Bridged or failed damp proof course?
October 30th, 2012
Bridged or failed damp proof course?
Another familiar story of a damp investigation for suspected rising damp on a property in York, North Yorkshire where a client contact us with regards to providing a survey and quote for a damp proof course.
The property is a 1960’s three bedroom right semi detached house constructed under a pitched roof with concrete roof tile covering and external walls of cavity construction incorporating a horizontal bitumen damp proof course (DPC). The area of concern was the front extension a 1980’s addition, which incorporated an integral garage and porch constructed under a pitched and hipped roof with concrete roof tile covering and external walls also of cavity construction incorporating a horizontal upvc damp proof course.
Within the porch the left side elevation wall was spoiling, the plaster was blistering and salting and the wall paper decorations had deteriorated. The metal plaster beads and nail heads used to fix the skirting boards were also corroding and elevated moisture levels were recorded within the timber skirting boards in excess of 26% Wood Moisture Content (WMC). For those of you who have read our blog on moisture meters, will know readings taken from the timber are quantitive meaning they represent the actual amount of moisture in the timber, which in this scenario is excessively high, so where has all this excess moisture come from and why is the plaster spoiling?
A profile of readings taken internally from the spoiled wall finishes indicated a rising damp profile i.e. moisture extending from the ground upwards however, as previously mentioned I found a modern horizontal upvc damp proof course within the extension. Upvc damp proof courses are standard in all new construction because unlike other systems they can be laid continuously and do not deteriorate with age unlike other alternatives. So if the building has a modern damp proof course made from upvc why are there symptoms of rising damp if it is almost guaranteed the damp proof course material would not fail.
My inspection externally revealed the damp proof course was positioned appropriately with 150mm clearance to the external ground as required by building regulations and standard codes of practice, so there was little chance of the damp proof course being bridged by standing water or rain water.
A small pilot hole was also drilled into the external leaf wall to allow inspection within the cavity with the use of an optical borroscope ensuring the cavity was clear from blockages such as; debris and mortar droppings which can occur during or post construction. The borroscope inspection also revealed the cavity was clean and there was no evidence to suggest a bridge of the damp proof course.
Looking at the relationship between the damp proof course and the internal floor level however revealed a problem. The floor level internally was low in relation to the DPC which meant the damp course was positioned approximately 40mm above the floor. The property had low profile skirting boards which forces the plasterers to plaster almost to the floor and more often than not into contact with the floor. This is where the bridge exists! The plaster internally extended beyond the damp proof courses position, thus bridging the damp course. This allowed damp beneath the DPC to migrate into the plaster and wick above leading to rising damp symptoms, hence why we established a rising damp profile.
This was confirmed when the skirting boards and plaster were removed at the door reveal only to expose the hidden damp proof course sited high above the floor and bridged by the plaster. (see pictures below)
Whilst the property expressed rising damp symptoms which theoretically is the cause, ground water being wicked up through the structure and finishes, this wasn’t occurring due to the absence of a damp proof course or its failure, this was occurring due to a design/construction fault leading to a bridge in the damp proof course.
Inevitably some repairs are required, although the installation of a new damp proof course is not necessary as in this circumstance the original DPC is working just fine. Therefore the clients original thought and instruction to us of “I need a quote for a new damp proof course, as I have damp” was incorrect – it pay’s to instruct a qualified surveyor. Ideally the builders during construction should have installed a vertical membrane internally from the floor up to the DPC to protect the wall and finishes or used taller skirting boards and cut back the plaster above the DPC to prevent the bridge.
Moisture wicking through the plaster was also affecting joinery fixed to the wall allowing the timber skirting boards to absorb moisture causing nail heads to corrode and decay to occur. The plaster spoiling internally was severe due to the gypsum backing plaster used during construction which reacts aggressively with hygroscopic chloride and nitrate ground water salts.
This type of survey is a regular occurrence to ourselves, and all that is needed is a simple repair involving some replastering and protection to the wall upto DPC level . Unfortunately however I see too many damp repairs which have been miss diagnosed by others leading to a re-occurrence of the initial problem.
When in need of expert advice for damp diagnosis and repair don’t hesitate to ask, call “Yorkshires Leading Damp & Timber Specialists” Dryfix Preservation Ltd. We are full members of the Property Care Association, our surveyors are fully qualified and all our workforce are Qualified remedial Technicians.
To find out more about our company, our accreditation’s and understand what all the association badges mean visit our accreditation’s page on the website or read our blog article members of the associations that matter.
Many thanks
Russell Rafton
C.S.R.T / I.S.S.E
Dryfix Preservation Surveyor
