Cob blocks are suitable for the repair and rebuilding of cob and earth buildings.
When repairing cob/earth buildings become a necessity its important to work with sympathetic materials and techniques to ensure a good looking and long lasting result. Cob blocks are therefore the natural choice for repairing cob buildings.
Repairing cob depends on the size of the problem. Non-structural cracks can be repaired with cob bricks (or cut-down cob blocks), larger cracks or defects with cob blocks. Rebuilding and repairing cob structures with cob blocks offers advantages over masonry and aerated blocks in that they:
- will 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 ready dried and therefore 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.
When using cob blocks 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.
It is very important to control suction from the cob. Both the surfaces of adjoining cob and the cob blocks should be dampened with a light spray of water 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.
A cubic metre of wall rebuild will require approximately 90 cob blocks.
1,000 kg 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.
A cob block measures approximately 90mm x 220mm x 440mm.
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.
Larger problems may need to be tackled differently. The need for buttresses, tie-bars and underpinning is therefore best discussed with an expert familiar with cob.
Cob block prices are based on customers using our lime products.
We reserve the right to add 20p per block if purchased without lime products.