Building with earth has been a popular and comparatively cheap option throughout the ages. In the South West we have a rich tradition of cob buildings based on mixing sub-soil with straw. There is an old Devon saying that “all cob needs is a good hat and a good pair of boots”. A stone plinth usually provides the boots and the hat was traditionally thatch.
When earth and cob repair becomes necessary, it’s important to work with sympathetic materials and techniques to ensure a good looking and long lasting result. The first priority is to identify the cause of any cracking or damage.
How old is the damage?
- Has the wall been decorated and when?
- Can your neighbours remember any history of movement?
- Is it connected with changes to the structure? ( e.g. extensions, patios, paths, new roof, drains, defective renders, new doorways, windows etc.)
How severe is it?
Cob can often show cracks and defects because old buildings gradually move on shallow foundations. Recent movement is of more concern than an old defect.
If the cracks are located around doors and windows the damage is likely to have arisen from changing stress loads or rotten timber lintels. Movement in an elevation wall is often revealed by bulges along its length, internal gaps with partition walls, floor joists becoming visible. Movement in a gable wall can be a problem if the crack widens significantly as it goes up the wall.
Unfortunately modern cement renders, plasters, masonry paints and emulsions conspire to trap moisture in the wall and damp cob can lead to structural problems. Once the moisture level exceeds 12% the strength of the cob drops dramatically. But don’t be mislead by damp-proofing firms using surface moisture meters. Condensation and hygroscopic (water seeking) salts can both give high meter readings for basically dry walls. Measure moisture in the centre of the wall by taking a core reading for a true result.
Rebuilding and repairing cob structures with cob blocks offers advantages over masonry and aerated blocks in that they:
- match the existing structure for porosity and density, allowing moisture to move in a similar way.
- won’t introduce hot or cold spots where differential thermal movement can cause renders and plasters to crack.
- have a similar compressive strength to the original cob and can therefore accommodate general movement better without detaching from the original structure.
- allow recycling of material with savings in energy consumption.
- are pre-dried and won’t shrink away from adjoining surfaces.
Before starting any work ask yourself; ‘do I need planning permission and listed building consent?’ Once you have dealt with this, work may begin.
Cob Repair Method
When using cob blocks and cob bricks for repair, its important to ensure that they are bedded on flat surfaces as far as possible. Damaged and unsound cob in adjoining surfaces must be pared back. As well as chasing the blocks into adjoining cob, stainless steel helifix bars can be driven into the existing cob and bedded in the joints of the repair.
It is very important to control suction from the cob. Dampen the surfaces of adjoining cob and the cob blocks and bricks with a light spray of clean water before use.
3. Bedding Mortar
The aim of the bedding mortar is to spread the load evenly onto the block and it should be kept to a minimum thickness. Traditional lime mixes or blends of earth, sand and lime are intended to be of similar strength and porosity as the cob repair.
For 1 cubic metre of wall – 90 blocks of size 17.5″ x 8.5″ x 3.5″
1 tonne of ready mixed Unhaired Lime Mortar will lay around 150 cob blocks, depending on the shape of the repair and the thickness of the bedding joints
It’s also possible to put up shuttering and tamp down a drier mix of cob, but this method is generally slower as only a couple of feet can be rebuilt at a time, as each layer must have enough time to dry.
Discuss larger problems with a cob expert (such as the need for buttresses, tie-bars and underpinning).
Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.