About heritage cob and lime – And how we'll help you maintain it
There are around 35,000 cob buildings in Devon, some very ancient indeed. At one time or another most of them need specialist maintenance, which means avoiding inappropriate modern materials and techniques. When you get it right, your cob building will keep looking good for hundreds more years. Get it wrong and you can cause fundamental damage to the structure.
In today's environmentally-sensitive world, cob is experiencing a resurgence in popularity, being completely sustainable. Do you live in a cob house or thinking about buying one? Here's what you need to know about heritage cob.
What is cob – One of the world's oldest and best building materials
In Mexico it's called Adobe. Some call it 'rammed earth'. Cob is made from wholly natural materials; soil, straw or something similar, water, and sometimes lime. A traditional building material in Devon and Cornwall, where there's plenty of good quality naturally-occurring clay, It has been used for building since prehistoric times.
The word 'cob' comes from an old English word that means 'lump'. People traditionally used their hands and feet to mix the ingredients. Depending on the type of subsoil, you can add sand or clay to create the best-performing mixture. Many of Devon's cob buildings are made from a mix of sticky mainly deep red, grey or yellow local clay, subsoil, sand, straw and water.
Building with cob is labour-intensive, involving a lot of hard physical work. Once the foundations are in, a cob structure is built in layers. Every layer is left to dry completely before the next layer is added.
Cob is strong enough to bear height, which is why we get so many two storey cob buildings. You can either put the doors and windows in place as you build upwards, or cut them out of the finished structure like cutting biscuits out of dough. A traditional-style cob wall will come in at about 60cm thick and provides a really good thermal mass, keeping the inside cooler in summer and warmer in winter. It's actually better than the minimum U-values specified for domestic buildings.
Cob doesn't like getting wet. It has a habit of turning back into mud and water, hence the many ancient cob farm buildings we see in Devon slowly melting back into the earth as the wind and rain do their thing. That's why a good roof is so important, one that protects the walls and foundations from the wet. You need at least 200mm overhang on the roof to keep the cob safe and dry, and cob walls are best left bare or rendered with lime to let them dry out naturally after getting wet.
Cob is fire-proof, which is why insurance for a cob building tends to cost less than for a thatched home, which comes with a much higher risk of fire. It's great in an earthquake, standing steady while other buildings fall. And it costs very little in itself, a cheap building material. The expense involved is down to it being a relatively time-consuming material to work with.
What is lime in building?
Lime and lime mortars have been used for building since Roman times, and many ancient buildings made using simple lime mortars still exist in remarkably good condition. These days lime mortars and lime plasters are becoming popular again, used to repair, rebuild and restore historic properties. So why do we recommend using a lime plaster on an old cob building?
Essentially, natural lime mortars used for lime plastering let your cob walls breathe. Permeability is fundamental in letting moisture escape, and to allow for natural structural movement. If there's a lot of moisture in your cob and it can't escape, it'll cause damp patches on your walls and can ultimately damage the structure of the walls themselves.
Modern cement-based materials are notorious for not being breathable. Using them on historic walling like cob and old brick damages the very fabric of the building, making it more of a challenge to restore. Modern materials ate hard, impervious, and not at all suitable for old, solid walls. We spend a lot of time removing modern rendering and replacing it with a natural lime render that lets the cob underneath breathe and dry out.
Trained by Historic England - Your local heritage cob and lime specialists
We specialise in cob rebuilding and repairs, including heritage cob and lime, lime plastering and lime rendering, all using the right sort of natural hydraulic lime. We can also insulate your lime plaster walls for you. Whether it's cob lime and putty rendering, plaster lathe work, listed building restorations, renovations and related repairs, we'll be pleased to give you a fair, expert quote for the job.
