A Tale of Two Leaks

December 12, 2013

By Philip O’Sullivan

Every so often a job comes along that is different.  This one involved leaks around showers in a large two level accommodation building with concrete floors.  What made it both interesting and difficult was the inherent risk of the closeness of the ground water table to the floor slab.

Showers were wetter on the ground floor than the first.  The ground floor leaks were generally more pronounced at both ends of the building.  Investigations included drilling observation water wells through the ground floor slab at each end and in the middle of the building and monitoring water levels over time during different weather conditions.  The ground water level was found to be relatively stable.  It was suggested there could be overloading of the subsoil drains causing occasional fluctuations.  Investigation trenches were dug, but nothing to support this as a cause was found.  We then used video cameras to inspect the subsoil drainage system and the adjacent stormwater system.  We found the subsoil drainage was working well, except in the middle of the building, where these drains were dry.  Here we found the ground water was leaking into the stormwater system, thus lowering the water table.

We destructively investigated one of the showers.  The under-tile waterproofing membrane was too thin, but we could not rule out the ground water being a contributing cause given the correlation between dampness to showers on the ground floor and the groundwater level.  We were faced with a dilemma!  Should we repair the showers with the risk of ground water causing further damage?

One of the showers had to be decommissioned due to mould contamination.  We were monitoring the moisture content in the bottom plates adjacent to the shower areas and found this particular shower was drying out.  This strongly supported the view that leaking through the membrane was the problem, but we still had a nagging doubt, given the close proximity of the water table.

This decommissioned shower then became damp again.  The volume of water indicated a plumbing leak rather than groundwater.  We decided to remediate this shower followed by others and to monitor the moisture content of the bottom plates to determine if groundwater was a contributing cause.  As we progressed we became more and more satisfied that groundwater was not a cause of the dampness.

In total 80 showers were rebuilt.  Of these, 90% had membrane failure, 20% had plumbing leaks and there was no contribution from groundwater.  The monitoring program also confirmed the repairs were effective.

Moisture monitoring, when undertaken in the right circumstances and for the right reasons, can have benefits.  The more important lesson is that undertaking remediation, rather than persisting with investigations is usually the more reliable way of determining the causes of faults.

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