Why is my ceiling mouldy?

Mouldy ceilings worry people.

However, not all mouldy ceilings are caused by leaking roofs. Most mouldy ceilings have nothing to do with roof leaks.

If you have an ugly mould area in an isolated area of your ceiling – along with brown staining, then you probably have a roof leak.

Leaking and mouldy ceiling

The brown staining is the fine dust on your ceiling that has been dissolved by the rain water and is leaching through the plaster. The mould is because the ceiling is damp (usually due to the ceiling insulation getting soaked by the roof leak).

If there is just BROWN staining, it is not mould – because mould is a grey colour. When there is an occasional roof leak, the ceiling gets wet for a short while and causes a brown stain. But the ceiling usually dries rapidly before mould spores have a chance to grow on it.

The rule of thumb is: Any isolated brown ceiling stain (with or without associated grey mould staining) is a sign of a leaking roof. Therefore, if you have an isolated ceiling stain that has any tinge of brown, you should go down the SYDNEY ROOF REPAIRS journey to find out more about getting your roof leak repaired properly.


However, I am asked to inspect a lot of mouldy ceilings that has nothing to do with a leak.

If the whole ceiling is covered by a fine layer of mould, then it is probably not a roof leak. If there are patches of fine mould around the cornices with no brown staining – then it is just a damp room. Not a roof leak.

In my experience, if the plaster ceiling has good ceiling batts on top of it, it is much less likely to attract mould. If there are NO ceiling batts – then this is the major cause of mouldy ceilings. Some ceilings will show ‘bands’ of mould – especially around the edges of the room. I always find that there are no batts at the edges of the ceilings (because the insulator could not bother putting the last bit in due to the tight space.) when a ceiling has the mould band over the windows.

The bands of mould on the ceiling are directly below the section of roof where the ceiling insulator did not bother to install ceiling batts – because it is rather hard to reach.

An uninsulated ceiling will collect condensation because it is cold and create a nice damp surface for the mould spores.

Dampness on a ceiling is a good attraction for mould growth. In the winter time, ceilings are prone to dampness because room heating and the presence of our warm bodies send warm moist air upwards. This moist air hits the cold ceiling (more pronounced on ceilings that are not insulated) and forms condensation. When mould spores are around, they migrate to the moist environment and thrive. Very quickly, they multiply and soon you have a fine coating of mould all over your ceiling.

Some rooms are more prone to dampness. Bathrooms and bedrooms are the worst because they are more private and are often closed and unventilated.

The solution to damp rooms causing mould growth on ceilings is to make sure that you have good ceiling insulation. The insulation keeps the ceiling warmer in the wintertime and there will be less condensation. The room needs to be ventilated. And it is very important to kill off the mould effectively, remove it and apply a good quality ceiling paint.  I do not do any of these things – so find a good ceiling insulator and a good painter…