THE GREAT DAMP PROOFING ILLUSION - PART 1 - MATERIALS
Russell Rafton
Director at DRYFIX PRESERVATION LTD Yorkshire's Leading Damp and Timber Specialists / Director Coleman Diagnostics
The great damp proofing illusion. When on the surface all appears well but when scratched symptoms of a reoccurring problem arise.
This is an article that explains the difficulties that surveyors encounter daily when advising clients of potential damp related problems when none appear visually apparent. Electronic moisture meters when used on masonry can be very misleading but equally construction materials themselves can also be very deceptive. Gypsum for example when used in the restoration of older properties can often be problematic with the plaster spoiling, delaminating and salting, all symptoms usually suggestive of a severe damp problem but this could simply be the result of a chemical reaction.
Material science is so often misunderstood by architects who specify products, surveyors who investigate them and builders who install them. This survey is a prime example of materials that can provide temporary cosmetic facades and the potential risks of reoccurring problems when ill-considered repairs fail.
So here goes, let me explain.
Walls and plaster affected by rising damp often become contaminated with salt.?This occurs as moisture within the ground migrates through the walls via capillary action carrying with it soluble chloride and nitrate groundwater salts.?These salts are transferred into both the walls and plaster as moisture evaporates often leaving behind a salty deposit on the face of the wall which spoils both the plaster and decorations.
These salts are hygroscopic, meaning they absorb moisture from the atmosphere which also means that walls and plaster which are contaminated may be restricted from drying if the contaminated plaster is left in situ. The plaster simply keeps pulling moisture in from the air continually wetting the surface of the wall. ?It is for this reason that following the installation of a new damp proof course the existing contaminated plaster is normally removed and replaced with new. The new plaster system however will always contain a salt inhibitor which prevents the movement of any salts within the wall migrating into the new plaster finish. This protection is often achieved by applying an inhibiting additive into the backing plaster mix or by using specialist pre-mixed factory plasters.
Illustration drawing highlighting the hygroscopic effect of salt contaminated plaster
Have you ever wondered however why the specification for replastering issued by most manufacturers is usually a very dense and cement rich render? Well, this is because, in addition to preventing the movement of salts, the new plaster must also act as a barrier to the moisture within the wall whilst drying occurs. In an ideal world after the plaster is removed and a new damp proof course is installed the walls would be left to dry naturally however, in reality, the drying times required for this to have any real benefit would far exceed the expectations of any client. There are quotes kicking around the internet suggesting one month per inch thickness of wall although, from my experience in flood restoration I can tell you the recovery of wet materials is often far longer than most expect. Using a strong cementitious rich plaster during reinstatement?ensures much less chance of the moisture within the wall being a problem, this is provided that drying of the wall does eventually occur. But what happens when it doesn’t?...............
Say for argument sake, the damp problem was misdiagnosed, ?the wrong specification for repair was issued or perhaps the repairs were breached after installation?. The problem continues however the density and alkalinity of the replacement plaster means that dampness will not pass through the plaster easily. The plaster in effect provides a cosmetic fa?ade masking the problem behind. In this scenario, if not fully intentional or understood moisture within the walls may continue to have a damaging effect upon the structure, the building fabric or other closely related elements such as internal joinery.
This was the scenario on a recent survey. The client was experiencing what they thought were some minor issues with dampness in a few random locations throughout the property. There was nothing that visually appeared serious so rightly they presumed this would be an easy fix. The property they had owned and occupied for 12 years and during their purchase a survey did flag a minor issue with damp within the lounge although nothing considered too serious. The property had previously been treated for a damp related problem made obvious by the installation of a retrofit damp proof course within the external walls. There was however no paperwork to support these works. Recommendations were made to simply monitor the walls and as such, based on this advice they had no reason to investigate the issue further. Fast forward a few years however and further damp problems started to arise within other ground floor walls causing the painted decorations to lightly spoil. With exception to these minimal observations very little else was observed. It was these observations that prompted my clients to instruct a professional survey.
During my inspection, I noted excessive moisture levels within the joinery throughout the property, this included skirting boards, architraves and door casings. The damp problems reported by the client were sporadic in nature however the issues I observed within the joinery were much more consistent indicating a widespread problem. I also took instrumental readings with the use of an electronic moisture meter from the walls using surface probes and only recorded the occasional high reading. I found nothing consistent.
High readings taken from the ground floor skirting boards and other joinery articles
Low readings taken from the wall plaster surfaces
I then scanned the walls with my moisture meter in search mode, a non-invasive mode that sends radio signals into the wall. These signals can penetrate the wall between 10 – 20mm in-depth and as such, can be used to provide information to the likely condition of the wall beneath the surface plaster. Be careful tho, I see a lot of surveyors using their moisture meter in search mode declaring a crisis with dampness every time they find a high reading. Using an electronic moisture meter in search mode can be very missleading, so at the very least if this is the case, i'd advise you find other evidence to support your findings before putting the fear of god into your clients.
In this scenario, irrespective of the low readings obtained from the surface plaster I obtained relatively consistent readings using search mode along with a profile indicative of rising damp. These findings added more suspicion to my prior observations within the joinery. The plaster finishes were generally unspoilt and the wall looked dry however, I suspected these visual observations were not truly representative of the walls condition.?The skirting boards and other more vulnerable hygroscopic materials suggested the likelihood of reoccurring problem which otherwise was not apparent to the homeowner.
An electronic moisture meter in search mode - this is a relative mode meaning it is not a quantitative reading of moisture. Search mode on the Promitmeter MMS3 is on a scale that varies between 0 - 999.
The problem with moisture meters is these meters work specifically measuring conductivity between two isolated probes or disturbance caused to radio waves passing through a material. These meters are specifically calibrated to timber meaning readings taken are not quantitative on masonry, thus will not accurately measure the quantity of moisture within a material other than timber for which they are specifically designed for and are accurate up to 30% Wood Moisture Content (WMC). Therefore, readings taken from timber materials; skirting boards etc are quantitative, therefore more accurate and reliable in non-disruptive investigations.
If you would like to know more about how electronic moisture meters work and how accurate they are here's a link to an article that explains all: https://www.dhirubhai.net/pulse/truth-moisture-meters-protimeter-russell-rafton-/
If you're also in the market for a new moisture meter here's a link to my gear reviews, which evaluates a range of the best professional moisture meters on the market:
Professional Moisture Meters Which is Best, PART 1 - https://www.dhirubhai.net/pulse/gear-review-professional-moisture-meters-which-best-part-rafton-/
Professional Moisture Meters Which is Best, PART 2 - https://www.dhirubhai.net/pulse/gear-review-professional-moisture-meters-which-best-part-rafton--1f/
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Electronic meters used on masonry and other materials will however allow us to map a pattern of readings to assess the source/pattern of dampness present. Due to the working of these meters, readings can however be influenced by other conductive materials or surfaces so they can be very misleading.
As described in the BRE guidance document 245 “Rising damp in walls “Diagnosis and treatment” electrical moisture meters do have a valuable role to play as preliminary surveying instruments and will identify areas where further investigation is required. To obtain more conclusive proof regarding the source/cause of dampness the most satisfactory approach is to take samples of masonry and plaster for analysis.
As such, to further our inspection I was granted permission from the homeowner to extract samples of masonry from the wall for onsite moisture analysis. I then processed these samples using a calcium carbide / speedy meter. The results of the analysis revealed excessive moisture within the walls at around 13% M.C indicating that the walls behind the plaster are very wet as readings taken from the skirting boards earlier indicated. In this scenario, the wall plaster was providing an illusion that all was ok when in fact there was clearly an unwanted damp problem suppressed within the walls.
If you don't know what a speedy / calcium carbide meter is, here's an article that will help - https://www.dhirubhai.net/pulse/damp-diagnosis-using-calcium-carbide-meter-speedy-russell-rafton-/
Samples extracted for analysis onsite
Calcium carbide / Speedy meter test results
So, I should just explain here that I'm not entirely against the use of cosmetic facades to suppress certain problems provided they are intentional and the consequences are evaluated prior and understood. In this circumstance however, the fa?ade provided by the plaster was working however I doubt this was intentional as excessive moisture was observed within the joinery and symptoms of damp were returning elsewhere. The cosmetic fa?ade here was never intentional it was just an advantageous consequence of the prior repairs. Advantageous for who however is the question?....
What is likely however is that the problems I've now discovered are symptoms of the prior issue that former homeowners have already attempted to address. The purpose of this article isn’t to explain the cause, my final diagnosis or my proposed repairs but to simply introduce you to some of the challenges I face with diagnosis during my surveys.
The anticipated repair to this property is both difficult and disruptive and has been caused by an issue partly introduced during construction and through alterations post-construction by former homeowners. I’m anticipating the cost of this repair to be between ￡20,000 - ￡30,000 a sizeable amount for any homeowner and a considerable figure for an issue that could and should have been investigated during their purchase. All too often I find pre-purchase surveyors placing a value on the anticipated costs of repairs or advising monitoring when there is no understanding of the cause, this is ludicrous. If you have no idea of the cause how can you possibly budget for a repair? My advice is simple, take nothing at face value and if you're unsure recommend further investigation.
I hope this article has however gone some way in explaining that surveying properties for damp related problems is a far more complicated subject than most appreciate. Damp surveys are often difficult due to the restrictive nature of the survey itself, the limitations of the instruments used but also because the construction of buildings is complex particularly when there have been prior interventions.
If you've made it this far, I hope you found this article useful and if you have any questions or queries please don't hesitate to get in touch.
Happy surveying
Russ
If you would like to learn more about damp, moisture mapping or damp diagnosis you may?also find the following articles useful:
Moisture Mapping
Industry Perception Study
Moisture Content Analysis
Russell Rafton - Dryfix Preservation Ltd Senior Surveyor
Dryfix Yorkshires Leading Damp & Timber Specialists
A 5 x Industry Award Winning Company with the Property Care Association.
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Complete Preservation
2 年Another previous lash up then ??
Building surveyor.
2 年It's an interesting article. For me what's often overlooked is the presence of salts in walls which do have an effective dpc and the effect this can have. The ground is not the only source of salts. Therefore the presence of salts does not confirm they are hydroscopic ground salts.
Surveyor at Paisley Surveyors Limited
2 年Magic Post Russell?? It's a shame the contractor didn't do the same further investigation work that you have done before they went ahead with the chemical injection and replastering works. The surveyor should have tested the timber skirtings and recommended further investigations. He should also have spotted that those injection holes are wrong.