Cob

Print Friendly

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 repairs do become a necessity its 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 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 loadings 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. The moisture needs to be measured in the centre of the wall by taking a core reading. New concrete floors with damp proof membranes can also cause damp to be concentrated in the walls.

Repairing cob depends on the size of the problem. Non-structural cracks can be repaired with cob bricks, larger cracks or defects with cob blocks. Rebuilding and repairing cob structures with cob blocks offers advantages over masonry and aerated blocks:

  • They will match the existing structure for porosity and density, allowing moisture to move in a similar way.
  • They won’t introduce hot or cold spots where differential thermal movement can cause renders and plasters to crack.
  • They have a similar compressive strength to the original cob and can therefore accommodate general movement better without detaching from the original structure.
  • They allow recycling of material with savings in energy consumption.
  • Cob blocks and bricks are ready 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 this has been dealt with, work may begin.

1. Preparation
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. Differing methods have been suggested of getting a mechanical fixing to the adjoining surfaces. 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.

2. Damping
It is very important to control suction from the cob. Both the surfaces of adjoining cob and the cob blocks and bricks must be dampened with a light spray before use.

3. Bedding mortars
The aim of the bedding mortar is to spread the load evenly onto the block and it should be kept to a minimum thickness. The mortar can be a weak mix of earth/lime/sand mix of varying proportions dependent on whether the cob blocks are to be exposed and weathered to match or rendered or plastered. In the latter case a 4/1 mix of coarse sand/lime putty will be suitable. These mixes are intended to be of similar strength and porosity as the blocks.

4. Quantities
For 1 cubic metre of wall, – 90 blocks of size 17.5″ x 8.5″ x 3.5″
1 tonne of ready mixed 4/1 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.

Larger problems may need to be tackled differently. The need for buttresses, tie-bars and underpinning is best discussed with an expert familiar with cob.

Safety
Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.

Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.

Leave a Reply