Dealing with dampness

Dealing with dampness

Renovations, Tips and Advice

When we construct houses and other buildings, we take steps to protect them from dampness. In spite of this, buildings are often affected by water and dampness, and we end up having to take further action to stop the damaging effects this moisture has.

There are four main types of dampness, each of which has to be dealt with in a different way.

  1. Rising damp that moves upwards vertically,
  2. Falling damp that moves downwards vertically,
  3. Damp that spreads horizontally, and
  4. Dampness caused by condensation.

Rising damprising damp causes and cures

Even though the Building Code of Australia specifies that impervious damp-proof courses (DPCs) must be laid at the base of walls, below floor level, moisture does sometimes rise up through various materials. For example, you might notice alkaline salts from mortar or from the earth in clay bricks staining the surface of internal walls just above floor level. But if you spot it early enough, you can usually scrub the surface with warm, soapy dishwashing liquid and get rid of the stains. The problem is that if the dampness continues, the stains will reoccur.

Sometimes you can identify rising damp by efflorescence which looks like a white, chalky powder on the surface. Sometimes, if a lot of crystals form, it can look quite white and furry. Efflorescence is common on the surface of brick paving and sometimes shows on clay or quarry tiles laid on a new concrete slab. Generally, it disappears as the concrete cures and gradually dries out. But if rising damp continues, for example, if the correct DPC has not been laid, the moisture can cause surface finishes including paint, wallpaper and even timber panelling, to lift. It can also damage brickwork and drywall cladding, causing it to gradually wear away in places.

Because there will always be moisture in the soil underneath buildings, if the DPC (whatever it is made of) is damaged or if it breaks down at any point, the result will inevitably be rising damp.

One of the most common reasons for Australian buildings developing rising dampness has been identified as inadequate waterproof compounds being mixed into a mortar DPC.

It may be that a poor quality product has been used, or more usually that the quantities are not correct, often because those doing the job have skimped, or because they are not properly trained.

Whichever DPC is used, it usually blocks the upward movement of moisture for a while. But if the ground level at the base of external walls rises (because of soil washing down a bank for example), or if ventilators that would normally provide good air circulation under the floor get blocked, then moisture may be forced upwards.

If a plastic membrane has been used, and this is damaged in some way, it will also result in rising damp. If a plastic membrane is used at the base of walls, the building code states that this must stick out of the wall on both sides. This is to prevent a mortar bridge being created at the face of the mortar joint which would also cause rising damp.

If rising damp is caused by damage to the DPC, it should be repaired as soon as possible. If the area affected is relatively small, this process should not be too difficult. But, be warned, if a new DPC has to be installed, the process will undoubtedly be costly.

The two main methods used to install a new DPC are to either physically insert a new DPC or to use chemicals to create a new DPC.

Physically inserting a new membrane involves cutting out a mortar bed course below floor level and inserting a new membrane into the horizontal gap that has been made. The easiest way to do this is to use the patented method that utilises a plastic bag that becomes the membrane. The bag is inserted into the joint and then pumped full of a quick-setting mortar so that it is able to support the wall above.

If a chemical solution is preferred, holes are drilled into the bricks or mortar below floor level and the chemical is injected into the holes. Provided the job is done correctly, the chemical will form a DPC that is highly resistant to moisture, which will prevent future rising damp – without you having had to have structural work done.

If rising damp damages plastered walls (which is usually does), it is best to remove all the damp plaster and some of the plaster above the level of the rising damp. Leave it to breathe for several months before replastering.

Falling dampness

If pipes, roofs and gutters leak, we often get what we call falling dampness. The big difference between this and rising damp is that it is a lot easier to stop falling damp because all you have to do is repair the pipes, roofs, gutters and anything else that is causing the problem.

Just be aware that while leaking water can be damaging in itself, more often than not, it soaks into the ground and becomes part of any rising damp problems as it makes its way back up to the surface. So it is imperative to fix any leaks and drips as soon as you notice them.

Horizontal dampness

This type of dampness moves in a horizontal direction through walls at any height. The most common reason for this is bad bricklaying because there is not enough mortar in a joint, so water is able to accumulate in hollows, or because excess mortar has fallen into the gap in the centre of a cavity wall.

As long as the water evaporates more quickly than it penetrates, it shouldn’t be a problem. If it is a problem, painting on a waterproof coating to the outside of the wall should help. Just be sure this is done correctly, according to the manufacturer’s instructions. If there are cracks or gaps in the mortar, these should be repaired before the coating is applied. If the wall was plastered, you may need to strip that section of plaster and replaster.

Condensation dampness

This type of dampness is really very basic and causes cold glass (as in windows), walls and metal to become wet when moisture condenses. It happens all the time, especially when people are cooking or when other appliances like tumble dryers create heat. It is not normally a problem.

Anything that reduces humidity will reduce condensation dampness.

At the end of the day, if you are aware and act promptly, treatment of dampness is not particularly difficult or expensive.

Dion Seminara Architect


Hi, I am Dion Seminara, practicing architect and licensed general builder for 20 years as well as an environmental sustainable design (ESD) expert. I graduated from Queensland University of Technology (QUT) with honours, QLD in 1989. Registered as an architect in 1991 and registered as a builder in 1992, I am also a fellow member of the Australian Institute of Architects (AIA). Having received 12 ArCHdes Residential Architecture Awards, LJ Hooker Flood Free Home Design Award and the 2016 AIA Regional Commendation for Public Architecture, my expertise with both residential renovation (to all types of houses, especially Queenslanders, 50s/60s/80s), new contemporary homes and luxury residences has earned me a reputation as one of Brisbane's architectural specialists in lifestyle design architecture, interior design and landscape design.