Why Plaster Fails After Damp Treatment

Homeowners often find that weeks or months after successfully injecting a damp proof course (DPC), the wall plaster continues to blister, bubble, and peel. This frustrating problem is rarely a sign that the DPC has failed; rather, it is a consequence of hygroscopic salt contamination within the old plaster work.

Understand Hygroscopic Salts

Groundwater contains dissolved mineral salts, specifically chlorides and nitrates. As water is absorbed into the masonry wall over months or years, these salts travel with it. When the moisture evaporates into the room, it leaves the salts behind in the plaster. These are "hygroscopic", meaning they actively attract moisture from the surrounding air. Even if the wall is now dry at its core, the salts on the plaster face absorb air moisture, making the wall feel damp and ruining decorations.

Why Lightweight Gypsum Plasters Fail

Standard modern gypsum plasters are highly porous and completely defenseless against hygroscopic salts. They act like a wick, bringing salts directly to the surface, where they expand and crystallize, destroying paint coats and wallpaper adhesives. For a damp treatment to succeed, the salt-affected plaster must be physically chipped off and stripped down to bare brick.

The Correct Plastering Regime

Following a DPC injection, the brickwork must be re-rendered using a specialised salt-retardant backing-plaster system, or a traditional lime-based sand-and-cement render containing a salt-inhibiting waterproofing additive. This acts as a barrier, preventing residual salts deep inside the masonry from migrating out, ensuring your new decorations remain in pristine condition permanently.