Renovation & Design


The ongoing battle against mould

QUESTION: Our ceramic tiles above the bathtub have been leaking, so we're going to have to remove them and replace the wallboard. We know there is mould behind the tiles because there are black areas in the basement ceiling where the pipes come through.

What disinfectant should we use to kill the mould after we open up the wallboard? Is bleach safe to use, and in what proportion to water? What precautions should we take when removing the wallboard? Thank you.

Elizabeth & Stephen Bahr

ANSWER: One of the most discussed issues related to hazardous materials in homes is mould. I get numerous questions from clients, readers and others wanting more information about this often hidden issue. There are many misconceptions about the dangers and treatment of mould if it is found in homes. You have identified a couple of these items and I will address them and others while answering your questions.

The first thing to note is moulds are everywhere and we can never eliminate them. They are critical components of our ecosystem and we would surely drown in leaves, dead trees and other organic matter if moulds were not present to help break down these natural items.

There are thousands of different species of moulds and only a dozen or so are known to be harmful to humans. Unfortunately, a few of the moulds that produce toxins can be present in our living areas. Some of these love the building materials we use in our homes, especially drywall paper.

The solution to mould, when found in our home environments, is to eliminate it. This can be done either by cleaning or removal and discarding of affected materials.

If you find a small amount of mould on a painted surface, concrete foundation wall or floor slab, or bathroom fixtures, cleaning may help prevent a quick return. Using common household cleaners, with a scrub brush or disposable cloths, should be enough to remove this surface growth. If there are much larger infestations, or if the mould is embedded in the wall coverings or other building materials, further repairs may be needed. Physical removal and discarding of mouldy or rotten drywall, plywood, paneling or insulation will be the only way to eliminate the growth.

Finding the true source of the issue -- moisture -- and eliminating it will prevent a quick return of mould. Whether it's leakage from foundations, roofs or windows, or from a one-time event such as sewer backups, stopping the water is the key.

In a situation like your bathroom, when old tile grout wears out and water from showering gets behind the surface, removal of moisture-damaged materials is required. Black stains on the subfloor beneath this area don't necessarily mean you have a serious mould issue. Removal of the damaged tiles and wall coverings will allow you to see inside the wall cavity to assess the extent of the problem.

You should wear breathing protection, like a good-quality disposable mask or respirator, when smashing out the old shower walls. Also, make an attempt to contain the debris to the bathroom, avoiding contamination of the rest of the home with any active mould spores. Cover heating registers and exhaust fans with plastic and put all debris in sealed garbage bags.

Once the wall is opened, you should be able to easily determine the extent of the moisture intrusion into this area. If the wood or the backside of the wall coverings from the opposite room are stained or black, the next steps must be taken. Probing all the wood with a sharp awl or screwdriver will help determine the extent of the damage. If the probe barely penetrates the surface of the wood, even with some visible staining, the framing may be salvageable. If the probe easily penetrates the surface and buries well into the wood, damage is extensive. Removal and replacement of damaged wood or drywall/plaster is the only choice.

Wood rot is a type of mould and may be physically indistinguishable from other more dangerous types. Either way, removing moisture-damaged framing or subflooring is the only solution.

If there is just surface staining or damage to the studs, you may have to do little labour prior to redoing the shower walls. There are liquid treatments that are supposed to help prevent mould re-growth, but they do not kill existing mould. Mould cannot be killed, just removed with the affected material. Using bleach is not recommended because chlorine bleach is toxic and if there are toxic mould spores present, mixing these toxins can be hazardous. Removing surface growth with household cleaners may be all that is required.

Once the new shower walls are installed and sealed, moisture will not be able to get to the wall cavity, preventing further mould growth, even if dormant spores are present.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at



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