The bridging of a d.p.c. is a problem usually associated with new buildings, and is caused by mortar droppings and brick bats falling into the cavity of the brickwork. There are however numerous other ways in which the d.p.c can be bridged, especially in older masonry walls
The following are some of the common types of bridging that can occur:
(a) Bridging by earth
(b) Bridging by path or floor
(c) Bridging by rendering
(d) Bridging by repointing; and
(e) Bridging by additions
Bridging by earth:
This is very common where street levels have been raised and flower beds and gardens have been filled to bring them to a new level. In addition, the subfloor vents are often covered, reducing ventilation.
Bridging by path or floor
Concrete paths and veranda’s laid without membranes are often raised the d.p.c, again often in cases where general surrounding levels have been raised.
Bridging by rendering
Many old buildings are cement rendered at some stage of their life and the render may mistakenly cover over the d.p.c. This can result in dampness due to moisture rising through the cement render past the d.p.c.
Rising damp by repointing
This may occur when badly decayed mortar joints are re-pointed at d.p.c. level without waterproofing agents in the re-pointing material.
Bridging by additions
The bridging of d.p.c’s occurred to meet up between the different sections of work, or there was a complete lack of d.p.c. in the new work, which was keyed into the old at a level above the existing d.p.c.
Bridging by cavity construction
In cavity construction’s similar bridging may occur. However dampness will only occur where moisture can move from the outer leaf to the inner leaf, such as in cases where the cavity has been filled in above the level of the d.p.c. at the base of the wall.