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Mike
Wye & Associates |
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Guidesheets: Cob
Repairs
Please
click on the links below to find other guidesheets:
COB BUILDINGS ~ A RICH TRADITION
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. |
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 offer advantages
over masonry and aerated blocks:
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They will match the existing structure for porosity and density, allowing moisture to move in a similar way.
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They won't introduce hot or cold spots where differential thermal movement can cause renders and plasters to crack.
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They have a similar compressive strength to the original cob and can therefore accommodate general movement better without detaching from the original structure.
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They allow recycling of material with savings in energy consumption.
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Cob blocks and bricks are ready dried and won't shrink away from adjoining surfaces.
We supply cob blocks made from a variety of soils, producing blocks of red, yellow or brown colouration if the need to match the existing cob is important.
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, treated hazel spars
or 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 beding 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 dependant 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 square metre of wall, 225mm thick - 20 blocks of size 18" x
9" x 4"
For 1 cubic metre of wall, - 80 blocks of size 18" x 9" x 4"
1 tonne of ready mixed 4/1 lime mortar will lay around 160 cob blocks,
depending on the shape of the repair and the thickness of the bedding
joints
Its 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.