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My approach to damp diagnosis

October 11, 2010

The definitive book on damp

Many surveyors (having found damp in a property) insist the applicant obtains a damp report produced by a damp and timber “specialist”. There are many risks for the applicant associated with this referral and almost no risk for the surveyor. I believe the referral to be unnecessary in many cases since the “specialist” rarely concludes something other than “Rising Damp”.

My DIY method of damp diagnosis is based on a simple and logical focus upon the SOURCE(S) of dampness and the DAMP PATHWAYS. Damp mechanisms are not considered except for problems associated with condensation. My approach to dampness follows closely the methods described in the book Diagnosing Damp by Parrett and Burkinshaw. This is the definitive book on damp and is a must read for surveyors.

THE CASE STUDY

The damp example described below was from a house built in around 1880 and is of traditional construction. The damp was found in the ground floor living room, either side of the fireplace and extended to the flank wall.

MY METHOD

Whilst undertaking a Homebuyer Survey, I use a simple resitance meter (protimeter) to identify any surfaces that give high readings. The meter is lightly pushed into the surface (usually walls) to “test” the electrical resistance of the surface. If I get a high reading it merely is a signal that the wall has a low resistance to an electric current.

Many materials have a low resistance to current including, breeze blocks, foil backed wall paper, water and my sweaty finger!! Due to the sensitivity of the resistance meter, high readings must be interpreted very carefully and no conclusions can be made from this investigation.

Dampness to the first 80cm of the wall

The damp wall in question was visibly very damp, it felt wet and it was cold to the touch. The damp stain either side of the chimney breast in the living room continued to about 80cm up the wall.

At this point it’s tempting to call for a further investigation and move on. However, surveyors must “follow the trail of suspicion” which means making some attempt at dignosing the problem on site themselves!  In other words, DIY Damp Diagnosis.

Alas the “Rising Damp” conundrum seems to spook some surveyors and the issue is just past on to a “specialist” who often turns out to be no more than a remedial damp-proofing salesman. This is a great shame because DIY damp investigating is great fun and you can save the client much aggrevation and cost by identifying very simple remedial work that will greatly improve the problem.

I digress, back to the damp house…

During my damp investigation the floor plan is used to note the positions where surfaces have been checked. High moisture meter readings are noted and abbreviated with the letters “RR” (Relative Reading) to indicate the surface checked is not timber.

Having found dampness either side of the chimney and marked the location and “RR” meter readings I will continue the investigation around the property.

Once the initial damp investigation is complete, the damp areas will be re-visited as part of my “following the trail of suspicion” process. I will then try to identify items of disrepair that would make the wall either damp or prevent it from drying out.

The damp chimney breast was part of the left flank wall and I was eager to get outside and look at the wall from the other side. My approach tries to keep things simple, I ask myself 2 questions:

Where is the damp coming from, THE SOURCE; and

How is the moisture entering the property, THE PATHWAY.

Painted flank wall of house

I’m also very interested in anything that is preventing the wall from drying out. Modern materials usually prevent the building from drying out by entrapping moisture within the building fabric. Cement mortars and modern external paints are very impermermeable and entrap dampness, thus slowing the rate of drying to a minimum.

On this particular flank wall I noticed the following:

1. Very high external ground levels caused by crazy paving.

2. Flank wall painted with modern paint rather than limewash.

3. Brick wall pointed in cement mortar rather than lime mortar.

4. Air bricks very small and partially covered by crazy paving and paint.

5. The chimney flue has been capped and not vented, although a gas fire has been installed with flue liner.

Very high ground levels and blocked air brick

Having considered all these issues I came to the view that the internal dampness wasn’t caused by one single issue, more likely several factors working together to allow the build up of moisture. The most serious issue perhaps was the very high external ground levels caused by the crazy paving, which was providing a quick and easy route in for the moisture.

The real irony was that this flank wall was facing south west and after midday it would get full sun for a good 6 hours in summer. The intense radiation from the sun would dry out the wall in no time!

RECOMMENDATIONS

1. Lift the crazy paving, removed 18cm of subsoil and re-lay the crazy paving.

2. Remove the modern paint, raking out the cement mortar and repoint with lime mortar.

3. Vent the chimney flue top and bottom and use the gas fire in winter.

4. Increase the size and number of air bricks at the base of the wall.

Implementing these recommendations will allow the flank wall to gradually dry out over a number of years. JOB DONE!

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3 Comments
  1. hi, excellent blog. how do you determine that the wall was cement mortar and not lime. is it the hardness? or something else?

    • jp

      The use of lime mortar is so rare these days that it is more than likely to be cement mortar.

      There is a simple test for lime and cement mortars. If you scrape your finger nail along cement mortar the nail will wear down, while if you scrape your nail along lime mortar the lime mortar will wear down.

      You refer to my post “My approach to damp diagnosis”. Having just got my front door frame finished, I think I’ll have another go at explaining my approach to dampness so people can understand that it isn’t rocket science, but simple observation and logic.

  2. ma701ss permalink

    Could you also have recommended creating a french drain instead of lifting the paving? Also, pleasse could you email me? I am a surveyor and would like to talk to you about something else!

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