04:51:00.000-07:00

As you can see the walls are nearly up. We are putting up the loft floor (gorgeous thick beams), and are then weeks away from the ring beam and roof. We have had lots of visitors lately, who have inspired us (by being suitably impressed) in these final stages. Thanks all.

Here are a few pics of the glass bottles I used to put in the walls as a kind of poor mans stained glass. The bottles are put top to top, taped, and then wrapped in thick aluminium flashing, which is also taped up into a cylinder, long enough to span the 500mm wall. If you do this take the added few mm that will be added on by plastering, i.e. I made the cylinders 540mm. Also if you are putting the cylinders in anywhere on a curve in the wall, it will end up being effectively thicker than 500mm, so best to custom make these as you go along. Yes yes, I made up 20 of these bottle cylinders and then had to untape some to make them longer.

I'm a bad blogger but we have been workin!!

2011-02-23T06:38:00.000-08:00

Dear faithful followers. Communication is not my greatest forte, but even I was shocked when a friend said he looked at the blog and it was last updated in May 2010. Well, we're still here, still building inbetween full-time jobs and yes slowly getting to the top of the walls. Michiel, a colleague from Netherlands helped us one day in December, and was a regular powerhouse. Thanks Michiel!

Spring 2010: Building starts again

2010-05-04T06:15:00.000-07:00

Check out 'Natural buildings globally':

Hi All

We're building in windows and doors and getting closer to floor level one. Meanwhile, since we've had a number of potential cobbites asking us questions I thought I'd give you a few vital costs and indices:

Should you use cob?: There are other options - strawbale, compressed earth brick, adobe bricks, rammed earth...bricks do allow you to have hired help work unsupervised making bricks while cobbing, strawbale and rammed earth require supervision at least
Cost/m2: about R1500 (this is an estimate as we have not finished yet), see www.tierraprojects.co.za/publications.php for comparisons of cob with other natural methods
Time: about 3.33days per volume (m3) (with 500mm walls and 2-3 people)
Costs of clay: this should cost nothing - get Ross Demolition or other earthmovers to dump clay containing earth on your site - carry a water bottle around with you so you can leap from your car and do a quick soil test
Cost of river sand (if needed): you can use your own subsoil if it is sandy but it should have a wide range of particle sizes for good cob and should ideally not be old sand dunes with high salt contents. River sand is about R300/m3
Cost of straw: depends on the harvest that year as well as the time of year - the longer after harvest in summer (if wheat) the more expensive. We bought all 200 bales at R5/bale from a farmer in Stellenbosch area and paid about R500 for trailer hire = R1500 for a 200m2 house
Labour: Cobbing with 2 is possible if you can build full time but if you only have the weekends then (if you don't want to loose faith) you need help. The going rate is R130-R150 per person. There are several skilled labourers in our area now and you can contact me for their details.
Sanitation options?: Consider a composting toilet (see http://www.humanurehandbook.com/) or urine diversion system (see http://www.southafrica.info/about/sustainable/toilets.htm). Although we built our own composting toilet for our small cabin that we live in on site while we cob, the toilet in the main house will be a urine-diversion system because it is south facing (a design blunder based on the thought that we would use the so-called EnviroLoo, later rejected) and thus may not receive enough solar radiation for composting. Also the urine-diversion loo is less maintenance and requires less space. The urine can be used diluted on plants or drain into our french drain while the dried faeces are sterile and can be used in a compost or directly as a soil conditioner. Considering one does not want to fool with faecal coliforms and wonder 'is it sterile or isn't it?' the urine diversion offers better security around sanitation than the composting option - simply because there could be that one batch that did not get to high enough temperatures when composting ...and diarrhoea/cholera are not pleasant. Either way, best to keep your human waste away from vegetable gardens.

Atlas Plastics make the urine-diversion pedastal and you will have to build the underlying chamber.

Summer progress!

2010-02-16T05:55:00.000-08:00

Hi again...our camera broke in early summer, hence the lack of photos and updates. However we have been busy(ish) and are up above the windows with first lintels installed 2 weeks ago. Still need to replace camera so will have to put up with cell phone pics for a while.

We are now building 2-4 times per week. Please drop in if you like, we should be building on most weekends, but just call me on 0849515535 to check and we'll give you an informal intro to cobbing. Besides that we are going to hold a course in April (see below).

NEWS! We'll be holding a workshop in April and will deal with general design, cobbing in practice, inserting windows and doors, and building a compost loo.

Winter progress...

2009-08-30T04:29:00.000-07:00

Despite the usual Winter torrential rains in Cape Town, we have managed to make some progress, but spent a heck of lot of time wrapping up the big mud pie. However significant milestones were:

Our curtain drain (0.5m trench behind house, sloping downwards, slotted pipe (wrapped in bidum membrane) goes in and then filled with building stone and covered with landscape membrane). I put this in to prevent any moisture build up around foundation and wall since back garden is higher than house floor level.

Our first windows!! Note batons on side of window with nails in - to key into cob. Otherwise pretty much like setting a window in a 'normal' house.

Built in shelves & niches - at last, the fun stuff starts...can't wait to do the lounge built-in bench..

2009-07-06T04:43:00.000-07:00

Cob inaugeration
THANK YOU all who came and made the day! Also thank you for bringing talismen for the wall. We will make a collage of the pictures taken and hang it on the wall where the talismen are. Thanks also of course to the weather gods who eventually allowed us a fab day in the middle of winter, and on the weekend nogal.

Ok, yes so this was supposed to happen when we laid the first cob. However, it was great to have a party in a lounge that one could almost imagine as a lounge, just minus windows and roofing and a few meters of wall - really just minor stuff.
xHeidi

Winter sets in...

2009-06-24T08:34:00.000-07:00

Hey faithful followers
Our house has spent much time since May wrapped up like a christmas present. Alas the combination of the seven-day weather cycles lately (carefully timed to rain on weekends) and our prime building time being weekends! So far we are up to 1m all around the house and starting to feel like a building.

We have several other things we're doing in the rainy weather: designing niches and bottles around windows (as little 'light bricks') as well as sealing windows we bought. Andy bought several beautiful sash windows from Ross Demolition (ie seconds) and I bought a couple of new ones to fill in where needed. Also we have our local engineer Paul Giloi designing an exposed-truss roof for us so that we can maybe get that up and even build a bit during the Cape storms. It would also protect the walls a bit and involve less plastic wrapping. Its our last major expense tho and not sure we are up to it just yet...

Ready for windows and built in furniture

2009-04-10T08:11:00.000-07:00

Lounge wall so far..we're now ready for inserting several windows in the lounge and bathroom. Note the armitures for connecting cob built-in furniture to the wall, and also that Andy is spading up wall-trimmings to be re-used for another cob mix - the beauty of cob! Imagine doing this with concrete.

On the right - kitty stamp of approval..thanks Drooler.

2009-01-29T02:30:00.000-08:00

COB WORKPARTIES AND WORKSHOPS
CALL US FOR THE LATEST DATES

BOOKINGS 0849515535

Our lounge so far

Andy shows level of roof for our 'hidden cave' bath room

(yes, mum that's right, the bath is off the lounge)

Melissa and Amy stompin!

Our mountain friends Troy and Fi

In the gathering race towards greeness, pause and visit us (Mud Hutters). We are cobbing a house in Welcome Glen, Cape Town and are open for:

Cobbing Workparties
Open-structured learning about cobbing on selected weekends 9am-5pm. R50 for organic refreshments or bring your own. Call: Heidi 0849515535 or Andy 0768651889

Cobbing Workshops
Structured course learning about cobbing with theme-based modules (Council permission, foundations, soil tests, cobbing 101, alternative cob mixing methods, adobe brick making, planning utilities, costs, natural plastering, sculpting with cob, green roofs). Dates and cost to be announced. Call: Heidi 0849515535 or Andy 0768651889

Cob oven building
One weekend later and you can be the proud owner of a cob oven. Call us if you would like to build your own cob oven. No previous experience necessary. We will come over and facilitate as well as muck in. Dates and costs upon request. Call: Heidi 0849515535 or Andy 0768651889