Kevin McCabe

COB BUILDING SPECIALIST

We are a small, prestigious company established in East Devon, with over 35 years’ experience. We specialise in the high-quality repair of historic buildings, particularly those containing cob — and, increasingly over the last 25 years, the construction of new cob buildings.

WORKSHOPS

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8th May 2021 09:30 Dingle Dell, Higher Ridgeway, Ottery St Mary. EX11 1TJ

FAQ

Why use cob?

As well as the important aesthetics of form, colour and texture and an automatic blending with the local environment (as long as your sub-soil is locally sourced), Cob has several other important characteristics. Firstly, walls are usually very thick, typically 2-3 feet (600-900 mm) – and this, as well as being very good insulation against heat and cold, is a massive heat store. So there is a fly-wheel effect on thermal movement. In other words, cool nights and warm days are evened out as well as when doors and windows are left open, resulting in a change and cooling of the air in a building. When they are then closed, the building warms up again through the heat stored in the thermal mass of the walls. This means that, typically, a cob house will use approximately 20% less energy to heat compared with a typical modern house meeting the same building regulation insulation requirements.

Another important property, often not appreciated, is the humidity store of earth walls. They can easily absorb moisture from the atmosphere and release it again when the air dries out, thus resulting in an overall higher background humidity than a typical new building. This is much more comfortable and healthier to live with, not drying out airways and resulting in less susceptibility to throat infections such as colds.

There has been much research to endorse what I’m saying here, particularly by “CRATerre-EAG” at the school of architecture of Grenoble in France, and Professor Germont Minke of Kassel University in Germany. Most of the new building projects in these countries have been done in either rammed earth (a form of building not native to the UK using a drier mix of earth and shuttering, resulting in a quicker drying time but much less sculptural characteristics), or in adobe (earth bricks). Minke has also experimented with “sausages” of an earth mix on the inside of a highly insulated wall simply to get the benefits of some thermal mass inside the insulation and mainly the humidity store. For me, however, there is nothing more satisfying than the massive three-dimensionally sculptured forms of cob, as I hope you’ll agree when you look at some of the photos available on this site.

Repairing Cob

Cob is a remarkably durable material provided it is treated correctly. In practice, this means keeping it dry. Unfortunately, as ordinary portland cement has taken over from lime in the building trade, largely during the second half of the 20th century, builders and home owners have often made the mistake of thinking cement (being very strong and waterproof) will protect the cob well. However, the reverse is true. Although cement will indeed keep off driving rain well, it is very brittle and often cracks due to movement in the much more flexible substrate. These cracks then let in water which is trapped by the impermeable render. Thus, rather than evaporating away when the rain stops, this water builds up in each subsequent storm, usually sinking to the bottom of the wall where in extreme cases it can eventually lead to total failure of the cob, i.e. collapse. This in a wall that might have sat quite happily for over four hundred years prior to the addition of a cement render.

The answer is to stick to either no render at all or a lime putty-based render, as has been used literally for thousands of years. The same applies internally; either lime or (even better) earth plasters work best. Most of my work repairing cob buildings over the last thirty years has been to strip off cement render and carry out necessary repairs, sometimes replacing existing repairs in a more compatible medium, namely cob blocks and an earth/lime mortar. The render or plaster can then be replaced with lime or earth mixes, before being decorated in lime wash.

Is cob more expensive?

No. The actual cob work, with the aid of modern machinery, is surprisingly cheap. It will depend on site accessibility and how much adjustment is required to the onsite sub-soil, but usually the cost of the cob itself, including labour and materials, will be no more than £300 per cubic meter. In practice, this means between £25,000 and £40,000 for a typical three bedroomed house. There are wider footings to consider and a stone plinth which brings the price up a bit and, of course, if you have many curved walls it makes most building processes (except the cob itself) more time consuming. In fact, the walls of a new house are usually around 25% of the cost of the whole building. In the case of cob, it’s usually about the same. Half of this 25% to put in footings DPC and build a well-insulated stone-faced plinth to a height of around 750mm and about the same cost again for the cob work up to roof height.

The main criteria in the cost of a new-build project, apart from the initial site cost, is how high the finished spec is, For example, in the case of Keppel Gate, we have bespoke sweet chestnut mullioned windows, oak floor boards, under floor heating, an Aga cooker, hand-made oak kitchen, marble bathrooms, porcelain floor tiles and hand-made oak doors with hand-made iron furniture. Therefore, this was not a cheap build. Nevertheless, because of the efficient design, using the wide roof space to create a three-story building, the square meter build cost in 2001 was only around £800. This equates to around £1600 at today’s prices, which for such a high spec is in fact very reasonable.

So what are the disadvantages of using cob?

Well, the fact that it is mixed wet means that, in a thick wall, it will take some months (depending on site conditions usually 6-9 months) after building, for the walls to finish shrinking. Most of the shrinkage will occur in the height of the wall. This means in practice waiting around a week of good weather between each lift (a lift being two feet (600mm) or so in height) before it is sufficiently dry and strong to take the next lift. Therefore, three months is ample time, allowing for a typical English summer to build a two-story building. Although the roof structure can go on as soon as a week after the wall plate height is reached, it’s best to wait about six months before fitting windows and door frames. Otherwise there is a danger of the reveals either side of the opening shrinking and then the lintel crushing the window frame. In practice, this means it is usually fifteen months from start to finishing a cob house.

Over the last twenty-five years, I have built over 30 buildings of one sort or another – and all of them except two rather complex split-level part-retaining walled buildings and Dingle Dell itself, have been finished well within this time frame. Although this is longer than a typical new estate house, it’s not actually especially long for any bespoke one-off design building. It’s also worth noting if any green oak is used in the structure for lintels or roof structure then this will continue to shrink for much longer than the cob (about one year per inch thickness of timber). This means it is a good idea to delay the final rendering for a further year if possible. The extra weathering on the cob surface during this year also serves to create a better mechanical key for the render.

Another disadvantage is that usually site space is at a premium these days and the simple fact that to pass the latest “part-L” insulation performance requirements for building regulations, unless insulated the exterior walls will probably need to be three feet (900mm) thick! This means the whole building will take up a significantly larger footprint and also need a larger roof area all around for the same internal space. However, for me, as long as the site is not really tiny, this is a small price to pay.