BRIDGED DAMP PROOF COURSE EXPLAINED
May 24th, 2019
Most people would assume as a damp surveyor i’d only inspect old period properties, most of which are constructed absent from an original damp proof course. The truth is, however, that’s simply not the case. The majority of surveys I undertake are on much more modern properties, buildings that most would consider quality construction, with adequate protection against age old problems such as dampness.
Even tho damp proof courses have been in use now for over a century still the practice of design and implementation is constantly under going review and scrutiny, therefore it’s not uncommon to find perfectly good damp proof course failing due to construction or design errors.
To find out more about some of the problems we encounter on a daily basis, read the following article. A study examining perceptions within the industry and the most common causes of damp.
So what exactly is a breached damp proof course and how does this occur?
A breached damp proof course is caused by either part of the structure or fabric of the building allowing moisture normally retained below the DPC to progress above the damp proof course. Common causes are solid floors or fabric such as internal plasters or external renders and sometimes via a build up of debris within wall cavities.
Typical symptoms include low level dampness and spoiling of the plaster. Generally this will be wholesale if the breach is a constructional issue or can be local in the case of debris within the cavity. Profiles will normally all be retained at low level up to around 300mm dependent upon cause of the breach. Often you’ll find the skirting boards suffer most in the scenario of an internal breach as moisture is drawn into the fabric plaster at the floor and wall interface. Often skirting boards situated at the floor wall junction will decay and reveal exceptionally high moisture levels when tested.
So how do you identify a breach?
First thing to do is locate the buildings damp proof course and its position in relation to he external ground and internal structure. Most internal breaches will occur via the floor or plaster, and if so, the floors are likely to be of solid construction. Unlike timber suspended floors where the damp proof course will be sited below floor level with solid constructed floors the DPC should always be sited above finished floor level and should be visible. If you’re unable to locate the DPC visually inside, then it’s likely you’ll have to undertake some disruptive work to find it.
Here’s a typical example of a 1950’s built semi-detached property in York which should incorporate adequate protection from ground moisture. The building incorporates a physical bitumen damp proof course within the outer and inner leaf walls. The floors internally are of solid concrete construction with asphalt protected floor surfaces. Internally the property displays symptoms of damp, wholesale throughout the ground floor walls, with a profile, in general, emanating from floor level upwards to around 300mm. Damp throughout the property is causing light blemishing and spoiling of the plaster and décor. The moisture content of the adjacent timber skirting boards is exceptionally high, varying between 25% – 30% moisture content. These symptoms are all typical of a breached of a damp proof course and normally clearly distinguishable against say, the absence of a damp proof course or complete failure.
Damp proof course located marginally above internal floor level and appropriately sited in relation to the external ground.
During construction a concrete floor was cast and plastering undertaken internally. One of the last elements to be implemented was the asphalt floor screed which provides a membrane to the surface of the concrete floor. Due to the era of the build the floors are generally laid in direct contact with the ground absent from a damp proof membrane, an era prior to the introduction of plastics. The asphalt screed is commonly laid after the plaster work has been undertaken internally, therefore the asphalt does not meet the floor and wall interface and protect the DPC.
Many properties of this era often have a shallow skirting board profile which often forces the plaster to extend into direct contact with the floor and in most scenarios breach the damp proof course installed upon the inner leaf wall, located marginally above internal floor level. The above is a prime example. As can be seen from the pictures, the moisture content of the skirting boards is exceptionally high and evidence of wet rot decay can be seen on the reverse side. The plaster also extends into contact with the floor, concealing the buildings damp proof course.
Exceptionally high moisture levels recorded within the adjacent timber skirting boards.
Decay observed to the reverse of the skirting boards at the floor / wall interface.
Plaster extends into direct contact with the floor. The damp proof course is not visible as it should be.
A section of plaster has now been removed in the following picture to expose the damp proof course.
To aid in diagnosis and differentiate between a breached damp proof course and a failed DPC, a sample of the plaster has been removed, ground down and placed through a calcium carbide chamber, revealing a moisture content of 5.4%.
Calcium carbide test result from the plaster analysed at 5.4%
A sample of mortar was also removed from the first mortar bed joint, above the buildings damp proof course and again tested with a calcium carbide meter, revealing a moisture content of 1.8%.
Calcium carbide test result from the wall analysed at 1.8%
The results of these tests clearly indicate that the majority of moisture present is within the plaster. The plaster extended beyond the buildings DPC and into contact with the solid floors. The solid floor incorporates no membrane therefore the plaster will absorb moisture from both the floor slab, evaporation at the floor/wall interface and form the brickwork below the DPC. The result is moisture creeping from the floor junction into the plaster. The wall is inherently dry with only slight dampening from the adjacent wet plaster.
The above scenario could quite easily have been alleviated had the plaster been cut back above the DPC or the asphalt screed been installed up to the wall interface protecting the DPC. This is a prime example of problematic DPC installations from this period where consideration was given to protecting the structure from capillary bound dampness although the design and implementation lacked the detail need for this to be effective. The devils in the detail as they say!
Just to a point to mention. The asphalt screed provides a membrane to the surface of the concrete floor preventing moisture within the constructional slab causing decay to the floor coverings. It is essential that this surface membrane is maintained in sound condition. Commonly you may find stress fractures within the floor and membrane often around door openings in addition to damage at the walls interface commonly caused by carpet fitters nailing gripper rods to the screed. Any damage in this scenario should be repaired.
Damaged asphalt membrane caused by nailing carpet gripper rods.
I hope you found this article useful and informative.
If you would like to learn more about moisture analysis or calcium carbide meters you may also find the following articles useful:
Moisture Content Analysis – //www.dryfix.net/blog/explained-moisture-content-analysis-using-the-gravimetric-process/
Calcium carbide Meters Explained – //www.dryfix.net/blog/damp-diagnosis-calcium-carbide-meter-speedy-meter/
Russell Rafton – Dryfix Preservation Ltd Director / Senior Surveyor
Dryfix Yorkshires Leading Damp & Timber Specialists
A 4 x Industry Award Winning Company with the Property Care Association.