Stone Wall Foundations for a Cob House (with Lime Mortar)


Here at Eco Caminhos, Brazil, we have just started our second eco build. We are building an ecolodge in a beautiful location up in the hills near Nova Friburgo, Rio de Janeiro.

We started by digging the foundations down to the hard bedrock, sloping the foundation towards the edge of the hill to make sure water drains out, and filled the trenches with gravel.

We set our guide strings, and planned where the water pipes go, and got going with the wall.

The first layer of stones. You lay these in the ground without mortar. Just shuffle and push them till they are stable and flat.  With this eco lodge we are using limecrete instead of concrete. The manufacturing process of concrete is extremely detrimental to the environment- requiring the burning of the raw materials at very high temperatures, releasing a lot of c02. Lime is a far less damaging mortar, and works better with natural stone due to its ability to self-heal and the fact that is is more flexible than concrete so can handle small shifts within the building over its lifetime.

Our mortar is made using the following quantities; 3 sand, 1 clay, 1 hydrated lime. You mix them well and then add water. We dry stack our stones and only actually use the lime afterwards to hold in the packing stones from above. Using this method you can make a mix of lime which will last a while as long as you keep it wet and in a secure container. So no big rush at the end f the day to finish the Portland cement before it dries. The lime we use is pictured below


So, lets begin…


You want to be looking for a number of things when building a natural stone wall or foundation:

Try and build up in layers from the bottom up. Finish a layer by making the top surface flat. When using square stones it is pretty easy but when using random shape stones it is SO DIFFICULT (our first build was using varied shape rocks). When you make your top surface flat it makes the next layer MUCH MUCH easier. I really focus on making your top surface flat. It is easy to neglect it and be tempted to place a rock because it fits but that bit of extra time spent making a rock flat will save you much more time on your next layer.


Don’t ever have lines running vertically. Always cover you joints or you wall can slip over time. Lines running horizontally are good, it means you are building in nice even layers. Think of a standard clay brick wall, your horizontal lines are always level, and your  vertical lines never run more than one brick…as shown below


Make your stone really secure before adding mortar. Then you just need to use a tiny bit of mortar to hold it in place.


Always have a bucket of chips and thin rocks on hand to use to firm up your main rocks. Makes life easier. Spend ten minutes at the beginning of the day smashing up some nice fragments. Use your awkward shaped rocks to make the chips from.

Making Chips

If you have a big variety of shapes and sizes of rocks, it helps to order them first into size, put aside the big flat rocks (I call them dictionaries, they’re beautiful. Commonly called ‘tie stones’) as you need them about every 50-60cm to tie the back and the front of the wall together

Keep about a metre of space between your wall and your stones, kit etc. A tidy work space helps you build efficiently. Reduce stress. So on. So forth.

Never drop a stone in to place as you’ll displace the stones underneath. Place it down slowly and move it carefully in to place.

When building a foundation wall you effectively are building two walls next to each other at times, so make sure you pack the space between well, and try and inter lock the two sides sometimes to join them together and create a strong overall structure.


When using triangle stones, I find it is best to raise them up from underneath rather than laying them flat and trying the make the surface on top flat. Take advantage of the flat surface you already have rather than trying to lay in flat then make a level surface on top with smaller stones. Below is an image that will hopefully explain what I was trying to say…


And remember, stone building can be stressful if you’re not a master. So, chill out, walk away, look at the trees and mountains, go back and work later.

rock smash

And last of all, this is the video we watched before embarking on our semi-dry foundation wall.

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