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should extend outwards a minimum of 900 mm (more in highly
reactive soil) and should have a minimum fall away from the
building of 1:60. The finished paving should be no less than 100
mm below brick yent bases.

Ic is prudent to relocate drainage pipes away from this paving, if
possible, to avoid complications from future leakage. If this is noc
practical, earthenware pipes should be replaced by PVC and
backfilling should be of the same soil type as the surrounding soil
and compacted to the same density.

Except in areas where freezing of water is an issue, it is wise to
remove taps in the building area and relocate them well away from
the building — preferably not uphill from it (see BTF 19).

It may be desirable to install a grated drain at the outside edge of the
paving on the uphill side of the building. If subsoil drainage is
needed this can be installed under the surface drain.

Condensation

In buildings with a subfloor void such as where bearers and joists
support flooring, insufficient ventilation creates ideal conditions for
condensation, particularly where there is little clearance between the
floor and the ground. Condensation adds to the moisture already
present in the subfloor and significantly slows the process of drying
out. Installation of an adequate subfloor ventilation system, either
natural or mechanical, is desirable.

Warning: Although this Building Technology File deals with
cracking in buildings, it should be said that subfloor moisture can
result in the development of other problems, notably:

+ Water that is transmitted into masonry, metal or timber building
elements causes damage and/or decay to those elements.

¢ High subfloor humidity and moisture content create an ideal
environment for various pests, including termites and spiders.

* Where high moisture levels are wansmitted to the flooring and
walls, an increase in the dust mite count can ensue within the
living areas. Dust mites, as well as dampness in general, can be 2
health hazard to inhabitants, particularly those who are
abnormally susceptible to respiratory ailments.

The arden
The ideal vegetation layout is to have lawn or plants that require
only light watering immediately adjacent to the drainage or paving
edge, then more demanding plants, shrubs and trees spread out in
that order.

Overwatering due to misuse of automatic watering systems is a
common cause of saturation and water migration under footings. If
ir is necessary to use these systems, it is important to remove garden
beds to a completely safe distance from buildings.

Existing trees

Where a tree is causing a problem of soil drying or there is the
existence or threat of upheaval of footings, if the offending roots are
subsidiary and their removal will not significantly damage the tree,
they should be severed and a concrete or metal barrier placed
vertically in the soil to prevent future root growth in the direction of
the building. If it is not possible to remove the relevant roots
without damage to the tree, an application to remove the tree should
be made to the local authority. A prudent plan is to wansplant likely
offenders before they become a problem.

Information on trees, plants and shrubs

State departments overseeing agriculture can give information
regarding root patterns, volume of water needed and safe distance
from buildings of most species. Botanic gardens are also sources of
information, For information on plant roots and drains, see Building

Technology File 17.

Excavation

Excavation around footings must be properly engineered. Soil
supporting footings can only be safely excavated at an angle that
allows the soil under the footing to remain stable. This angle is
called the angle of repose (or friction) and varies significantly
between soil types and conditions. Remoyal of soil within the angle
of repose will cause subsidence.

 

Where erosion has occurred that has washed away soil adjacent to
footings, soil of the same classification should be introduced and
compacted to the same density. Where footings have been
undermined, augmentation or other specialist work may be required.
Remediation of footings and foundations is generally the realm of a
specialist consultant.

Where isolated footings rise and fall because of swell/shrink effect,
the homeowner may be tempted to alleviate floor bounce by filling
the gap that has appeared between the bearer and the pier with
blocking. The danger here is that when the next swell segment of the
cycle occurs, the extra blocking will push the floor up into an
accentuated dome and may also cause local shear failure in the soil.
Ifit is necessary to use blocking, it should be by a pair of fine
wedges and monitoring should be carried out fortnightly.

This BTF was prepared by John Lewer FAIB, MIAMA, Partner,
Construction Diagnosis.

 

The information in this and other issues in the series was derived from various sources and was believed to be correct when published.

 

The information is advisory. It is provided in good faith and not claimed to be an exhaustive treatment of the relevant subject.

 

 

Further professional advice needs to be obtained before taking any action based on the information provided.

 

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