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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 ●● cupping of strip timber flooring ●● patchy brown-coloured stains beneath clear floor coatings close to or in contact with the soil, party wall or other foundation ●● dampness in carpet and under rubber underlay The subfloor may have wet soils from: ●● blistering of sheet vinyl ●● groundwater or a high water table ●● persistent mould growth within the building ●● surface water draining to and accumulating under the building ●● excessive internal condensation, especially on windows. ●● springs ●● water seeping through foundation retaining walls ●● 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: ●● no, or insufficient, ventilation ●● waterpipe and wastepipe leaks into the subfloor ●● existing ventilation openings are blocked or obstructed ●● internal wet area leaks such as a leaking shower ●● 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