Repairing flood damage to houses

By - , Build 85

New Zealand has a highly variable, and volatile, climate. Early this year, the lower half of the North Island experienced unseasonal heavy rainfall, causing widespread floods. Des Molloy checked out the repairs on some affected homes.

Devastating floods hit the lower North Island in February 2004.
Devastating floods hit the lower North Island in February 2004.
Everything must be removed before the repairs can start.
The plasterboard has been removed from at least 300 mm above the high-water line.
New insulation can be installed before the walls are lined.
The framing must be dry before the lining goes back on.

When a house has been flooded, the householders will be advised when they can return to it. This gives them a chance to recover items of immediate concern, but does not necessarily mean the house is safe or can be occupied. The local Territorial Authority (TA) will most likely have placed an ‘uninhabitable’ notice on the affected house, which means it cannot be lived in until the notice has been lifted. In the meantime much cleaning and drying needs to be done.

Check for contaminants

Before starting the clean-up, check the property to make sure there is no immediate danger to the people involved. Floodwater is often contaminated by dead animals and sewage, so strict health precautions must be followed. Bury all accumulations of faecal matter and animal bodies as a first priority and drink only purified water until the normal water supply is safely drinkable.

What can’t be dried must be disposed of

Everything that is wet should be taken outside for cleaning and drying whenever weather permits. Dispose of any absorbent items that have been saturated by contaminated waters and cannot be cleaned. Leave nothing inside the house that can trap moisture and prevent or slow the drying-out process.

Remove wet plasterboard

Gypsum plasterboard has a low tolerance to water and will almost always have to be removed and replaced if it has been immersed in water. Depending on the wall finishes being reinstated, it may be possible to remove a strip 300 mm wider than the flood-damaged zone. For a high level of finish, a better result will be achieved if full sheets are removed and replaced after the framing has dried.

Remove wet chipboard

Items made from composite wood materials, like MDF or particle board, have a better but still low tolerance to being immersed in water and will need replacing if the flood water has caused the fibres to swell. If particle-board floors are under water for less than a week they may remain serviceable but the floor must be dry before its strength can be checked. Drying of the floor can be helped by sanding or using a heat gun to remove any varnish or sealer. Make sure there is good ventilation both inside the house and under the floor, and lift water-resistant floor coverings.

Everything must be removed before the repairs can start.
The plasterboard has been removed from at least 300 mm above the high-water line.

Wait for everything to dry

Once all the wet materials have been removed and the house has been thoroughly cleaned, drying out can begin. Be warned that the drying out, particularly in winter, can take 3–4 months or even longer. Re-lining should not be carried out until the moisture content in the timber wall framing has dropped to 12–16%.

Check the electrics

Appliances within the house must also be removed for cleaning if they were covered by floodwaters. They may be able to be carefully cleaned, decontaminated and checked before being reinstalled and/or reused. It is probable that flooded electrical outlets and fittings will need to be replaced, but the wiring may well be usable. Fixed electric and gas heaters, hot water cylinders, central heating systems, central vacuum systems, wood burners, and toilet pans are other items that will need to be cleaned and checked before re-use.

Start reinstating

When reinstating insulation, take the opportunity to upgrade to the best practice recommended by SNZ/PAS 4244: 2003 (Insulation of lightweight-framed and solid-timber houses) if possible. (This recommends R2.6 for walls, R3.3 for roofs with non-solid wall construction, and R4.6 for roofs with solid wall construction.)

New insulation can be installed before the walls are lined.
The framing must be dry before the lining goes back on.

BRANZ recommends all reinstatement work be carried out in accordance with the requirements of the current building code, including upgrading to current bracing requirements wherever possible. Where appropriate, add tie-down straps between the studs and bottom plates. Check the number of foundation holding-down bolts and add to them if necessary.

Under the floor, insulation that has been removed should be replaced. Under most floors the perforated foil can be fixed directly to the underside of the floor joists, but adding timber battens will prevent fixings from pulling through. The use of purpose-made polystyrene panels or glass-fibre insulation fitted between the joists is an alternative but they must be a tight fit between the floor joists.

If the remedial work involves replacing ‘like for like’ there may not be a requirement for a building consent, but the TA should still be consulted.

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Articles are correct at the time of publication but may have since become outdated.

Devastating floods hit the lower North Island in February 2004.
Devastating floods hit the lower North Island in February 2004.
Everything must be removed before the repairs can start.
The plasterboard has been removed from at least 300 mm above the high-water line.
New insulation can be installed before the walls are lined.
The framing must be dry before the lining goes back on.

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