Flood damage to walls

Identifying problem areas of flood-damaged walls.

Cracks in plasterboard

Cracks in plasterboard may derive from a number of sources including:

  • damp wall cavities
  • differential pressure during the flood
  • movement of foundations (framing/stump)
  • physical damage when wet.

Repairs should not be made until the wall cavity and the rooms are well dried out to ensure that new plasterboard is not damaged by moisture. If cracking occurred due to the flood, there is no reason why further cracking should happen.

If damage was associated with differential movement, it is important that sub-repairs are made (if appropriate) and sub-floor stability returns before making any repairs to lining. Sub-floor stability may only return when the sub-floor has dried out significantly (a period of a few months depending on weather).

Damp wall cavities

A wall cavity needs to be thoroughly cleaned of mud immediately post-flood and allowed to dry out. However, once this has been done and damp wall cavities persist it may be associated with either:

  • trapped mud or moist material
  • moisture
  • inadequate ventilation.

These problems need to be resolved and the wall cavity then once again opened up (by removal of skirting boards). The wall cavity should then dry out over a period of three to six weeks.

Differential pressure

In a fast rising flood, the water level in the wall cavity may be lower than both within the house and outside. As a result, a differential pressure may be set up between the wall cavity and the interior. This pressure is sufficient to crack the plasterboard along its bottom edge. This problem could be avoided if a small gap is left between the skirting board and the floor.

Door and window jamming/warping

Warping and jamming of doors and windows immediately post-flood is common. This may be caused by either:

  • general movement of the building
  • warping of the door/window or the framing
  • damp wall cavities.

It is advisable to assess whether there has been general movement either by:

  • indirect observations such as general cracking in walls or bouncy floors
  • direct examination of sub-floor.

If there is general movement, this must be repaired before repairing doors, for example. If the damage is local, the window or door may, depending on the material, return to its correct shape with time. For timber products or hollow doors this is unlikely except if the flood was short. However, for solid timber it is quite probable. Therefore, it is recommended that ventilation be encouraged and that solid window frames or doors be allowed to dry for up to a month prior to repairs. If the window/doors have not returned to shape after this time, repairs will be needed.

Flaking and blistering paint on plasterboard

Flaking or blistering of paint on plasterboard may either arise as a result of the flood or it may signal a dampness problem associated with either:

  • damp wall cavities
  • hidden brickwork or chimneys.

Clean off the flaking or blistered paint, but do not immediately repaint. Ensure that air flows freely over the wall and that the wall cavity is well ventilated and cleared of water and mud. Wait at least two weeks and then if the plasterboard is dry it is probably safe to repaint. If it is still not yet dry, check for damp wall cavities or hidden brick work. Note, depending on environmental conditions, it may take many months for structures to dry completely. While unsightly, it is best to delay painting while drying is completed.

Hidden brickwork and chimneys

In older houses it is not uncommon to find one wall which has a mould problem when all other walls are dry and free from mould. Frequently this is associated with a brick structure (most frequently chimneys) which has been covered over by the lining. This structure is most often drawing moisture up from the damp sub-floor and sub-floor soil. If the sub-floor and soil dries out, the problem of the mouldy wall will often disappear. However it must be remembered that it may take up to two or three months for the sub-floor to dry out completely. If the problem of mouldy walls persists despite drying out the sub-floor then it may be necessary to place a damp-proof course in the brick structure. Expert advice should be consulted in this circumstance.

Often old chimneys have been plastered over. The brick work will not have a damp proof course and thus will permit moisture to move from ground up to the wall and can damage the plaster.

See also Flood damage to floors: Sub-floor moisture.

Lack of ventilation

If a general mould problem persists after following the procedures to clean wall cavities, then it is likely that the cavity has insufficient ventilation to dry out. This problem may be associated with 'blocked cavities' or just to general low level of air movement.

Cavities may be blocked locally or generally. The most common form of general cavity blocking is caused by tapping the sarking over the cavity at the ceiling. The sarking should be tapped back so the cavity freely vents into the roof space.

Local blockages may be associated with windows where the window bases effectively closes the cavity.

Lack of cavity ventilation is best remedied by removing any general blockages. If the problem still persists, add additional vents under windows if mould growth occurs there locally, or just below the ceiling if mould growth is more general.

Skirting boards should be removed (for a period of up to two months) to further encourage air movement.

Mould on plasterboard

Mould may occur as a result of the high humidity immediately post the flood. Immediately following the flood, deposited mud and silt should be removed from floor and wall cavities. Following this action (and if wall linings are not to be removed), mould should be cleaned off as soon as possible using bleaches or methylated spirits. If the mould re-occurs (after two to three days of cleaning) it indicates that there is an ongoing problem.

This could either be:

  • damp wall cavities
  • hidden brickwork or chimneys.

It is necessary to resolve the ongoing problem before proceeding further. Once the ongoing problem is resolved the mould can then be cleaned and should not re-occur. If it does, then expert assistance may be required. Once the wall is cleaned of mould then it should be allowed to dry prior to painting.

Movement of framing and stump

Subsidence of foundations may occur following a flood and may lead to a number of problems in the dwelling including:

  • cracks in walls
  • door and windows jamming.

Such subsidence may result from the inability of damp soil to sustain the building load. Alternatively, the foundations themselves may be damaged from the flood. A common occurrence is the failure of timber stumping. Often such stumping was already in poor condition prior to the flood and the flood simply 'finished it off'.

The best way to limit such problems is to ensure that:

  • foundations are in a good condition prior to the flood
  • sub-floors drain properly
  • water does not remain trapped underneath the house.

Poor tiles, glues or backing

Bathroom walls should have a moisture resistant backing and membrane. In the absence of such a membrane, and if a backing is not moisture resistant, then the backing sheet can accumulate moisture leading to subsequent tile failure.

Good quality adhesives and moisture resistant tiles should not fail in a moderate flood. However, glues and tiles are of variable quality and poor tiles/adhesives will fail. Therefore it is recommended that quality backing, adhesives and tiles be used.

If a failure has occurred, and a substantial fraction of tiles have failed then the whole system including backing should be replaced. If only a few isolated tiles have failed then these can be replaced individually. However, be sure that the backing is dry prior to replacement and use moisture resistant glues.

Trapped mud, moist material or moisture

If the internal lining was removed, it may still be possible that mud may have accumulated on top of noggins at various positions up the wall.

A horizontal mould patch on the internal lining may indicate that mud remains on the noggin. Extra efforts should be made to clean this mud off by hosing down from the top of the cavity.

If steel framing is being used, mud may accumulate in the U-profile of both the bottom plate and the cross beams. This will be difficult to clean out.

If the problem persists, either with timber or steel framing, then the lining should be removed to permit direct removal of mud.

Wall tiles lifting

Wall tiles lifting or cracking may be associated with:

  • damp conditions
  • poor glues/tiles/backing
  • differential movement of foundations leading to wall movement.

In the case of differential movement, tile cracking will just be one of a number of systems (cracks in walls) and tiles should not be repaired before the foundation problem is resolved.

If the wall is damp and/or mould growth is evident, it is likely that the problem is associated with moisture induced glue failure or moisture induced tile expansion. Bathroom walls should have a moisture resistant backing and membrane.

In the absence of such a membrane and if the backing is not moisture resistant then the backing sheet can accumulate moisture leading to subsequent tile failure. Good quality adhesives and moisture resistant tiles should not fail in a moderate flood. However glues and tiles are of variable quality and poor tiles/adhesives will fail. Thus it is recommended that quality backing/adhesives and tiles be used. If a failure has occurred, and a substantial fraction of tiles have failed then the whole system including backing should be replaced. If only a few isolated tiles have failed, then these can be replaced individually. However, be sure that the backing is dry prior to replacement and use moisture resistant glues.

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