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Mould grows where the water gets in

Mould needs moisture, and in a house it arrives five ways: from the roof and gutters, from the pipes, out of the air indoors, over the floor in a flood, and up from the ground. NSW Health says to remove mould as soon as it appears and then to treat the cause, because the patch is likely to return until the moisture that fed it is dealt with.

General information from official health, building and housing sources, not advice about your home, your health or your tenancy. The page behind most of this one is NSW Health’s mould factsheet.

Water from a leaking roof, reaching the ceiling: NSW Health lists a leaking roof and a blocked rain gutter among the water leaks and plumbing problems to repair.Water from a gutter fault, into the top of the outside wall: Consumer Protection WA’s list of what a landlord should do to avoid mould developing includes cleaning and maintaining the gutters, so rainwater stays out of the roof cavity.Where a roof or ceiling leak shows: NSW Fair Trading’s own inspection list says to check for damp or mould along skirting boards, walls and ceilings.Where a gutter fault shows: a discoloured or bulging patch that may be cold or damp to touch, in the wall or roof cavity (Consumer Protection WA).Roof spaceRoof and gutters Shower steam, and the bathroom exhaust fan the building code sizes: the 2025 National Construction Code sets a minimum exhaust flow rate, 25 litres a second for a bathroom or sanitary compartment.Signs in a wet area: the SA Government’s inspection list says to look for mouldy walls, lifting tiles, peeling paint or pools of water.BathroomThe air Moist air reaching cold window glass: the national home design guide says condensation forms when humid air meets a cooler surface, such as a window in winter.Condensation and mould on the window: the national home design guide says aluminium frames form a thermal bridge into the home and are prone to condensation.Moist air reaching the outside wall behind a wardrobe: one of Homes Tasmania’s tips for a dry, well-aired home is to allow space between furniture and the wall so air can flow, and it calls this very important with wardrobes.Mould on the outside wall behind the wardrobe: one of Homes Tasmania’s tips is not to stack boxes or other items against outside walls, especially walls that do not get a lot of sun.A small patch on a bedroom wall: NSW Health says it is good to remove mould as soon as it appears.Bedroom Water from a concealed pipe leak, down inside the wall: SA Water says damp, warped or discoloured walls, floors or carpets can suggest a concealed leak indoors.Where a concealed leak shows: Icon Water says to look in the kitchen, bathrooms and laundry for dripping taps, washing machine and dishwasher connections, and damp patches on the walls.KitchenLaundryPipes Flood water in the home (the line marks no depth): Victoria’s health department says flooding can cause excessive mould growth, which has to be cleaned up before moving back in.A floor joist with staining or decay: the Queensland timber guide says a surface stain that scrapes off to sound timber underneath is not of concern, while wood that is soft and spongy or easily penetrated has decayed.Flood water Damp rising from the ground into the base of the wall: NSW Health says rising damp is ground moisture rising up a brick or stone wall.Where rising damp shows: the base of the wall and the skirting, where Consumer Affairs Victoria says damp brick walls can indicate rising damp or salt damp.Damp ground under the floor: Homes NSW says rising damp is moisture from the ground travelling up into a home through the bricks or stone in its walls.A damp subfloor: NSW Fair Trading’s inspection list says to lift carpet and check for rotting floorboards or damp.SubfloorThe ground

Water the building let in

Water the household made

The split follows where the water came from. Queensland’s tenancy authority says the property owner is typically responsible where mould is caused by problems with the property, such as a leaking roof or leaking pipes, and the tenant typically is where it is caused by the tenant’s actions; Victoria’s consumer body says the rental provider may be responsible where a problem with the property caused the mould and has not been fixed, and the renter may be where their actions contributed; and in Western Australia, Consumer Protection generally holds the landlord responsible when the mould comes from poor ventilation, for example windows that can’t be opened securely or no exhaust fans in wet areas, while a tenant may be responsible when the growth comes from damage they caused or from not keeping the home clean and ventilated. Who fixes mould in a rental

Where the signs show What to look for when buying

Flood water (the line marks no depth)

Water from a leaking roof, reaching the ceiling: NSW Health lists a leaking roof and a blocked rain gutter among the water leaks and plumbing problems to repair.Where a roof or ceiling leak shows: NSW Fair Trading’s own inspection list says to check for damp or mould along skirting boards, walls and ceilings.Water from a gutter fault, into the top of the outside wall: Consumer Protection WA’s list of what a landlord should do to avoid mould developing includes cleaning and maintaining the gutters, so rainwater stays out of the roof cavity.Where a gutter fault shows: a discoloured or bulging patch that may be cold or damp to touch, in the wall or roof cavity (Consumer Protection WA). Shower steam, and the bathroom exhaust fan the building code sizes: the 2025 National Construction Code sets a minimum exhaust flow rate, 25 litres a second for a bathroom or sanitary compartment.Signs in a wet area: the SA Government’s inspection list says to look for mouldy walls, lifting tiles, peeling paint or pools of water. Moist air reaching cold window glass: the national home design guide says condensation forms when humid air meets a cooler surface, such as a window in winter.Condensation and mould on the window: the national home design guide says aluminium frames form a thermal bridge into the home and are prone to condensation.Moist air reaching the outside wall behind a wardrobe: one of Homes Tasmania’s tips for a dry, well-aired home is to allow space between furniture and the wall so air can flow, and it calls this very important with wardrobes.Mould on the outside wall behind the wardrobe: one of Homes Tasmania’s tips is not to stack boxes or other items against outside walls, especially walls that do not get a lot of sun.A small patch on a bedroom wall: NSW Health says it is good to remove mould as soon as it appears. Water from a concealed pipe leak, down inside the wall: SA Water says damp, warped or discoloured walls, floors or carpets can suggest a concealed leak indoors.Where a concealed leak shows: Icon Water says to look in the kitchen, bathrooms and laundry for dripping taps, washing machine and dishwasher connections, and damp patches on the walls. Flood water in the home (the line marks no depth): Victoria’s health department says flooding can cause excessive mould growth, which has to be cleaned up before moving back in.A floor joist with staining or decay: the Queensland timber guide says a surface stain that scrapes off to sound timber underneath is not of concern, while wood that is soft and spongy or easily penetrated has decayed. Damp rising from the ground into the base of the wall: NSW Health says rising damp is ground moisture rising up a brick or stone wall.Where rising damp shows: the base of the wall and the skirting, where Consumer Affairs Victoria says damp brick walls can indicate rising damp or salt damp.Damp ground under the floor: Homes NSW says rising damp is moisture from the ground travelling up into a home through the bricks or stone in its walls.A damp subfloor: NSW Fair Trading’s inspection list says to lift carpet and check for rotting floorboards or damp.
  1. Roof space Roof and gutters Is it the roof?
  2. Bathroom The air Why does mould keep coming back in the bathroom and wardrobe?
  3. Bedroom wall and wardrobe The air Can I clean mould myself?
  4. Kitchen and laundry wall Pipes Is it a leaking pipe?
  5. Floor Flood water What to dry, clean and throw out after a flood
  6. Subfloor The ground Is it rising damp?

Water the building let in

Water the household made

The split follows where the water came from. Queensland’s tenancy authority says the property owner is typically responsible where mould is caused by problems with the property, such as a leaking roof or leaking pipes, and the tenant typically is where it is caused by the tenant’s actions; Victoria’s consumer body says the rental provider may be responsible where a problem with the property caused the mould and has not been fixed, and the renter may be where their actions contributed; and in Western Australia, Consumer Protection generally holds the landlord responsible when the mould comes from poor ventilation, for example windows that can’t be opened securely or no exhaust fans in wet areas, while a tenant may be responsible when the growth comes from damage they caused or from not keeping the home clean and ventilated. Who fixes mould in a rental

Where the signs show What to look for when buying

Flood water (the line marks no depth)

The whole house

Read the drawing

  1. Roof and gutters

    NSW Health lists a leaking roof and a blocked rain gutter among the water leaks and plumbing problems to repair, and Queensland’s tenancy authority says a roof has to keep the rain out and the ceiling and walls must not be affected by significant dampness.

    Is it the roof?

  2. The air

    Victoria’s health department names the common causes of indoor mould growth: condensation from cooking, showering and clothes drying, and rooms where air does not circulate. The national home design guide says windows and doors are common locations for condensation, and that a thermal bridge can lower a surface’s temperature enough for condensation to form.

    Bathroom and wardrobe mould

  3. Pipes

    SA Water gives the signs of a concealed leak: damp, warped or discoloured walls, floors or carpets indoors, and outside, wet paths, unusually green grass or damp soil.

    Is it a leaking pipe?

  4. Flood water

    After flood water has been in a home, Victoria’s health department says the mould it leaves has to be cleaned up before anyone moves back in, and Tasmania’s health department says the house should be cleaned and dried as fast as possible, within 24 to 48 hours.

    Mould after a flood

  5. The ground

    NSW Health says rising damp is ground moisture rising up a brick or stone wall, and that poor sub-floor ventilation or moisture in the sub-floor area makes it worse.

    Is it rising damp?

Small patches

Victoria’s health department says small or medium sized patches of mould can be tackled at home, and that household detergent or white vinegar is enough in many cases. NSW Health gives the dilution in parts: four parts vinegar to one part water for routine clean-up on a mouldy surface. The ACT’s health page repeats that mixture.

Where a patch does not come off and the item cannot be thrown out, NSW Health moves to bleach: 250 mL of bleach in 4 litres of water, with PVC or nitrate rubber gloves, safety glasses and safety shoes, in an area kept well aired. The ACT gives the same dilution and the same kit. Tasmania’s health department differs on strength: one part bleach to three parts water, and it asks for rubber gloves, safety glasses and a face mask, with the room kept ventilated.

The difference between the state pages is mostly what comes first. Western Australia’s health department starts with scrubbing: soapy water and a brush, working up to 50 cm past the edge of the visible mould, then lifting the loosened residue with a damp cloth or a vacuum with a HEPA filter, then drying the area. The same page also notes that the scrubbing does the real work, because every trace of the mould has to come off physically or it will grow back. Each of the health pages that give a method ends in the same place: find the water, or the patch returns.

Can I clean mould myself?

When the pages say stop

  • WA Health draws the line for cleaning it yourself at one square metre. Contamination no greater than that, new, isolated and visible, is part of what its guidelines call a simple mould problem, which also requires that no other hazards such as asbestos or toxic chemicals are involved; larger than one square metre, or found in more than one room, it calls complex. The same guidelines call the figure a recommendation rather than a health threshold, and say a larger area may still be dealt with without professional help.
  • Victoria’s health department says some people should not take part in mould removal or be in the room while it happens: pregnant women, children, and anyone with a weakened immune system or a chronic lung disease.
  • Consumer Protection WA says a mould problem that covers a large area, or that keeps coming back after cleaning, is one for a mould removal professional.
  • In a home that has taken water, the Queensland timber guide treats soaked timber as a structure question: wood that is soft and spongy or easily penetrated with a knife has decayed, and lining and insulation that were under water usually have to come out and be replaced. In houses built after about the mid 1970s the lining may also be the wall bracing, so where it is stripped the same guide says temporary diagonal timber braces should be nailed to each wall until permanent bracing and lining go back.
  • After flood water, Victoria’s health department says to treat a home as contaminated with mould if it has been under water for more than two days, if mould can be seen, if water has caused damage, or if a strong, musty smell is present.

If the building might contain asbestos, the Queensland building commission’s guide to flood-damaged timber gives its own advice for sheeting that may contain it: do not sand or cut those products, and leave their removal and disposal to a specially licensed (Class B) contractor. The federal safety agency’s advice for householders is plain: it is not recommended that you handle or remove asbestos yourself, and in some places the law goes further: in the Australian Capital Territory any removal needs a licensed removalist, unless it is part of minor routine maintenance or other minor work, and in Queensland only a removalist holding a Class A licence may remove friable material. The agency adds that a professional is easier, safer and often cheaper, and that the waste has to be disposed of at a landfill that is licensed to take it.

When should you call a professional about mould?

The whole house