Damp Genius was recently instructed to carry out an impartial survey on a mid terraced house in the Plymouth area. The house was a fairly typical property and of 9″ brick wall construction which was rendered to the front and rear. Once inside the house, it was immediately clear that some previous damp works had been carried out around the bay window but these were done before the current owners bought the house and no record of it existed.
Once inside the front room, it was hard to miss the deep yellow coloured salt band 1.2m above the floor level and the immaculate plaster beneath it. A full survey was carried out in accordance with our standard methods following BRE245 guidance.
When a building is treated for a damp proof course failure the standard approach is to hack off plaster up to 1.2m, drill holes along the mortar line between the brick, squirt in some new damp proof course fluid and then replaster. It is this new plaster which both causes and hides the damp problem in this house.
When drilling this wall to take samples (more on this below) it was very clear that a hard sand and cement render has been applied. The drill struggled through the cement and then glided easily into the wet bricks behind. This sand and cement render would have had a salt neutralising waterproof additive put in the mix to prevent salts from damaging the new plaster. It certainly achieves this as the pictures show! The trouble is it also stops this solid walled property from being able to breathe at all. The damp in the wall is now trapped with no escape since the property has also been rendered externally with impenetrable sand and cement.
In this situation, on a solid walled property which is rendered externally with sand and cement, it is impossible for the wall to dry out. Even if the damp proof course had worked (it didn’t, more on that later too) the residual moisture would still be likely to push up and over the render to create a salt band stain halfway up the wall.
Has The Damp Proof Course Injection Failed?
To understand if the damp proof course injection was working or if the salt staining was simply residual damage it was necessary to take accurate moisture meter readings behind the sand and cement plaster. This task could not be done using your standard 2 pronged electronic moisture meter since these commonly used tools react to salt giving false readings, plus it could not reliably look behind the render. Instead plaster samples were taken at three intervals up the wall and these analysed on site using a Speedy calcium carbide meter to get a total moisture content.
The results of this calcium carbide damp meter testing found that the wall was still wet at the base but this diminished the further up the wall from the floor the samples were taken.
So what does this mean in plain English? Well, the wall is still wet, so the rising damp treatment has not worked. The impervious sand and cement plaster and damp proof course injection has done its job perfectly and kept the damp hidden away in the wall for many years without actually solving anything.
This type of cover up bodge is extremely risky. The worst thing that could happen is the damp wall sends more and more moisture down into the floor joists, on top of the cost of the dpc injection, the homeowner now has to fork out many thousands of pounds to have the floor replaced once it has rotted away.
What Caused Damp In The Wall?
This building was constructed around 1910 out of 9″ solid brick walls. All buildings within England at that time had damp proof courses installed as standard and knowing this area of Plymouth it is fair to say that it probably bitumen. These do not really fail without some serious structural movement. Some in the damp industry will say that DPCs break down due to freeze and thaw action but after many years looking at damp problems I am not convinced this happens. Is it really reasonable to believe that bitumen (a warm thing compared to bricks) will freeze in the middle of a 9″ wall with sufficient regularity to break apart a 3mm thick piece of tar? What path would the rising water take since it cannot travel up bitumen or the new air gap?
In this instance damp proof course (DPC) failure is nothing to do with the damp problems found in this wall. Looking outside the house it was clear that the render was taken right down to the concrete ground creating a bridge over and around the damp proof course (original and also the newer injection). Then at the base of the wall the situation was compounded by a rainwater downpipe which was leaking and discharging onto a piled up layer of stones and earth. Once on the ground this water, along with any rainwater directly landing on the area, was settling to soak down through the cracks at the wall/floor junction.
The whole situation was being further compounded by the Ordinary Portland Cement render on the outside of the building trapping moisture in the wall. Combined with the internal sand and cement plaster the minor amounts of water in the wall were becoming trapped then building up over time.
There is no record of this damp treatment or photos of the area we before so it is impossible to know what the wall looked like; however, it is likely that it was originally a relatively minor damp staining problem at the very base of the wall which if treated with some sensible external works could have been resolved years ago.
Damp Survey Recommendations
The invasive damp survey showed the problems are being caused by a combination of external defects and hard sand/cement render trapping moisture into the wall. The recommendations to this client included:
- Remove all the debris from the front area.
- Repair the guttering downpipe to prevent further leaks.
- Repair any cracks in the concrete base.
- Internally our recommendation was to remove the existing plaster from floor to ceiling height and then leave this for as long as they can bear to let the wall dry out.
- Once dry, use a salt neutraliser to prevent further salt problems.
- Replaster using a breathable plaster mix.
These recommendations will solve the issue long term and result in a drier, more pleasant living space which can be enjoyed by the family for many years to come.
