Download Good riddance to

Maintenance
FEATURE
SECTION
Good riddance to
damp subfloors
Many homes with suspended timber floors suffer from damp. Once the
cause has been identified, there are ways to fix the problem.
BY TREVOR PRINGLE, ANZIA, BRANZ PRINCIPAL WRITER
OF THE 1.7 MILLION or so dwellings in New Zealand, about 1.2
million have suspended timber ground floors.
BRANZ House Condition Surveys carried out in 1994, 1999, 2005
and 2010 all identified a lack of subfloor ventilation and subsequent
higher moisture levels as a problem for houses with suspended floors.
Many homes too damp
Of the houses surveyed, 66% did not meet current ventilation
Blocked or obstructed ventilation openings
are a common cause of problems.
requirements, while 33% had less than a third of the current requirements. The resulting high level of moisture was a significant factor
in other deterioration, such as corrosion and decay.
In many houses with suspended floors, the timber or particleboard
is too close to the ground, restricting the potential airflow through
the subfloor. This makes the timbers and flooring more susceptible
to moisture damage when the ground is wet.
For villas and bungalows, in particular, it is not uncommon for
untreated subfloor timber joists and bearers to be in contact with
the ground.
Measurements show that, on average, 0.4 litres of water can
evaporate from a square metre of ground in 24 hours. On sites that
are damp or where water is being introduced into the subfloor space,
it is significantly more.
Look for signs of damp
Leak from hot water cylinder.
The most common indication that all may not be well below a floor
is a musty smell inside a building.
Build 135 — April/May 2013 — 63
FEATURE
SECTION
Maintenance
Other signs of subfloor problems are:
●●
the subfloor space airflow is obstructed by framing timbers too
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cupping of strip timber flooring
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patchy brown-coloured stains beneath clear floor coatings
close to or in contact with the soil, party wall or other foundation
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dampness in carpet and under rubber underlay
The subfloor may have wet soils from:
●●
blistering of sheet vinyl
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groundwater or a high water table
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persistent mould growth within the building
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surface water draining to and accumulating under the building
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excessive internal condensation, especially on windows.
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springs
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water seeping through foundation retaining walls
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water being wicked up from the ground through brick or concrete
Often too little ventilation or too much moisture
walls or by stored materials and accumulated debris.
There are several common reasons for high levels of subfloor
moisture.
masonry foundations
●●
downpipe discharge draining under the floor.
Ventilation issues can include:
Check also for leaks such as:
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no, or insufficient, ventilation
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waterpipe and wastepipe leaks into the subfloor
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existing ventilation openings are blocked or obstructed
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internal wet area leaks such as a leaking shower
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roof and wall cladding leaks, which may indicate a more serious
problem exists with the building.
Damp subfloor can cause problems
A damp subfloor can contribute to:
●●
unhealthy moisture levels within living areas
●●
deterioration of materials such as particleboard and timber
flooring, floor framing and corrosion of fixings
●●
tarnishing of foil floor insulation or wetting of bulk insulation,
reducing its effectiveness
●●
high moisture levels and condensation in the roof space of brick
veneer clad buildings where the veneer cavity is open to both the
subfloor and the roofspace – effectively creating a chimney effect
Leaking pipes can be obvious.
that transports moisture from the subfloor to the roofspace
●●
fungal growths within the subfloor.
Steps to reduce moisture
There are a number of checks to work through when remedying
new path, wall
and drain
dampness in the subfloor:
●●
Check the subfloor for signs of leaks from showers, wastes or pipes.
Fix any leaks before doing other work.
●●
Check that downpipes are connected to the drains and there are
no leaks into the subfloor from a blocked downpipe or stormwater
drain – check when it’s raining or use a hose to fill the downpipe.
●●
Provide adequate ventilation – at least 3,500 mm2 of net clear, open
area for every 1 m2 of floor area is needed. Lack of ventilation is
Figure 1: Reshaping the ground contour around the building will stop water accumulating in natural
depressions under houses.
64 — Build 135 — April/May 2013
the most common cause of elevated subfloor moisture.
●●
Cover the ground with a vapour barrier such as a polythene sheet
to reduce the amount of moisture evaporating into the subfloor
space. While this reduces the ventilation demand, it should be
used with at least minimum levels of ventilation.
●●
For buildings within a depression, reshape the outside levels to
reduce the potential for surface water to flow under the building
and install drainage channels or subsoil drains (see Figures 1–3).
●●
For springs, capture and pipe the water to the stormwater system.
house
natural ground
water
Figure 2: Before remediation – water drains to the house subfloor.
house
new wall
drainage channel
and path
Figure 3: After remediation – water is captured and drained away.
Build 135 — April/May 2013 — 65