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DIY
CRACKING CHECKLIST
If
a crack appears overnight in your previously intact brick house,
don't panic. It is not the first step in the total disintegration
of your whole home.
Most cracks require
cosmetic treatment only. The majority are not indications of grave
structural damage. Remedies can be as simple as watering the garden
a bit more or less, or just waiting for the season to change.
However it can
be valuable, if only for peace of mind, to be able to recognise
and evaluate the type of cracks. You will then have some logical
basis for deciding which method of correction will be appropriate.
Unfortunately
lack of knowledge can lead to the adoption of unnecessarily expensive
treatments which may be suggested by firms biased in favour of
using their own patented and profitable 'cures'.
An independent
assessment should be sought first from your local council, Archicentre's
Inspection Service or a structural engineer.
The form it takes
Cracking can be vertical, horizontal,
cogged, stepped or a combination.
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The form it takes is sometimes a
clear indication of the problem.
For example,
a diagonal crack starting at the corner of a door opening
is sure to have been created by movement in the footings.
It is also informative to watch what the crack does over
various seasons.
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1) Types of Cracking |
Footings and foundations
These terms are commonly confused. In
this Technical Information Sheet, 'footings' refers to the structure
below the floor level that rests on the soil. 'Foundations' are
the soil or rock state on which the footings rest.
What type of foundations is my house
built on and can they move?
How much foundations move depends on
the type of rock or soil and how that type is influenced by changes
in moisture, temperature and imposed loads.
The movement on
the foundations is rarely uniform over the whole of the building
site, and different movement under different parts of the footings
creates stresses in the walls resulting in cracks.
Foundations are
usually classified as either reactive or non-reactive to changes
in their moisture content.
Reactive soils
are typically clay soils, but also include the 'black soil' and
'black earth' found in Queensland, South Australia and western
New South Wales. All of these are plastic soils, shrinking rapidly
as their moisture content decreases and swelling as it increases.
For example, reactive
clays may swell and retain moisture when saturated which can cause
deformation, particularly in modern houses with concrete slab
floors, and shrink and collapse when water is removed from them
by excessive evaporation or by the action of trees.
Non-reactive soils
are such soils as rock, gravel, shale, phyllite or sand. Their
volume does not increase or decrease depending on the moisture
content.
The Building or
Engineers Department of your local Council, Shire or Building
Authority should be able to provide information for you on the
type of soil your house is built on. This information can be useful
as cracking caused by drying-out of normal reactive soils can
often he simply remedied, whereas owners of houses built on very
reactive soils or on filled land which has subsided permanently
may have to pursue more elaborate solutions such as underpinning.
Different types of soil move, but
what causes them to move? What are the remedies?
Movement in foundations is caused principally
by:
1. Moisture movement in reactive soils.
2. Uneven settlement of the foundations.
3. Sliding surface layers.
4. Building on variable foundations.
5. Additions to existing buildings.
6. Excessive vibration.
1. Moisture movement in
reactive soils
Experts suggest that change in
the water content of clay-type soils cause up to 90% of all cracking
problems in houses. So what factors affect the water content in
the soil?
(a) Water extracted by trees and shrubs
The greatest damage occurs when trees are planted
after construction .
Contrary to popular belief, it is rarely the
growth or uplift of the tap roots of big trees which disturb the
foundations. The damage is done by the plants extracting considerable
quantities of moisture from the soil that reduces the volume of
the soil, causing footings to subside in that area and cracks
to appear in the masonry.
This phenomenon is known as tree drying settlement.
The way it works is the tree takes water from the soil by a system
of fine hair-like roots. The moisture is then transported to the
trunk of the tree via the main tap roots. The astonishing fact
is that, depending on the type and age of the tree, this fine
root system can extend over several hundred metres and the amount
of water transpired daily by a tree can be several hundred litres.
A more commonly accepted formula is that a tree's
root system will extend for a horizontal distance equal to its
height. In a line of trees, where competition for water exists,
the horizontal root spread is one and half times the tree height.
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Of course, if the water available in the
soil is sufficient to meet the tree's transpiration rate,
no damage will occur. But a dry season, even in a temperate
zone, can alter this balance.
Diagram 3 shows the typical damage resulting
from a large tree having been planted too close to the footings
at a corner of a brick house.
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2) Root spread of a typical tree
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Some remedies
Now the good news.
If you take moisture out of clay soils, you can put it back.
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In one case seen by Archicentre a row
of large trees within 3 metres of a house had, during
a prolonged dry spell, caused substantial cracking and
distortion of the wall nearest to them. The trees were
removed and the area flooded with water. Within two months
the clay soil had absorbed the water and heaved some 35mm
to the original position, closing nearly all the gaps
completely.
3) Left: Cracking due to tree damage
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So, if some cracks appear in your house for the
first time during a dry season, the best thing is to water the
soil and do nothing more. Wait until after the next wet season
and see if they close up by themselves. Try and keep the soil
near any large trees constantly wet and don't be tempted to fill
the cracks with anything rigid which will prevent them closing
naturally, as more cracking could develop in response elsewhere.
If cracks are more serious, generally large enough
to insert one or more fingers, the tree, shrub or root system
responsible for the damage should be removed. Pruning is of short-term
value only. Local flooding of the area will accelerate the heave
recovery of the foundations.
Alternatives to cutting down the trees are root
barriers, made from concrete or other impermeable materials such
as fibre cement sheet wrapped in plastic and inserted to a depth
greater than the surface root system of the appropriate tree,
between the tree and the affected footing.
Another remedial system aimed at retaining the
trees, is to drill holes into the soil with an auger approximately
1.5m deep and close to the wall which has subsided and cracked.
Into these holes is poured water to a constant level to get moisture
back to the soil, and also copper sulphate or other root suppressant
chemicals to repel the approach of the tree's roots.
In conjunction with this, holes are drilled into
the soil on the side of the tree away from the house. These holes
are then filled with water and nutrients that will attract tree
roots in their general direction.
This system does require maintenance in keeping
the holes filled with water (although this could be accomplished
with an automatic watering system) and replenishing supplies of
repellent chemicals and nutrients.
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Finally, we come to the remedy known as
underpinning, an expensive operation and only to be undertaken
if all else fails. Underpinning can mean two things: first,
the removal of soil below an existing footing and its replacement
with (usually) concrete. The second is the removal of the
lower defective part of the wall and its replacement with
masonry.
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4) Underpinning
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Although underpinning is not a complicated operation,
it is slow and awkward and must be done properly to be effective.
The new footing system must be designed properly to start with
if problems are to be avoided. It would be wise to ask to see
the Engineer's drawings of any proposal.
It would also be wise to obtain several quotations
from firms specialising in underpinning. Ask to see successfully
completed jobs and make sure a guarantee will be forthcoming.
It is essential that a building permit from the local council
be obtained for any underpinning work.
A word of warning at this point about organizations
which offers cure-alls for cracking. Archicentre has found that
some of these firms exaggerate the dangers involved with some
small cracks and recommend expensive patent treatments or unnecessary
underpinning where the cracks might simply respond to a bit of
water added to the soil.
Remember that a company offering a particular
treatment will probably be biased in favour of using it regardless.
A non-involved, experienced person such as an
Engineer or Architect or the local council should be consulted
first. They won't be inclined to overlook the more humble remedies.
Planting trees
When planning to plant trees, the idea is to
also consider the strength of the footings supporting the house.
For example, an old brick house with stone footings
on a clay foundation offers little resistance to drying settlement
whereas a well designed concrete slab may tolerate even a heavily
planted garden. As a rule of thumb, trees should be planted at
a distance from the house equivalent to their mature height, although
trees may be planted closer and culled as they grow.
Trees to be wary of:
The following is a list of common trees and
plants to be wary of. The list is not meant to be totally comprehensive
but can be taken as a general guide. Further information may be
available from your local building authority or plant nursery.
(Note: Height of tree and spread of roots can
vary dramatically from region to region depending on climate,
soil type, rainfall, etc.)
CLICK HERE TO SEE A LIST
OF TREE HEIGHTS
(b) Damage caused by solar radiation
While drying out
of soil by trees is the most common reason for cracking in brickwork,
some clays are also very susceptible to drying out by direct solar
radiation.
The northern facing wall is the most likely to
be affected and stepped diagonal cracking is the most common symptom,
usually occurring at the north east and north west corner of the
building.
Again underpinning should be considered only
as a last resort and more simple remedies tried first.
Mulching of garden beds around the house will
cut down loss of moisture by solar radiation. Another measure
is to provide an impermeable ground cover around the house together
with a vertical impermeable barrier or border which should preferably
be taken down to a depth where the moisture content of the soil
is constant, approximately 300mm to 600mm.
(c) Migration of moisture
The movement of moisture beneath a building
can produce the phenomenon known as long term dome and saucer
effect.
The dome effect is a slow heaving of the soil
caused by movement of moisture from the perimeter of the house
to its centre, and the saucer effect is moisture moving in the
opposite direction, from the centre towards the perimeter.
The dome effect causes the walls to tilt outwards
and the restraining influence of the roof produces horizontal
cracks on the outside wall. These will be wider on the outside
surface rather than the inner.
5) Long term dome effect |
6) Long term saucer effect
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The long term saucer effect causes the external
walls to tilt inwards and partition walls to sag. Cracking may
be similar to that seen with the dome effect but the width of
the cracks will be greater on the internal surface of the walls.
The most economical solution, and one which gives
good results where damage is not too severe, is to improve the
sub-floor ventilation so that the atmospheric conditions under
the floor and those outside are not too different.
2. Uneven settlement of foundations
Where a particularly heavy load
is placed on the foundation such as a large column, movement may
occur as moisture is squeezed out of the soil or the soil readjusts
itself. This consolidation will stop when the soil has finally
compacted enough to support the load.
Cracks, which may result from movement during
compaction, could be measured to see if and when the movement
has ceased.
Then a decision can be made as to whether the
cracks may simply be patched up or if the size of the existing
footing needs to be increased to support the load.
3. Sliding surface layers
Overloading can also cause shear
failure in the soil. The soil can slip in a downward, sideward
and upward movement allowing the footings to settle as a result.
A typical example may be caused by an excavation
on an adjacent site to a greater depth than the footing, thus
robbing it of lateral support and causing it to tilt towards the
hole.
Flooding or diversion of natural drainage channels
beneath the footing can produce shear failure as some soils, especially
clay, lose cohesion when too much water is added to them.
The cracks occurring will more often be vertical
than diagonal.
Service pipes in the ground should be carefully
checked to see that they are unbroken. This check should also
cover existing drains, down pipes and gutters.
4. Building on variable foundations
Finding part-rock, part-shale,
or part-clay on a flat site is possible but more probable on a
sloping site where part of the slope has been cut into and the
material cut out has been used as fill to extend the horizontal
pad for the house.
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The fill can compress more readily and
uneven settlement can 'bend' the house at the point where
the two materials meet. The resulting cracks will be vertical,
wider at the top than at the bottom.
The remedy may have to be underpinning
or some comparable method to spread the load more evenly.
7) Right: Cracking due to cut and fill
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Houses built on fill only will be subject to
settlement that will frequently be uneven. If the fill is loose,
but not uniformly so over the site, the cracking will be extensive
and unpredictable.
Loose but uniform fill can produce either stepped
or vertical cracks.
Pier and beam footings can be used to underpin
the building or, if the fill is reasonably compact, widening existing
strip footings may be sufficient.
Either solution requires design by an engineer
and implementation by a reputable underpinning contractor.
5. Additions to buildings
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Building an addition onto a house can impose
a load intensity on the soil different to that which is
there already and so cause differential settlement. Even
if the loading intensities are similar, the difference in
time between when the two settlements occurred can be enough
to create cracks.
These may be vertical or cogged and normally
occur near where the new work keys into the old.
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8) Cracking due to an addition
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Measuring the crack to assess when the new settlement
is complete and then patching the crack is probably the best method.
Again underpinning the new work should only be contemplated in
an extreme case.
6. Excessive vibration
Damage caused by vibration from
earth tremors, heavy traffic or pile driving is fairly rare, however
if the vibration is great enough to actually cause the foundation
to move, cracking can occur. In this case a rock foundation is
not the best as it can easily transmit vibrations to the building.
The cracks show up irregularly and if the source
of the vibration can't be removed, a possible remedy is the installation
of anti-vibration mats beneath the footings. If the source of
the vibration is some machine operated within the building, these
mats could be placed between the machine and the floor.
Some general maintenance tips
1. Don't plant trees or allow them to exist
closer to the house than their natural height unless their roots
are discouraged or contained in some way as in the systems described.
2. Keep the garden and lawns around the house
evenly damn throughout the drier months. Don't neglect one side
of the home just because nothing much grows in the area.
3. Regularly check existing drains, downpipes,
guttering and service piping to ensure no leakages occur over
the life of the building.
4. If you think your house needs underpinning
or other building work, make sure you have an independent assessment
undertaken first by an architect or engineer. Then obtain at
least three quotations and make sure you have a firm written
contract with the company. Archicentre can help you here with
an inspection and recommendation.
All the above information relates to cracking
associated with movement of the foundations caused in the most
part by drying out of the soil.
Statistics compiled by Archicentre show that
cracking from other causes is relatively rare, but can occur when
various elements of the building itself move. For example movement
of steel frames with brick infix panels or the shrinking of concrete
elements.
Cracking can also be due to the expansion or
shrinkage of the clay brickwork itself.
Remember that the best way of avoiding
expensive problems is to be aware of them before you purchase
your house. Archicentre has carried out thousands of home inspections
for home owners and prospective buyers, to help them make a realistic
appraisal of the property before buying, renovating or repairing.
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