Bristol Green House Clay and Lime


In the process of making this building I have used both clay and lime renders. Sadly, my experience with clay render has been perplexing. In this page I will try to deal with some of the issues raised. Throughout the build I have sought information on the processes I was about to employ from people with experience and from the web, but I have yet to find an exhaustive website on clay render. I would love this to be a definitive guide that makes all clear to the reader. In reality I think I will only achieve the opposite. Everything is as clear to me as muddy water. If anyone has answers to the issues raised, please do contact me.

Let's begin with clay and move on to lime. There are more references to these matters in the blog.

Tyre walls
Straw bale walls
I-beam roof
Living roof
Clay/Lime render

Richard Long making River Avon Mud Arc, Bilbao Guggenheim, 2000
Richard Long making River Avon Mud Arc, Bilbao Guggenheim, 2000

Mud, mud, glorious mud. We have a lot of it in our garden
and I intended to use it.

clay slip clay slip clay slip

In fact, like much of Bristol, beneath a thin layer of topsoil there is clay of a
very high quality. Just right, I supposed for rendering with.

The process is this:

Mix clay with water to make a smooth, sticky, clay slip. The slip is used as
a base coat on the straw bale walls to act as a keying layer.

To work out what kind of clay you have you mix the saturated clay (that's left
after the slip has been drained away) 50/50 with some fresh cow poo, then add
plasterers sand in different ratios and make some test patches.

clay render test patches clay render test patches
Testers as applied
and two weeks later
Our best ratio was found to be 1 clay to 1 poo to 5 sand. It set very hard without
cracks and appeared perfect for rendering straw bales with.

A clay rendered wall in a very dry climate. Photo by cer!se.
Clay render

The Mix

The final recipe was (your mix might differ with your local clay and sand):
1 part clay
1 part finely chopped straw (you could use 'hair' - either plastic or animal)
1 part cow dung (fresh)
5 parts holm sand (it's our local building sand)
and 7 cups of thick clay slip (just to make the mix more moist and pliable)

We started out mixing by foot. This was fun but strenuous and slow. We found the best mix was obtained to loud Tango music.

bare foot clay render mixing

Next session I hired a render mixer. It's like a giant dough mixer. And quite pricey. You can't use a conventional cement mixer as it will just make balls of clay in the sand. Apparently you can use a cement mixer if you make it vertical. I couldn't think of an easy and safe way to do this.


The process of applying the clay render is quite a pleasant one. There is a farmyard aroma, but it is a very natural product and very straight forward to apply. I prefer clay render to lime a lot. Lime is damaging to the skin and damaging to any surfaces it comes in contact with. Clay render just wipes off and it's good for the skin. Once the roof was on and the bales had been left to compress for a few weeks we began rendering the exterior. Just about anybody can master the art of applying render in just a few minutes. It's like playing mud-pies.

Applying first coat of clay render
Above, left. Applying first coat of clay render. Right, Scored first coat.
Below, edging strips for windows and the bottom of the wall, to be rendered in.
They help water drip away rather than stand on the edges of the render.

clay render

Why I used clay on the exterior

Oh woe is me. I knew there was a risk in rendering the exterior with clay. I have a strong streak of iconoclasm mixed with a habit of tilting at windmills and an urge to experiment. It's my building, I insisted like a petulant child, I'll try something counter intuitive if I want. It was a gamble and it has caused me huge problems. I can't complain that much, can I. I am in favour of finding low impact building techniques and clay is low impact building material. I only hope I don't put people off from using it.

Others have done it successfully and if you plan to do it, find out what they did right.

The problems

(See blog for more details) When I decided to render the exterior with clay it was suggested I try something called active silicate paint. The idea is that the paint chemically reacts with the clay and sand to create a tough crystalline waterproof layer that is also breathable. So I tried it and for some reason the reaction didn't occur. The paint people, Keim, claim it's my render mix that is at fault and this seems likely to be true for two reasons. 1, both Keim and Barbara Jones say that other have done this successfully and 2, When I dig in to the render it is powdery under the hard surface. Anyway the long and the short of it is that the paint isn't waterproof and the clay render becomes saturated when rained upon.

Why did this happen?

The nice salesman from Keim came down and did some tests with a glass tube that attached to the wall with blu-tack. Where the paint was perfectly smooth the wall was totally waterproof, but wherever there was the slightest imperfection or hairline crack the wall sucked in water. As to why the paint didn't react with the render, or why the render is powdery beneath the surface I simply have no answers. I wish I had time to research it. Part of me would like to be an esoteric building practices researcher full time, the other half of me (my wife) sees the need for me to earn a living. Keim were not able to tell me. They suggested that too much water in the render when it is painted means the reaction doesn't happen. We tested the level of moisture in the clay render both interior and exterior and his meter said the level was too high for Keim paint. This might indicate that Keim paint just wont work on clay render. Clay render is always going to have a high-ish moisture content as one of its properties is the regulation of humidity in a building. The mirror in a clay rendered bathroom is said to demist very quickly because the walls drink up the water vapour from the air.

What have I done to remedy this?

As the problem was hairline cracks my solution was to paint on three layers of lime wash, the first two with sand in to make a tough, gap filling cover for the dodgy paint. The recipes as follows:

First coat. 1 cup lime putty, 1 cup water, quarter cup sand.
Second coat. 1 cup lime putty, 1 cup water, eight cup sand.
Third coat. 1 cup lime putty, 1 cup water.

straw bales rendered with clay
Above. The finished wall with 3 coats of limewash and 3 coats of Keim, before the cracks appeared.

What would I do differently next time?

I will render the exterior in lime next time. If I were to be convinced that clay can be made to work I would do advance testing over a winter. Certainly if anyone plans to use active silicate paint, I suggest you get a free sample from the manufactures and do a full test involving two layers of render and a soak test.

The wall with the clay render was beautiful and when we took the render off the straw underneath was in perfect condition, even at the bottom of the wall.

Clay - pros and cons

I am pro-clay for the following reasons. It's free, it's already on site, it's non-industrial. The down sides are that it is not tough against knocks and if used exterior it must be treated to become waterproof.

Antonio Moura commented on this web page:

I found your blog during the research for an image of a cement mixer. Presently I'm finishing my dissertation about clay renders and plasters. I think clay is a very interesting building material and very sustainable as well. Just want to point out that the mixing time of clay plasters is important (Gernot Minke, Building with Earth) and the curing time as well (48 hours). Also in US and other parts of the world many people add some cocked wheat flour past to the mix, casein powder, etc. (see Adam Weismann & Katy Bryce, Using Natural Finishes - Clayworks a company in Constantine, Cornwall). The % and type of clay is also important. If you use too much water to make it easier to mix you might find that when it dries its dusty and crumbly (at least when the type of clay is Kaolinite).
In UK the climate is not appropriate to use clay renders externally but in old times many builders would cover the clay render whit a thin layer of lime render (lime was expensive) so many old buildings have clay behind the lime render (lol).

I should add that your experiment and sharing it online is invaluable.
Did you know that the first strawbale council houses were finished a week ago? see North Kestevan District Council website - they had some issues with lime renders as well. Actually many skills have been lost. Using natural building materials requires knowledge and experience. Just because these materials are natural and sustainable that doesn't mean they are easy, on the contrary...
My research on the subject drove me mad...

Good luck
Antonio Moura

Rendered happy at Piers Strawdio, June 2006.

Lime render

It is very easy to get lots of very good and consistent information about lime online. There isn't much info specifically about rendering straw with lime, but it seems to me the same principles as rendering lath apply.

Second coat of lime render
Above. Second coat of lime render. Small cracks are normal and can be filled by rubbing in more render. They close up when you add the lime wash.
Below. You can see the bits of straw that were added as 'hair' to strengthen the render.

Second coat of lime render

The Mix

There are several recipes for lime render depending on who you ask. One is a mixture of lime putty (sometimes called butter) and sand, another is a mixture of Hydraulic lime powder and sand and the recipe I went with uses both types of lime.

My mix was this, based on the recipe I learned volunteering on another job and I think it originates from the Alternative Building Co:

1 buckets Hydraulic lime (NHL2) (Apparently for rendering straw you need a 'weak' lime, ie lower no. = more flexible. NHL2 is poignantly classed as 'feeble', while NHL5 is 'eminent' - I got this off a French website)
3 buckets sand (lots of different sizes of grains
Half a spade of lime putty (This improves the texture, but apparently if mixed with NHL2 will weaken it further. Adding it has worked very well.
Half a bucket of straw cut to 1 inch lengths
As little water as possible to make it sticky. (The less water the less it will crack later and note that the longer it mixes the wetter it becomes)

Also note that mixing in advance and 'knocking up' by running it through the mixer again improves the feel and workability of the render.

Also note that loads of web references say DON'T use sea sand as it will leach salts, but the local sand here is Holm sand from the Bristol channel and has been used in render for generations. I tried a (more expensive) land sand and it was much courser and gave a rough finish to the wall.

Quantities - a rough guide

It is very hard to work out how much sand, lime and putty to get, but here is a very rough guide based on two coats of approx 1cm thickness each.

Surface area: 13m2
Approx thickness: 2x 10mm
Volume of render, therefore:
Sand used: 22 bags (550kg)
NHL 2 used: 3 bags (75kg)
Putty used: 2.3 tubs (58kg)

This means the render weighs 683kg, approx 52kg/m2
I needed approx 1.7 bags of sand and a quarter bag of NHL per m2

Working out person hours is even harder. We did the top coat in one day with 5 people. This is about 40 hours for 13m2, which is 3 person hours per m2. This seems reasonable enough, but be aware that before the base coat can be done there is a lot of time required to tape up the woodwork, protect the floor, put down material to catch the falling render, mix the first render and so on. I'd be interested to see any one else's calculations based on their experience.


You have to wear gloves and put a sticking plaster over any cuts in your skin as the lime will prevent them healing. It's very easy to apply if you get the right consistency in the mix. Protect any surface you value from splashes, lime will change the colour of wood. In some areas I rendered in a single coat, figuring it would take half the time. It isn't easy because normally the first coat covers most of the straw on the surface of the wall and the second coat covers the few bits that poke out. Doing it in one requires a lot of patience to cover all the stubble. It's probably not as strong in the long run too.

Two coats of lime render, not yet fully dry.  Below, the finished wall with two coats of lime wash
Above. Two coats of lime render, not yet fully dry.
Below, the finished wall with two coats of lime wash.

<empty>Two coats of lime render, not yet fully dry.  Below, the finished wall with two coats of lime wash

Use of hessian to keep the render moist

The render mustn't dry out too quickly if it is to keep reacting and hardening so it is recommended to keep the wall moist. It's hard to do this without spraying the surface with water. It is often suggested that by hanging hessian in front of the wall, this can be kept moist without having to spa ray the wall.

When I re-rendered the 7 bale wall I put hessian over one area and not over the other. I lightly 'misted' both areas daily. In practice this means spraying on water with a plastic bottle with a spray head or even a hose with a 'mist' head.. There was no difference in cracking between the hessian and no-hessian areas.

Recyclable building material

Lime causes the creation of less CO2 than cement, but it also has this amazing ability to be re-used. I made up too much render last summer and so kept it sealed in plastic and added it to the mix a year later. The only obvious effect is that the mix seemed to take longer as the re-used lime was less 'reactive' than the fresh NHL. I added a bucket full to every three of sand. If and when this building is demolished it will be entirely possible to remove the render and re-use it again.

Finish - Lime wash

We gave the building a lovely hand finish rather than using plasterers floats, which requires some skill. The look is very touchy-feely. I painted on two coats of lime wash which is so easy to make (one cup lime putty, one cup water - the consistency of skimmed milk) and very easy to apply. You can add your own colours, but I left it just white to maximise light.


Before the bales were rendered the acoustics in the studio were amazing. Pleasantly dead, perfect for sound recording without echoes. Since it has been rendered with lime the sound is very toppy and bass tends to muddy. I Have had to create several panels of 50mm insulation batts to deaden the sound back again. Someone has since suggested that it is possible to finish the straw with hessian. This sounds like a good idea if you want to use the space for recording. We have since recorded instruments in the space - a banjo sounds great, but electric guitars are harder to record. The frequency response is far from flat. I would be interested to know what clay render would do for the acoustics. I imagine it is far more absorbent that lime. The straw is a fantastic absorber of sound so that even if we have a band playing in here sound only escapes through the door rather than the walls.

truth window

Lime - pros and cons

It is easier and cheaper to mix than clay, very strong and reliable as a render. Water proof and easy to give a lovely finish. However it is quite nasty on the skin and wood.

Above. I bought a 6 inch long metal fly in a flea market (ironic, huh) and fixed it to the wall. This inspired Dee to add some sculptural elements in the lime render.