Is blown plaster caused by damp or poor preparation?

Question

Does this mean I have to do the whole thing again? Why is this even happening? I've done this before in our previous home and didn't have an issue

Philip · Answer

Hi it can be caused by both rising damp can cause plaster to blow of internally. And externally can happen if cracks appear and water and frost can get into the plaster a make it blow of .or both internal and external can be caused by poor preparation.  Hope this helps cheers phil @PMCC plastering.

Mike · Answer

It could be either. Most commonly it’s caused by damp.

• Moisture gets behind the plaster and breaks the bond with the substrate.
• Can come from penetrating damp, rising damp, leaks, condensation, or even trapped moisture from previous work.
• You’ll often see salts, staining, or crumbling backing plaster.

If the plaster is blown in patches near external walls, chimneys, or around pipes, damp is almost always involved

Poor prep can be a problem however.

• Dusty, shiny, or unprimed surfaces stop plaster from keying properly.
• Old paint, PVA slapped on too thick, or not using the right bonding agent can all cause delamination.
• If the substrate moves (loose render, blown backing coat, loose plasterboard), the finish coat will fail.

Poor prep tends to cause clean, dry delamination — the plaster comes off in sheets with no signs of moisture.

Did you blue grit the wall first? I blue grit every surface that isn’t plasterboard now days.

🧰 How to tell the difference

Here’s a quick way to diagnose it on site:

Blown plaster caused by damp usually shows up with a damp smell, salts, staining, or a crumbly backing, and you’ll often hear hollow patches on external walls or around things like pipes, gutters, and chimneys. When it’s down to poor preparation, the plaster tends to come off cleanly and dry, often peeling away in sheets if the surface was dusty or over‑PVA’d, and newly plastered walls are especially prone to this kind of failure.

Stewart · Answer

Blown plaster occurs when the plaster detaches from the underlying brick or block work, creating hollow-sounding, loose, or bulging sections. It is primarily caused by moisture (damp/condensation), poor installation (lack of sealant/bonding), or structural movement. It must be removed to prevent further deterioration and ensure a stable, dry surface for repairs.

SR Gill Builders
 +4
Key Reasons for Blown Plaster
Damp and Moisture Ingress: The primary cause in the UK, often from leaking roofs, damaged masonry, rising damp, or high condensation levels, which destroys the adhesive bond.
Temperature Fluctuations: Rapid changes in temperature can cause stress, leading to detachment.
Poor Application/Surface Prep: Failure to use bonding agents (PVA) or to properly prepare the wall (cleaning/sealing).
Structural Movement: Settling of the house or vibrations can cause cracks and separation.
Aging/Incompatible Materials: Older, brittle plaster is more prone to failure, especially if modern, incompatible materials are applied over it.

SR Gill Builders
 +4
Signs of Blown Plaster
Hollow Sound: A dull, hollow sound when tapping the wall.
Bulges or Bubbles: Visible raised, uneven areas.
Cracks and Looseness: Cracks, particularly on internal corners, or areas that feel soft or loose to the touch.

SR Gill Builders
 +4
How to Fix It
Identify the Source of Moisture: You must fix any underlying damp issues (e.g., failed DPC, leaks) before repairing.
Remove the Plaster: Chip away the loose/hollow plaster back to the brickwork.
Prep the Wall: Ensure the surface is clean and appropriate bonding agents are used (or wet the brickwork for sand and cement).
Re-plaster: Apply new plaster to a solid base.

Is blown plaster caused by damp or poor preparation?

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