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The issue of damp

John Conlin / 28 April 2017 ( 27 November 2018 )

Chartered Surveyor John Conlin advises on a host of damp issues, including discoloured walls, damp bricks and more.

Question: how do I remove black spots?

If our damp problem is identified as condensation, what would be the best method of removing the black spots?


The first thing is to cure the condensation by reducing internal humidity. 

Ventilation, avoiding drying washing on radiators or indoor clothes lines, using an extractor fan when showering or bathing will all help. 

The black spots are collections of fungal spores. Wiping with an household surface cleaner may erase the black spots but will not eradicate the spores. 

Use a proprietary spray such as ‘Polycell Mould Killer’. If you need to redecorate choose a paint containing a fungicide.

Question: is concrete waterproof?

Our garage and kitchen extension were built up to the boundary with our neighbour who has now built a raised concrete patio which has buried the damp proof course of our garage and extension. 

I have always understood that damp proof courses should not be covered, but my neighbour says covering damp proof courses with concrete is acceptable because concrete is waterproof. 

I am not convinced and should be grateful for your advice.


You are correct. Concrete is not waterproof; if it were, one would not need to incorporate a damp proof membrane in solid floors. 

Covering a damp proof course creates a bridge for both rising and lateral damp.

Question: why is our damp proof course failing?

My house, built around 1870 had a damp proof course installed about 50 years ago but now we have damp along the side wall.  

Do you have any suggestions?


50 years ago the damp proof course was probably a ‘pressure injection’ type. 

The chemical compound then used slowly becomes diluted by the moisture it is repelling. I suspect that you need a new damp proof course.

Modern nano particle technology has created damp proofing creams that are injected into holes drilled into the mortar surrounding the bricks using a simple hand held cartridge dispenser. 

 It is an easy job and does not require specialist skill or equipment.

Question: do damp courses wear out?

Do damp courses wear out? Parts of our outside walls feel damp to touch and a damp proofing firm have suggested we need a new damp proof course.

Is this true or a marketing scam?


Damp courses consist of a horizontal layer of lead, slate, bitumen or plastic in the outside brickwork just above ground level and rarely fail. Chemical damp courses are sometimes found in older property that had no original damp proofing and can be identified by rows of holes in the walls where the chemical was injected.

Occasionally, the chemical ages and becomes less effective. However, do not spend money on any work until you have checked that the damp you detect is not condensation. If in doubt ask an independent chartered surveyor to test and give impartial advice.

Question: why are our walls damp?

We have recently replaced old wooden windows with double glazed ones. Since then although the house is warmer we are experiencing damp walls due to condensation. Is there a solution to this?

Also is it safe to use a de-humidifier each night? Your advice would be much appreciated.


I think the most likely explanation is a lack of ventilation because your new windows, unlike the old ones, are draught proof and natural lifestyle humidity is building up.

If your new windows are equipped with ‘trickle vents’ or the window handles have a secure slightly open position try using that. It is also important to use an extractor fan in the bathroom and an externally venting cooker hood.

I, personally, would use passive dehumidifiers rather than electric ones: have a look at the Uni-Bond 360º.

Question: condensation or external damp?

We have black-spotted staining appearing at the junction of ceilings and outside walls. Our landlord says it is because of condensation due to lack of ventilation, but I think it is damp coming through from outside. Is there any way of checking?


There are sophisticated (and costly) tests, but try this simple DIY test. Cut a postcard- sized piece of kitchen foil and tape it, shiny side out, to the wall in a stained area. Turn the bottom of the foil up to form a little gutter with closed ends. At various times over several days, check for any droplets of water on the surface or collected in the gutter. If so, it confirms condensation.

Question: damp behind panelling

It was several months before we became aware that an outside wall had become saturated due to a defective gutter. Internally it is clad with old decorative timber panelling that, although seeming dry, has developed a musty smell.

I am worried that dry rot may be spreading unseen. Is there any way of checking without removing the panelling?


You are right to be concerned because damp, stale air in a confined space is an ideal environment for fungal development. Fugenex dry-rot sensors, which comprise a 90mm (3½in) rod impregnated with a reagent, only require a tiny hole for minimum cosmetic damage. As the fungus develops, the pH in the surrounding material (wood, brick, plaster etc) changes, causing the reagent in the rod to go from blue to yellow. If this occurs, more invasive investigation is justified.

Question: why is my masonry discoloured?

I had the outside of my house clad with roughcast about 8 years ago. One wall is becoming discoloured with what seems to be moss. A builder says water may be going down the back of the roughcast and it will cost £700 to redo it. Could you advise?


The fact that only one wall is affected may be significant. All forms of cement rendering (inc. roughcast) will absorb moisture unless they are coated with waterproof paint. If the discoloured wall faces away from the sun or prevailing wind it may not dry out as well as the others and the constant low level dampness will encourage the growth and spread of algae. Pressure wash the wall using an algaecide additive to stop re-growth then, during a dry spell, coat the wall with a colourless silicone water repellent such as “Thompson’s WaterSeal”.

Question: has my damp proof course been damaged?

I have a damp problem in the bedroom of my bungalow. I asked a damp-proofing firm to investigate, and they say I need a new damp-proof course as the present one has been damaged by water draining from my neighbour's land onto the path adjoining the wall of my bedroom.

Does that sound likely?


Like most things there are good and bad damp proofing firms but you are right to be cautious. It can be like asking a fox to count the chickens.

Providing the surface of the adjoining path or soil is at least a couple of inches below the line of the damp proof course, it is unlikely to have been damaged.

There are many other possible causes, so you really should get an impartial opinion from a local chartered building surveyor. You can find a surveyor on the RICS website.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.