What is a bridged Damp course?
The video was shot on a pre-purchase damp survey for a homebuyer in north Leeds, West Yorkshire. A RICS member identified that her was a potential damp problem and recommended our services because he needed expert advice for his client. He also recommend us due to our membership of the Property Care Association. It has a common problem caused by a bridged damp course. It’s a useful video to watch. Bryan explains and shows you how rising damp can be misdiagnosed, if a surveyor does not fully investigate the potential for other sources of moisture. Sources, which in this case, have resulted in the installation of a chemical DPC in the past. A DPC which may not be required and which could not work properly because the cause of the bridging hadn’t been resolved. In effect it was a waste of resources and someone’s money.
As you can see Bryan carried out a visual inspection to take into account the construction of the house, the presence of the original physical bitumen DPC, the mould on walls and the lack of ventilation in the kitchen and bathroom.
He used an electronic moisture meter properly and discovered a ‘rising damp profile’, which was isolated to one stretch of external wall.
Damp diagnosis should be methodical.
The video illustrates the methodical approach, which is crucial in damp diagnosis. Often dampness is caused by a combination of issues. A moisture meter on its own cannot differentiate these problems. Breaking down the information, in order to find out what is really happening rarely needs chemical analysis or flashy chemical moisture meters, gravimetric or thermography (though we use all of these sometimes, with great effect).
The evidence which matters here is:
1. There is a bitumen DPC already present (they tend to work well for a very long time)
2. The walls are cavity construction
3. There is evidence of some mould growth in a cupboard in the bedroom
4. Someone has applied polystyrene to the kitchen walls and there are mould stains behind it
5. The house is not occupied
6. The kitchen and bathroom lack any mechanical extraction
7. The walls outside are painted render and painted brick
8. The floors are suspended timber
9. The area previously treated still has a rising damp meter profile
10. Other walls are dry
11. The adjacent skirting board is damp
12. An external stair is present but it barely overlaps the damp area.
The above is quite a list, but this is what any half-decent damp and timber surveyor will be seeing and noting as he surveys. We’ll try to outline why these are important.
Bitumen damp courses have been used for many decades. They should last a long time and in practice failure or ‘perishing’ is uncommon, though not really rare. BRE 245 “Rising damp, diagnosis and treatment” confirms that most cases of rising damp, where there is already a physical DPC present are due to bridging.
These notes will give readers the fine details on how damp is investigated and diagnosis reached. Read on, or get in touch on Leeds 265 2752 York 566577 or Sheffield 224 5121 or email us via our contact page
We’d better just explain what is meant by a bridged damp course.
‘Bridging’ or ‘bridged’ is a surveying term which described a DPC which has been bypassed by damp. In effect damp can then rise up a wall, even though a DPC is present. Bridging is a physical defect, which is usually easy to see. Paths raised to a level very near a DPC (within 150mmm), can cause low level bridging externally. A path or soil, raised higher than a DPC causes severe bridging and the DPC may as well not be there, for all the good it’s doing. Another low level bridging defect is external render, applied over a DPC and worse still, in contact with the ground. These would be examples of a simple externally bridged damp course. Easy to fix too.
The second type of bridged damp course is internal. Internal bridging usually happens during construction; though retro-fit works can cause it too.
Solid concrete floors in older houses rarely had a damp proof membrane (DPM) under the concrete. If the floor is slightly damp, moisture can ‘wick-up’ the walls, causing rising damp. The damp course in the walls should stop this. However, if the supervision of the concrete floor laying is poor, the concrete may be laid a little higher than the damp course. This has a similar, though usually less pronounced effect, as a high external path does in external bridging. Rising damp is the symptom. Conversely the damp course could be above the floor, but the plaster may be covering it and be in contact with the slightly damp concrete. Moisture is wicked-up the face of the plaster, similar again to the external bridging cause by render in contact with the ground.
The third type of bridging is that which may be caused by problems within the cavity of a cavity wall. The external ground may be below the DPC, the solid floor internally too and, no render or plaster in contact. Still, the wall has rising damp symptoms. This can happen if mortar droppings or dense debris has accumulated at the bottom of cavities.
Our surveyors know all this, so the presence of the physical DPC, suspended timber floors (which don’t cause a bridged damp course) and the cavity construction, which may be susceptible to bridging due to mortar droppings are all very relevant.
Mould?
The mould is in two locations: in a cupboard in the bedroom and in the kitchen, behind polystyrene. What factors does the surveyor bring to bear as he theorises about the reasons for the mould?
First, the house is not occupied. This is relevant because if the mould is due to high humidity, caused by for example the lack of mechanical extract ventilation – the mouldy patches should be dry. This is because there will be few sources of excess humidity, now that the bathroom and kitchen are not in use; nobody is sleeping in the bedroom. In the absence of other causes of mould, such as a plumbing leak, he expects to get low meter readings from the mouldy patches. He actually gets very low readings from the mouldy stains in the kitchen, behind the polystyrene. However, he gets higher readings from the mouldy patches in the bedroom cupboard, on the side external wall. What does this tell him?
The use of a moisture meter as well as visual clues to access the causes of mould is outlined below. Read on for the details of why not contact us for friendly and knowledgeable help? Leeds 265 2752 York 566577 or Sheffield 224 5121 or email us via our contact page
Let’s take the kitchen first.
The mould stained plaster is dry. This is great evidence that it is either historic or is a transient source of moisture, which can come and go. This points to condensation. Condensation may form on a wall and evaporate away, leaving no trace, within hours. Penetrating damp or rising damp doesn’t do this. Once a wall is wet through its thickness, for whatever reason, it takes months to dry.
The presence of the polystyrene is also relevant. This is an old fashioned remedy for a damp wall. Usually applied to warm the internal face and reduce condensation. This tells the surveyor that the wall probably has a history of mould/damp problems. This, combined with the lack of high readings, lack of mechanical ventilation and the unoccupied state of the house are enough for the surveyor to determine with confidence, that the mould is related to condensation, which was happening when the house was occupied.
The mould in the bedroom cupboard.
The readings here are high. As soon as high readings are found the surveyor takes many readings to obtain a ‘profile’. The profile does not follow the mould pattern. In fact, the mould is also present along the return wall from the corner, but those stains are dry. Mould free parts of the adjacent side wall elicit similar high readings, as those from the mould stained parts. The mould is therefore not associated with the high readings and is probably a stain left from when the mould was growing. Condensation again and not surprising, as the cupboard will be prone to it; it keeps the corner of the bedroom colder than the adjacent room.
The conclusion must be that there is another dampness problem, which is not associated with the internal environment of the house. It could be rising damp, penetrating damp or a bridged damp course, amongst many others…. let’s get to work.
The entire accessible internal face of the side wall is profiled and an obvious rising damp profile is found. However, it is not uniform and the high reading terminate at varying heights above the skirting. How can the DPC fail in so patchy a manner?
The external stairs are in poor condition. They are damp of course. There is a bridged damp course behind the concrete stairs. However, this is the external skin of a cavity wall, so if the cavity is clear of debris, this shouldn’t really impact. In addition, the rising damp profile does not appear to get worse as the stair is approached. It is most severe to te front of the wall, a meter or two from the stair.
The skirting board is damp. Taking into account that there are dry mould patches and the house is unoccupied, this is the strongest indicator that there is an ongoing damp problem.
Condensation cannot be severe at the moment; it would need to be in order to saturate the skirting board. If the readings were due for example, to salts in the remedial plaster used by others in the past, this would not wet the skirting board. Our surveyor surmises there is a source of free water somewhere.
The conclusion on the exact cause of the dampness in the above house are revealed in detail below. Read on, or call us for friendly honest advice from real experts Leeds 265 2752 York 566577 or Sheffield 224 5121 or email us via our contact page
Methodically the evidence leads a surveyor to the hidden problem causing the damp.
We have rising-damp-like symptoms, in a wall with an apparently good DPC and a chemical DPC in it too. Condensation, though a historic problem in this house, is not happening at the moment. The possibility of a bridged damp course, due to a contaminated cavity must be considered.
A quick look with a boroscope, which all of our damp and timber surveyors carry, reveals the truth. The lower levels of the cavity wall are filled to varying heights, with compacted mortar droppings. This is the reason for the varying height of the moisture profile found earlier. The questions have been answered; here we have damp, caused by a bridged damp course
In addition, the low level brick areas are painted bricks with painted render. Moisture rising up through the mortar droppings will evaporate very slowly. In fact, as the house is poorly ventilated, it is likely that the damp rises even higher when the house is occupied. This is due to the higher vapour pressures internally, caused by modern living.
Recommendations
- Clean the cavity.
- Install great mechanical extraction in compliance with part F of the building regulations to the kitchen and bathroom.
The above would be the minimum required.
Additional works may be needed if the side wall remains damp. Why could this happen?
Well the passage of moisture from the ground has been going on in this wall since it was built. That is plenty of time for the concentrations of ground salts like Chlorides and Nitrates to build up in the brickwork and the plaster. The plaster had been replaced before Bryan arrived. However, it is damp now. It can’t be the right product. If it were it would not be damp (if the house was occupied this statement could not be made because the blocked cavities would present a cold bridge and condensation could happen in patches) It is likely that the salts will continue to cause dampness and may even get worse as the walls dry. Replacing the plaster with a salt resistant renovating product or sand/cement would be sensible.
Is a chemical DPC required here?
Not really, the rest of the walls are dry, so there is no reason to believe that the bitumen DPC failed. There is a slight chance that the DPC is damaged, missing in places or is beginning fail, but without taking up the floor boards and inspecting it the surveyor doesn’t know for sure
A chemical DPC was installed – why?
In fact, we installed a DPC in the inner leaf of the wall on this one. The reason was that our technicians were on site for several days to clean the cavity, replace some spalled bricks, hack off and replace the plaster and the skirting (which was decayed by wet rot on the reverse side). Installing a dryzone chemical DPC in this small stretch of walls was cheap and quick.
Taking into account the disruption and expense the client was already going though, he took the option of having the DPC installed, for a complete and guaranteed solution. This is one reason why there are sometimes chemical DPC’s installed when there is no definitive ‘proof’ that the existing DPC will not work, once the bridging defect is removed. If the DPC had been omitted in this case, it is likely that there wouldn’t have been any adverse effects. However, this is not certain.
Our client, considered the options and made an informed decision in this case. A long term insurance backed guarantee gave him the confidence that he wouldn’t be facing problems again. Unlike the poor vendor, who had to negotiate a substantial discount from the sale price of the house, to cover this work, despite having paid for work to resolve the ‘Rising damp’ in the past.
Whether he also installed efficient mechanical extract fans is unknown. The video was shot a few years ago, before we introduced our own ventilation installation and commissioning service. The ventilation was a strong recommendation and we hope it was done. If it wasn’t, the mould is probably back.
We hope that by watching the video and reading these notes you learned something about the complexity and simplicity of damp diagnosis. A bridged damp course is one of many defects, which can lead to damp. We know them all.
If you are concerned about any damp problems or, if you need a damp survey, or homebuyer survey please get in touch Leeds 265 2752 York 566577 or Sheffield 224 5121 or email us via our contact page
Vendors wishing to check their property before it is sold are welcome too – we can let you know whether a damp problem is likely to be raised during a sale. We help you decide on the best course of action (if any) – before the buyer’s surveyor arrives.
Still hungry for knowledge? Try one or two of our blog posts on surveying and dampness issues:
Rising Damp for beginners – how serious is a rising damp diagnosis
Browse our web site for tons of informative information on all preservation and building pathology matters or, visit our MD’s blog at PreservationExpert.
