Sources of moisture

DPC bridging

'Bridging' usually refers to the situation where moisture tracks around a damp-proof course (dpc) – usually via porous finishes. For this to occur there must be:

  • a dpc;
  • sufficient moisture in the wall below the dpc;
  • a finish of sufficient porosity;
  • raised external ground levels at or above the horizontal dpc; and
  • poor construction detailing, e.g. retro-fit internal solid floor and unlinked dpm to dpc.

Bridging may also occur in a cavity wall construction where moisture soaks from outer to inner leaf, for example through mortar droppings lodged on cavity wall ties. In true dpc bridging, moisture soaks upwards through porous material that spans the dpc. Dpc bridging is sometimes misdiagnosed. For example, wet soil is banked up high against the front bay wall of a house. The wall above the dpc is becoming increasingly damp because of this wet soil in contact with it. 'Bridging' is not the problem here; it is penetrating damp above the dpc height (figure 1).

Figure 1: Rainsplash has soaked the wall just above the plinth because of the garden soil piled against the wall; if the soil is removed the wall will dry out

Also consider the carelessly installed patio slabs that have taken the finished external ground level an inch or so above the dpc. Is it likely that moisture is mainly tracking from a wet wall zone below the ground, via the concrete slabs, and back to the wall above dpc level? Or is it more likely that the base of the wall is becoming soaked by rainsplash just above the new raised surface level? This is a case of penetrating dampness.

There are 8 separate examples of bridging of a dpc contained in BRE Digest 245:2007:

damp

If the damp course above the ground is concealed by a cementitious plinth, moisture could be transferred from a damp brick below the dpc to a brick above the dpc – and this would be true bridging.

Figure 2: After chopping off part of the plinth, the dpc was found to be one course of brick above the yard, but surprisingly no dampness was present due to the‘bridging’