Head Above The Parapet
A regular client with a portfolio of properties recently sent me some images of a particular damp problem to the rear kitchen extension of the property they had recently let. They knew there was some minor dampness to the head of the kitchen wall but expected that this was a result of the property standing vacant for a long time and they thought it would dry once occupied. To my mind there is no good reason for vacant properties to be damp but in any event on seeing the images it was fairly obvious that the property had a severe penetrating damp problem to the head of the rear kitchen wall.
They appointed me to investigate the problem and when I arrived I noted that it was a single storey extension with a parapet wall directly above the damp kitchen wall. It has got to the point that if I see a parapet wall I know it has defects and has been detailed incorrectly so I always open up to confirm.Out came the telescopic ladders and I accessed the flat roof deck to be met with the following site.
It was immediately obvious that the coping stones were too narrow for the application but I also noted that the mortar joints were heavily eroded and seemed to be comprised almost entirely of red sand. I scraped the joint and it was clear that the sand had only been mildly threatened with the possibility of any cement.
I lifted off a coping stone and as expected saw that critical technical details were absent. The was no hard support across the cavity and no physical damp proof course installed. The parapets would continue to take in rainwater until these defects were corrected.
There are a number of proprietary systems on the market now for securing copings, such as cavity lock systems, and there are even proprietary hard supports available to bridge the head of the cavity but in the absence of any of those systems I have detailed the method that can easily be achieved by any reasonably skilled builder with readily available tools and materials.
I always include a drawing of the correct technical detail in my reports so that this can be given to the contractor because it is rare to find builders who understand the required technical detailing, which is odd because parapet walls are becoming a very common feature on some new build sites. If you’re interested in that detail then here it is…