When a Boundary Wall Becomes a Room Wall: Understanding Damp in Boundary-Connected Rooms

Damp and moisture damage in a room connected to a boundary wall

A boundary wall was never necessarily designed to perform as the wall of a habitable room. So, what happens when a room is built directly against it?
From time to time, we come across rooms that have been constructed directly against an existing boundary or “party” wall. One of the most common complaints in these rooms is damp, often appearing as damp patches, peeling paint, blistering finishes, mould or salt deposits.
The obvious question is:
“Where is the damp coming from?”
The answer is not always as simple as rising damp.
This is where proper investigation becomes important.

The boundary wall problem

SANS 10400-K, Clause 4.2.4.3, states that no horizontal damp-proof course (DPC) shall be provided in free-standing walls.
This is an important consideration when an existing free-standing boundary wall later becomes part of a building.
A boundary wall and a habitable-room wall do not necessarily perform the  same function. When a room is constructed directly against an existing boundary wall, the wall may have no horizontal DPC to interrupt moisture movement from the ground.
There may also be differences in foundation construction, ground levels, drainage and exposure to moisture.
The result is that moisture within the existing masonry can become a problem for the newly constructed room.
But is it actually rising damp?
This is where things get interesting.
At Safeguard, we don’t believe every damp patch should automatically be labelled as rising damp.
Damp in a boundary-connected room may involve:

  • moisture rising through porous masonry;
  • lateral moisture transfer from the existing boundary wall;
  • higher external ground levels;
  • rainwater entering or penetrating the boundary wall;
  • defective drainage or plumbing;
  • bridging of an existing DPC;
  • moisture trapped between two walls;
  • salt contamination within the masonry or plaster; or
  • a combination of more than one moisture source.

That is why the visible damp patch is not necessarily the source of the problem.
The first step should be to investigate the wall, the surrounding ground levels and the construction details before deciding on a remedial treatment.

When a new wall is built against the boundary wall

Where regulations and approved plans permit construction close to the common boundary, the new wall should be considered as a separate structural element, with its own appropriate foundations and damp-proofing provisions.

Simply building a new wall directly against an existing boundary wall does not  automatically isolate it from moisture.
If the original boundary wall contains moisture, placing another wall directly against it  can create a pathway for moisture to transfer into the new construction.

This is particularly important where there is direct contact between the two walls.

So how do we deal with the damp?

There is no single “boundary wall product”.
Email: info@safeguardchem.co.za

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