The causes of damp walls and mould are varied, with this article starting with penetrating damp (also known as lateral damp) which can occur at any level. It’s very common with older properties that have solid walls but not newer properties which have cavity wall insulation.

Exterior causes of penetrating damp are those of:

  1. Blocked rainwater gutters, hoppers or down-pipes which overflow, causing the adjacent wall to become saturated;
  2. Defective concrete flaunchings around disused chimney pots that have been capped or sealed. This allows water to enter the unused chimney and then penetrate into the building;
  3. Defective flashings around chimneys, flues, soil-and-vent pipes and ventilation terminals, and at abutments of roofs to vertical walls. Whilst these defects usually cause leaks and not damp, sometimes water finds its way into the roof construction and emerges elsewhere as penetrating damp;
  4. External wall cavities that have been bridged by rubbish left in the cavity or by mortar droppings left on wall ties or insulation batts during construction;
  5. Exposure to driving rain where water penetrates the brick outer leaf which may then be led across the cavity, locally saturating the inner masonry leaf and as well as internal plaster or plasterboard finishes;
  6. A wall tie, which is designed not to allow moisture to cross it as it is coated with mortar when the wall is built;
  7. Pipework which extends through the wall;
  8. Window, door frames and lintels which have been poorly fitted with damp proof courses;
  9. Cracked brickwork;
  10. Deterioration of mortar joints (pointing) with the resultant gaps in the wall providing moisture and cold air a direct route to the internal walls causing first damp then mould problems. Cold air can also cause what is known as a ‘cold spot’ where the internal wall has a different temperature to the wall surrounding it with condensation likely to form when the warm moist air in the room touches the cold area, resulting in black mould growth;
  11. Deterioration of water tanks.

Interior signs that point to penetrating damp are those of damaged plaster, watermarks and damp patches that won’t dry out which may also be seen to expand during heavy rain. There can also be black mould growth and the place will have that particular damp, musty smell.


Rising damp is most likely to be found on the ground floor of very old houses or blocks of flats. It is rarely seen in flats above the ground floor.
Where external walls are constructed of permeable or absorbent materials such as brickwork, stone or plaster, and there is either no damp proof course or it has become defective, such constructions allow moisture from the ground under and around the building to rise up the walls until internal plaster finishes become saturated. This then leads to mould growth and the degradation of the finish. Eventually the fabric of the wall will begin to degrade too.

The usual height at which rising damp will be seen in plaster finishes is usually up to 1 meter above ground floor level, and it usually shows as tide marks because of the evaporation of water and salts from the ground. If these are not present then another common sight is that of brownish/yellow damp patches or staining above the skirting board and up to around the same height.

Loosening and curled up wallpaper or peeling paper at the skirting board is another clear pointer as to the problem.

Exterior Remedy

The usual solution for this kind of damp is to drill holes at regular intervals around the perimeter of the affected area (just above external ground level). A damp proof course  is then applied (DPC) with a pressurised chemical injection into the holes. The chemical is absorbed into the masonry, forming an effective barrier to moisture traveling upwards by expanding into a line across the brickwork. The wall is then re-rendered using a waterproof rendering system

Interior Remedy

Before the affected interior can be redecorated, the decorations and plaster-work must be removed to get rid of most of the hygroscopic/deliquescent salts which will have accumulated. If decorations and plaster are not removed, any salts that stay in the masonry will return and show through newly decorated surfaces. Walls take time to dry down and the lower part of the walls are always likely to remain damp because of the limitations of the chemical injection systems in terms of controlling the rising damp.  So chemical injection damp-proofing is a 2-part process: the injection to ‘control’ the rising damp, and the replastering (inside) to prevent future show through- the processes are inseparable.


Blocks of flats have a number of pipes running through walls, under floors and through ceilings and leaks are usually obvious, such as large amounts of water coming through ceilings, appearing under the floor, or wet patches showing on the wall or floor, even when the weather is really dry.

There are a number of potential causes of plumbing leaks with the following information sourced from the ARMA downloadable fact sheet ‘Water Leaks’

1: Broken Or Perished Seals

These are found around baths and showers and which are white flexible beads which run around  the bath/shower and allow water to run back to the baths/showers. When the mastic seal is damaged, split, loose or curling away from the wall or bath, water can freely run down the back of the bath/shower, and eventually make its way through the ceiling and into the property below, often through a light fitting.

2: Degraded Seals

When found around the kitchen worktop and around the sink, the gaps they create can often cause water to penetrate down the back or into other properties;

3: Gaps In Grouting

These can allow water in behind tiling causing damp patches, tiling to come loose and again, the risk of water leaking into the property below.

4: Incorrect Appliance Installations

These can things like a cold feed to hot supply, or ill-fitted power shower pumps etc, causing joints to fail.

5: Unsupported Pipes

They can lead to sagging then to blockages and unpleasant smells;

6: Pipes With Too Many Bends

The incorporation of too many bends along with an inadequate number of fixings can lead to noise and performance issues.

7: Poor And/Or Over-Notching To Joists:

Poor and/or over notching to joists can cause floors to become ‘springy’ and in extreme cases could potentially lead to floor failure and collapse.

8: Badly Supported Tanks and Pipes

These can be susceptible to leaks or splitting.

9: Leaking Toilet Overflow Pipe

A constant sound of running water from the toilet overflow pipe can cause damage to the exterior of the building because water is escaping onto the ground below. This will cause the wall to stain, damage mortars and quite possibly lead to water ingress elsewhere. Vegetative growth can also likely occur.

10: Vibrations Of Appliances

The outlet connections of dishwashers and washing machines (found under the kitchen sink) can come loose due to frequent vibrations so these connections should be regularly checked as should the washing machine hose as this is a major cause of leaks.

11: Plumbing Corrosion Or Joints Not Fully Water Tight

Corrosion can be internal as result of more than one type of metal being used in the plumbing, or external  which is often as a result of concrete or cement coming into contact with copper pipes. It can also be caused by flexible tumble dryer vents extracting into a room rather than outside of it. It typically manifests itself as a small isolated damp patch, without any brown staining, that gradually grows, ‘bubbly’ plaster at the edge of the patches with water appearing if the leak is near (or above) the surface.

Other areas that can cause plumbing leaks are no insulation on cold-feed pipes, leaking hot/cold supply pipes, supply and waste pipes (including traps), hot water pumps or boilers, repairs to individual central heating systems and dripping taps (the latter fo which can often be repaired by replacing a washer).


Add one (or more of above) with poor or no ventilation then any damp walls will struggle to dry out. This will be regardless of whether the original source of moisture has been repaired because inadequate ventilation causes moist air to accumulate. When the dew point is reached (100% relative humidity which is the percentage of water in the air based on a scale of how much water the air can hold), condensation starts on the cooler surfaces first and usually in a particular order:

  1. Window condensation because windows are less thermally efficient than the surrounding walls, especially in older buildings. which makes their internal surface temperature cooler by a few degrees than that of the walls. Shows up as beads of water on the window;
  2. Pools of water on the windowsill;
  3. Beads of water on external walls;
  4. Damp patches on external walls;
  5. Damp patches on ceilings;
  6. Damp patches in the corners of rooms;
  7. Damp internal walls;
  8. Black mould on window frames, window recesses and windowsills;
  9. Black mould on walls – particularly corners and recesses with little air flow;
  10. Black mould in cupboards;
  11. Black mould on curtains and clothes;
  12. Wet internal walls.

As well as black mould, affected areas can also show as patches of mildew and wallpaper coming loose from the wall. Mould can also be white and fluffy.  All this will be accompanied by a musty, damp smell.
Additionally, if the plaster has suffered exposure for a long time and if it is split or cracked, crumbling away or if when tapped on, it sounds hollow, then it has blown off of the wall’s surface below and the entire affected area will need to be removed.

The process for treating black mould is as follows:

  1. Clean the area with a solution of anti-mould cleaner to kill the spores and help prevent mould regrowth. This must be carried out before redecoration as just washing off the mould with detergent then redecorating will not kill the spores and mould regrowth is likely, even through anti-condensation paint. This will be most effective where the underlying problem of condensation has been remedied;
  2. Leave the anti-mould cleaner for the recommended time, then thoroughly clean the area, preferably with sugar soap (mixed with water to the manufacturer’s specifications) or detergent with water to remove any grease then rinse well.
  3. Apply anti-condensation paint which is a special paint which is hygroscopic, allowing the paint to ‘breathe’ and makes it ideal for kitchens, bathrooms and shower rooms.

So what can be done to reduce or eliminate it in the future?

  1. There must be some ventilation provided to all rooms to enable moist air to escape;
  2. Excessive use of heating must be reduced, especially if point 1 also applies;
  3. Each litre of oil used produces the equivalent of about a litre of liquid water in the form of water vapour so no portable paraffin or flueless gas heaters must be used;
  4. Condensation can also be produced where there are no cooker hoods/extractors over the cooker. Much of the steam will be removed if they are installed and vented to the outside air and 6″ or 8″ in diameter;
  5. If condensation is produced on cold pipes it can be prevented by using moulded insulation which is thermal insulation pre-moulded to fit plumbing pipes and fittings;
  6. The internal surface temperature of walls can be increased by internally insulating with dry lining which is a system for the cladding of internal faces of buildings i.e. walls and ceilings.
  7. External walls can be insulated with an insulated render system;
  8. The addition of more extractor fans such as in the bathroom;
  9. Upgrading the heating system.


The risks presented by a water leak extend far beyond simple water damage, and every leak, no matter how small, requires a quick remedy to remove any potential danger. This is because water, (and the contaminants it contains) acts as an electricity conductor which means it can carry an electrical current to whatever it touches, including people! It can also cause a fire risk because even if a leak is small, water may find its way into light fixtures and other electrical wiring. In most cases this will cause a harmless short but touching a switch when water is pooling in a light fixture for example can result in an electric shock but even worse, result in sparks that start a blaze, sometimes with fatal costs.

Our block is a 1930’s build and needs effective management of both the inside and exterior. The latter is our remit, and so it is regularly checked over and any required works are placed in the order in which they are to be carried out. Any residents directly affected by water ingress are placed at the top of the list.

What we cannot control is ventilation and plumbing issues within each individual flat. Even when exterior works are completed, they will not work as stand-alone measures for fixing condensation issues if there is not enough interior ventilation. A number of landlords on our block have automatically blamed the tenants and ourselves for any and all damp issues and it is indeed true that I have walked into some flats that were like greenhouses with the heating cranked up and washing drying inside. However, this does not cancel out the fact these same landlords have covered up fireplaces, (without adding air bricks), replaced original wall vents with smaller ones or taken them out altogether. We’ve even had one landlord paint over a kitchen vent!

We on the other hand have left our fireplace open and we have little or no damp or condensation issues at all!


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