What is cob?
Cob is roughly 75% sandy aggregate, 25% clay and some straw.
Where do you get the materials from for cob?
Ideally you use the sub-soil on your building site. Most of your sub-soil for the making the cob will hopefully come from your foundation trench. You do NOT use the top soil – this contains organic matter which would decompose in your cob walls and cause structural instability.
What do the sand and the clay do?
The sandy sub-soil ideally needs to be sharp and contain angular stones and gravel. The sandy sub-soil gives the cob its strength. The clay sticks the sand particles together.
What happens if I add too much clay?
Too much clay will make your cob crack when it dries. Not enough and your cob won’t stick together properly.
How do I know if I have the right cob mix?
This is the bit that is hard to get from a book. Our Cob Building Courses will give you hands on experience which will allow you to get a ‘feel’ for the material and gain the confidence you need to go away and build your own cob wall, garden studio, cob house or cob pizza oven. We will show you how to make a test brick to check how the cob dries, share the drop test with you and various other tips.
Why do you add straw to cob?
Straw gives cob tensile strength, regulates the rate of drying so it does not dry too quickly and crack, and provides insulation.
Cob is made of earth. What happens to it in the rain?
Cob houses in the UK with an overhanging roof, brick or stone stem wall and a lime render should last forever and be unaffected by the rain. Cob dries rock hard and is much more durable than you may imagine. We have experimented with leaving cob walls exposed all winter through the snow, ice and rain and found there has been very little erosion of the wall. Of course your wall needs to be protected properly eventually but there’s definitely no need to panic if your cob walls get wet during construction.
Does cob need lots of up keep?
No. Cob is low maintenance and can be repaired very easily and cheaply if damaged over the years.
What about building regulations – does cob pass building regulations?
This is not an issue at all. Cob houses have been standing strong for thousands of years. Cob is non-combustible, load bearing and safe. We have experience in dealing with building regulations and will deal with that side of it for you if we are building a project for you. We also teach all you need to know about building regulations on our 4 day complete cob building course. There is plenty of evidence documented about cob houses which you can share with your local building regulations officer. You can also choose to use a building regulations company that specialise in cob and sustainable builds – you do not have to use your local council. We can recommend firms to you.
How easy is it to get planning for a cob house or cob garden studio?
It is just as easy to get planning permission for a cob house as it is for any other construction type – in fact it might be easier as cob ticks so many of the planning officer’s ‘green boxes’ that they need to take into consideration these days. Depending on the size and nature of your cob structure, planners are only really interested in what your house looks like. Many cob houses are welcomed by planners as their beauty wins out. You can also face your cob house with bricks or render – you can make it fit in with the local vernacular. Cob houses can be built in any style you want.
Garden studios usually don’t need any planning permission, unless you live in a conservation area. Different areas have different regulations though. It is always a good idea just to give your local council a ring and simply ask them – you’ll be surprised how accommodating they’ll probably be. We can also advise you on any planning issues.
What kind of roof can I have on a cob house?
You can have any kind of roof you want – tile, slate, thatch, sedum. Anything goes!
How environmentally friendly is cob?
Cob couldn’t be more environmentally friendly. Usually the earth dug out to produce your foundations is usually enough to make the walls of your house. Any added ingredients such as clay or sand can be sourced very locally. And timbers for lintels or doorframes can be made from reclaimed wood and or driftwood. No chemicals are used on your cob house and certainly no cement. Cob houses have almost zero embodied energy.
What about U values and insulation?
Cob walls have an excellent thermal mass – they hold onto the free energy from the sun and slowly release this back into your house. They act like storage heaters, providing you with free heating. However cob is not amazing in terms of insulation and so residential cob houses have thick walls to offset this. However you can build your walls thinner if you have straw bales in your cold north wall – straw bales provide exceptional insulation that will help ensure the heat emitted from your cob walls into your home can not escape.
On our 4 day complete cob building course we teach you to use this passive solar design combining both cob and straw bale walls in your home or studio.
How much do cob houses cost?
If you build your cob house yourself then you can spend very little. Your walls will cost you nothing as you use earth from your foundation trench which is completely free! You can also easily use reclaimed windows and doors which will cost you less – cob is very versatile and easy to work with.
If you commission us to build a cob house for you it will cost slightly less than a conventional build. It will also be cheap to run once built as if it is designed using passive solar principles it will require very little heating.
What are the benefits of building with cob?
Please see our All About Cob Houses page for the low down on this.