How to Test and Make Exterior Render using Cow Poo

The Quick Answer…

Find a good ratio of cow poo, sand and clay. We ended up using 1 earth, 1 poo, 2 sand, water and add linseed oil at 4% of total volume to increase water resistence. Remember that the linseed oil will make your mix a lot wetter so maybe better to add that first and the water second. The linseed will also change the colour a lot. It changed ours from a mid-brown to a nice beige gold colour.

There are many other things you can add to increase the water resistence such as cactus juice, PVA glue, wheat paste and milk protein however we did not test these so I can only speak for Linseed Oil and cow poo here.

Also worth noting that our earth is amade up of about 70% sand. So when I say 1 earth, 1 poo, 2 sand (which is like saying 25% earth, 25% poo, 50% sand) you much remember that the 70% of that 25% earth is sand, and the other 30% clay. Hope that makes sense….

And the full process…

The advice on how to make this mix was given by three friends of ours here at Eco Caminhos, Nova Friburgo, Brasil. Thank you to Cobi, Tomek and Pedro.

We have just started to apply the natural exterior render on to our first cob house here in Brasil. We have done tests and it lasts over a minute when directly sprayed with a hose. It stinks, but its beautiful, and here is how we made it…

Step 1. What are you going to need?

  1. Cows who eat grass. Or alternatively, a friendly neighbor who doesn’t mind you collecting the cow poo in the morning
  2. Sand. A mix of coarse and round grain. Filtered to approx 3mm, or less if you want super smooth render
  3. Clay, again fine-filtered.
  4. Buckets for collecting and measuring poo and sand
  5. A mesh filter for sand a clay
  6. A clean water source
  7. Trowels and spades for collecting poo etc
  8. A barrel to make your main mix
  9. Linseed oil for extra water resistance (we are only using it on the walls which get a lot of driving rain)
  10. Trowels, spatulas etc
  11. A large brush to wet the cob wall prior to application

Step 2. How to do your tests

It can take about two weeks to get your tests ready for this particular mix so make sure to leave enough time.

You are going to need to do a number of tests before you find your ideal render. As the materials and the way they react to the environment are going to be different everywhere in the world. Even if you are living in a similar climate to us here, I would not recommend copying our mix as the clay, sand and poo you will be using will not likely be the same.

Filtering the clay using a mesh

We put together our poo:clay:sand ratios…

The advice from all three of our pros varied slightly which gave us a great starting point. You can see the advice in full in the archive post

The ratios I came up with based on their advice were as follows: (all ratios written as poo:clay:sand)

Cobi Ratios

  • 1:4:4
  • 1:4:5
  • 1:4:6
  • 1:4:7

Tomek Ratios

  • 1:2:2
  • 1:2:3
  • 1:2:4
  • 1:2:5

Pedro Ratios

  • 1:1:1
  • 1:1:2
  • 1:1:3
  • 1:1:4

As you can see the sand increases in each mix. You can do as many mixes as you like. All the advice seemed to point to a particular preference in poo:slip and then the addition of sand to taste, so that’s why we chose these ratios.

We collected the cow poo…

Here is one of our volunteers getting to know the poo.

The cow poo is used for its enzymes, as they help increase the water resistance of the render. The grass in the poo also acts as a great fibre for reducing cracks and increasing overall strength. The advice we got from our three friends were as follows:

  1. Make sure you use fresh poo, you can’t use it if it has been composting and drying as the enzymes will be deactivated
  2. Go and collect the poo in the early morning, you want to catch the ones they do right after they wake up.
  3. If you can get poo with urine on it, even better
  4. Make sure the poo is free of stones and dirt, only take the top of each one
  5. You can leave the poo for a couple of days up to a month to ferment, it is best to leave it with the clay for a few days to help the binding process
Try not to start a cow poo fight with anyone who particuarly enjoys playing with cow poo. You will always loose

Then we made the test mixes

We used old milk boxes for our mixing containers. We cut off the bottom of a small water bottle and that represented ‘1 part’. So for example, for mix number one, we took 1 part poo, 4 parts clay slip and 4 parts sand.

As a note, before we combined the ingredients in the milk box, we first made the filtered clay in to a paste which resembled a nice plaster (by adding water) and did the same with the poo.

We made our mixes and to apply them to the wall you need to take a large wet brush and brush the cob wall in one direction till the clay has formed a nice shiney patch to work on to. Use a trowel or other plastering tool to firmly apply your render to the wall. The more force you use to push the render and the more you work to smooth over holes, the longer your render will last.

Make sure to mark each mix with a number and keep track of which number corresponds to which ratio!

Then we waited some time…

You may want to do tests on your north and south facing walls to see how your mixes react to shade and direct sun. We left ours a week before doing tests to see which was strongest. After  a week we could see that the high clay mixes we cracked so we eliminated them immediately. The high sand mixes were very rough and brushed off easily with the hand, so we eliminated them too.

The main test in the end for us really was how long the render could withstand a hose being sprayed directly at it. In the end we were spraying mix number 9 for over a minute before it came off. Ten however, lasted a good length of time, but also had a higher sand content, which we wanted, so I remade 9 and 10 to double check, and this time added linseed oil at 4% of volume. Linseed oil is known to really increase the water resistance of render.

You can see the full test results in the archive post. They include information on how easy the render was to apply, how easy to remove with a pick and how long each lasted under the hose.

I applied 9 and 10 to a larger space on the side of the house that faced the sun. After a few days they were completely dry and I tested them again. This time 10 lasted longer, and after 1.5 minutes of direct water spraying from less than a metre away we decided to stop, smile, have a high five and a coffee.

We had created a really strong render. When you look at it close you can see the grass fibres interlinking and it is incredibly beautiful.

So we made our final mix…

Our final mix, if you would like to know, was 1:1:2. So that’s 1 part poo, 1 part clay, and 2 parts sand. First you want to combine the clay and poo and mix well, leave them for a few days to combine properly. Please note, our mix got darker and darker green as we left it for two weeks before adding the sand. This made it really difficult to replicate the red/brown that our boss wanted. If you aren’t too fussed about colours then leave it longer (up to a month) to ferment and bind with the clay.

Then you add the sand…We encountered a few problems at this point. I chose to use an oil barrel to make the final mix but it turns out this was way too big. Or at least I filled it up too much. In order to mix it properly you need really to make the mix fill it up half way

Another problem was that the colour changedslightly with every new mix. As we had to do over 10 mixes to get round the whole house, we had a lot of different colours going on. This is difficult to rectify as the cow poo which makes up such a large volume of the mix, is always going to be different colours, depending on which cow you took it from, whether they were ill and how many minutes ago they did the poo. All I can recommend for this is to maybe use less poo in your mix, make large mixes and have a lot of people applying them maybe, and also weigh all your other ingredients such as earth, sand and colour. It is ok as a rough guide to measure your dry ingredients by the bucket however as there is a lot of air between the sand and earth grains, you are always going to have a slightly different quantity, so weighing is more reliable.

So anyway we were mixing with three of us for about half a day, with spades, hoes, power drills etc. We finally created a mix that seemed consistent so we added the colour tint and started to apply. Make sure to wet brush the wall before application to create a good suction between the render and the cob underneath.

And that is how to make render using cow poo

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Advice and Notes

Thanks to Cobi, Tomek and Pedro for your time

“Adding cow dung to the plaster adds strength and make the wall a bit more protected from the elements. Its all about experimenting but as a thumb rule we use up to 1/4 by volume. Its should be fresh (not composted) if its come with urine even better. It should be clean (no stone etc, if it has it should be sifted). Test batches should be made (earth sand and manure for example 1:2:1/4, 1:3:1/4 and so on) the mix should be left to ferment at least 24 hours (up to a month its ok…) and tn applied. Some people mix a bit of linseed oil in the plaster to enhance workability and hardness (more or less 1-2 % by volume) there is a lot of information on the subject but its mostly all about experimenting and finding the right mix.”

“Sure, cow poo is fantastic for plastering. Definitely the only way to use it is when it’s fresh, I usually collect it in the morning when it’s still very fresh, mix it with sand, water and earth and leave it for 2 days to let it ferment (in closed buckets, the smell is strong though) and the it works best (when you leave it with water and earth/clay it binds better and all clay particles are soaking water what gives amazing texture pleasant and easy to work with) The best you get from caw poo is fermentation so if the poo is old and dry it doesn’t work that well… Also it has all those little fibers that bind everything together. Caw poo is protecting the plaster from water erosion, i could see it clearly when making tens of material tests and test it for water protection. I would make few test and start from proportion clay:poo 2:1 and then add as much sand as you think is appropriate. If you finish it with piece of plastic (like cutting circle from yogurt cup) you will close all the pours and make it really strong for water but you can spend like 2hrs per 1square meter… it takes long time as you have to work with it while it’s drying, if you will live it it will crack a bit. If you make it right you can get really shiny finish but maybe not so color consistent… Also you should use very fine sand not only in last layer but also 1 before as when you will water the wall to put last layer the sand will mix a bit between layers.”

“You have to soak the shit in the clay slip and mix it well. Try first half and half but you may want more slip, it depends on how thin is your slip.
After make some tests with different proportions of sand, chopped straw and shit/slip mix. You can add also linseed oil 4% of all mix. 
The proportion should have more sand (good sand is sharpen edges)
Make the different test in the wall like 50×50 cm and let it dry. And see which one is harder to remove.
If you don’t like any use less shit in your slip.”
Notes
Test Results: Application of Render. The notes after each ratio signify: ease of application, ease of removal, how long the render withstood the hose, and how the finish looked
Ratio Key – poo:clay:sand
  • 1:2:2. Easy application. Rubs off easy. 7 seconds. Smooth
  • 1:2:3. Easy on. Easy off. 25 seconds. Very smooth
  • 1:2:4. Tough to apply. Very easy off. 23 seconds. Rough finish
  • 1:2:5. Tougher on. Weak when dry. 18 seconds. Very rough
  • 1:4:4. Easy on. Quite tough dry. 9 seconds. Smooth finish
  • 1:4:5. Easy on. Quite tough dry. 10 seconds. Smooth Finish
  • 1:4:6. Tough but workable. Tough dry. 17 Seconds. Grainy
  • 1:4:7. Bit tougher on. Quite tough dry. 7 Seconds. Rough
  • 1:1:1. Easy on. Easy off. 67 Seconds. Smooth finish
  • 1:1:2. Easy on. Quite tough dry. 30 seconds. Smooth finish
  • 1:1:3. Easier on. Easy off. 12 seconds. Grainy
  • 1:1:4. Tough on but workable. Very easy off. 8 seconds. Rough.

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