CASE STUDY No: 5 PDF Print E-mail
‘Rising Damp was present but a new damp proof course was not required’
This was a property with extensive rising damp problems in most walls but a new damp proof course was not considered necessary.

Instructions: Inspect ground floor walls for possible rising damp.


Background:

The property has been owned by my client for several years. In the Home Buyers report my client commissioned prior to purchasing the property it stated that increased moisture was detected on a moisture meter at the base of most ground floor walls and in places the timber skirting boards recorded a moisture content of 22%. Nothing visible was present on the decorative surface so my client proceeded with the purchase. Complete re-decoration is due to take place and my client wanted to establish the extent and cause of any rising damp present prior to doing this.


Inspection:

The property was built in 1951 of 11” cavity brick walls under a tile roof. Gutters are half round plastic discharging into a combination of plastic and cast iron down pipes. Replacement windows and doors had been fitted and generally the external condition and external ground levels were good. The floors throughout the ground floor are asphalt covered solid construction. The property had not been decorated since purchase and to the eye, there did not appear to be any obvious visual evidence of internal damp problems.


Using a surface resistance moisture meter increased moisture presence was detected at the base of virtually all ground floor walls in contact with the ground up to an average height of 250mm above floor level. The kitchen had original ceramic wall tiles fitted on the walls but a moisture meter in radio frequency mode revealed a similar pattern of moisture behind the tiles where access was possible.


I was able to locate the presence of the original physical bitumen felt damp proof course in the external elevations as well as in the dividing walls of the under stairs cupboard area where the damp proof course in the inner skin of brickwork was also visible (Fig: 1).

 

dpc
Fig: 1 Original DPC visible in under stairs cupboard

 

The pattern of moisture presence readings obtained i.e. the relatively low and uniform height up to which readings were recorded above the floor and the sharp cut off between wet and dry, combined with the presence of a physical damp proof course lead me to believe the probable cause was what I refer to as a ‘detail’ fault.


With my clients permission I removed the section of skirting board between the base of the stairs and front entrance porch dividing wall. The rear surface of the skirting was affected in places by wet rot and wood boring weevil. Removal of the skirting revealed that the original wall plaster extended down past the asphalt floor surface and was in contact with the concrete slab beneath the asphalt which was also damp (Fig: 2). I removed two sections of wall plaster from behind the skirting position in an attempt to expose the bitumen damp proof course where I anticipated it would be but without success. I removed one further section near to the base of the stairs and there it was – the edge of the damp proof course (Fig: 3). Perplexed by the absence of the damp course some 350 mm away I raked out two perps (join between two bricks) and eventually located the bitumen damp course approximately 30mm in from the edge of the brick.


dpc2

dpc3

Fig: 2 Skirting board removed – plaster found to be in contact with floor Fig: 3 Plaster removed and DPC exposed

 


Conclusion:

The pattern of damp readings obtained had already roused my suspicion of a possible cause. The removal of the skirting revealed the original porous wall plaster was bridging the damp proof course as well as being in contact with the damp underlying concrete slab confirming my suspicion that the cause was what I categorise as a ‘detail fault’. In addition, at the point where this exposure took place, the damp proof course material had ‘veered off line’ for a short distance as well.


Based upon the evidence available at the time of my inspection I concluded that the cause of the rising damp in most walls at ground floor level was likely to be due to the original physical damp proof course being bridged by the original porous wall plaster which was also in direct contact with the damp concrete slab beneath the asphalt floor coating. This was a defect ‘built into’ the property at the time of construction in 1951.

Remedy:

As the cause of the problem had been built into the property since 1951 it was very likely that ground salt contamination, which is hygroscopic, was present in the wall plaster where it was affected by damp. The following was therefore recommended:


•    Remove all skirting boards throughout the ground floor level.

•    Remove all plaster from all ground floor walls up to a minimum height of 750 mm above floor level and expose the edge of the original damp proof course.

•    Remove random bricks to examine inside the cavity of external walls for any accumulated debris which if present should be removed.

•    Vacuum clean the gap between the asphalt floor and walls and apply a bitumen material in the gap, up to and past the original DPC line.

•    Replaster in accordance with the specification provided and once the new plaster had dried adequately fix new skirting boards.

 

It was not necessary to install a new damp proof course.

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