Damp management and remediation

DPCs and floor membranes

Most properties in the UK built after 1877 possess a wall damp-proof course (dpc), traditionally located at least 150mm above ground level (see A History of Building Control). The wall dpc has 2 functions:

  • it helps prevent moisture soaking up past it from below the ground; and
  • it acts as a barrier to water entering the wall between the ground and the dpc externally.

Evidence of this second role can be clearly seen if porous brickwork appears to be frost-damaged between the ground and the dpc where rainsplash has soaked porous masonry. Bricks just above the dpc are protected from undue soaking and remain in better condition. Cement-rendered plinths can help shield the wall base, but they may also trap moisture behind.

Also note that ground level means 'finished ground level' because sometimes contractors lay paths and pavings that compromise this necessary minimum height of dpc even before the building has been completed.

Example

Problem: an estate of huge housing blocks on the Fulham Palace Road in Central London has gone through many changes since it was built in 1900. Some of the ground floor dwellings are suffering serious dampness problems because external areas were hard-surfaced for car parking, probably 20 or 30 years ago. The surfacing has almost completely concealed the cement plinths along some elevations, and the base of the walls are stained green by organic growth from the persistent rain splashing off the tarmac. There is no longer any sign of the dpc. Ground floor flats often suffer from extensive internal mould as a result.

Solution: reducing external yard levels back to their original height may well be the ideal long-term solution, but it is expensive and disruptive as over the years door thresholds, railings, external landscaping and even drainage inspection covers may have been adjusted to suit higher yard levels. Nevertheless, in a case such as this the surveyor must make it clear to the client that, although costly, reducing external ground levels may be the only sure way of remedying the dampness problem at source. A retrofit dpc could be installed at a higher level in the wall, or internal finishes of wall and floor could be linked to provide protection internally from penetrating damp, but such options address symptoms of the dampness problem rather than the cause and would probably cause damage to the building and further problems in years to come.

When any re-surfacing around buildings is being considered, surveyors should always insist that existing surfaces are grubbed up so that the new surfaces finish at the correct height in relation to the dpc. Also, physically check the dpc position by chiselling away a section of plinth. (It is often assumed that dpcs are located at the top line of the cement plinth, whereas in fact they are often lower down; see figure 1.)

Figure 1: The staining (a) up to about 750mm high all along an inside wall actually looks like a tide mark - a wavy stain with bands of green, brown and black. Outside there was a green stained tarmac pathway that was being subjected to a high bouncing rainsplash, with the bottom zone of the wall green too, and suffering from some frost damage to the brick face. Over the years the pathway has been raised to well above the dpc level, and the dpc could not be traced after hacking off a small section of cement plinth (b). In cases such as this, try excavating a trial hole near the wall so that you can more directly learn the construction of the building near the wall base before specifying remedial works

In a modern building a damp-proof membrane (dpm) for the floor is usually lapped (or 'married up') over the wall dpc during construction. In an older building that has a raised timber ground floor there may sometimes be a marrying-up of wall dpc to sleeper wall dpc, and the ventilated floor void will help the base of the wall to breathe. In this sort of construction there will usually be a difference in the levels of external ground and internal ground (the level on which the timber floor sits, or, in more modern constructions, the concrete slab). The interaction between the bases of walls and the floor construction must be looked at closely in any survey in order to understand whether the walls and floors are performing well enough.

Where a solid concrete ground supported floor replaces an existing raised timber floor in an old building, there is usually a lack of effective linkage between the existing dpc and the new floor dpm - a source of potential problems.

Bear in mind that dpcs and dpms also affect the way moisture moves downwards. Moisture soaking into a wall or floor from above the dpc or dpm may soak downwards by gravity to collect on top of the damp course, to then spread laterally above it. Such moisture (typically from a penetrating damp source) must be able to evaporate away effectively to prevent defects caused by a significant dampness build-up. What makes diagnosis of rising damp particularly problematic is that increasing amounts of moisture present towards the base of a wall could mean that moisture is rising against forces of gravity, but also that the moisture is falling from a source above, under the influence of gravity.