Damp in New Extension

New Rear Extension Affected by Damp & Decorative Spoiling

Following on from my last blog, regarding damp in a three month old extension, I recently encountered a similar issue, whereby a newly constructed extension, was again affected by damp and decorative spoiling within a few months of the extension being completed. The affected wall, would have once formed the rear wall of this building, before the recent addition of a new rear kitchen extension.

New or Pre-Existing Damp?

The homeowner saw the obvious coincidence, that the dampness must be related to the new construction works, whereas the constructor, thought this was a pre-existing problem with damp, and nothing to do with the new extension.

We did see obvious decorative spoiling to the wall, with the emulsion paint, blistering and flaking from the plasterwork, and further recorded high relative surface readings using the Protimeter MMS2 in pin mode.

To establish, whether this was new or pre-existing damp, we then carried out testing for moisture at depth, both at wall base, and to other areas which seemed worse affected by decorative spoiling. The highest reading obtained was 1.5% total moisture content. This is a fairly insignificant level of moisture, and certainly not high enough to cause decorative spoiling.

Note Decorative Spoiling To Wall Despite Low Moisture Reading

Often, new plaster has been applied without an air gap, at the wall base, so it is in direct contact with the solid floor slab. Since plaster is significantly more permeable than the underlying masonry, then it can wick up moisture, and cause spoiling of the plasterwork. To rule out this potential issue, we removed a small section of skirting board at wall base. As can be seen, this was not the issue.

Plaster stops short of solid floor

The Problem with Newly Plastered Walls

Since there is no significant problem with moisture at depth, and the plaster has been applied correctly, then what can we conclude?

Hard plastered walls generally take circa 6 months to dry out, and the general rule of thumb, is that you should only apply a single mist coat of emulsion, until the plaster has dried out, after which, the walls can be fully decorated. After speaking to both the contractor, and the homeowner, it was clear that the walls were fully decorated within a week or two of the plaster being applied. It was therefore my opinion that construction moisture had caused the decorative spoiling.

Is it the Contractors fault?

Not necessarily, and in fact the homeowner was clear, that she wasn’t prepared to wait 6 months for the walls the dry, and accepted, the inherent risk, in rushing to apply decorative finishes to walls that had just been plastered. The one piece of advice I’d give to any contractor, is that you should insert a caveat into your contractural arrangements, to state that newly plastered walls should be given 6 months drying time, prior to being fully decorated. In this case the contractor did not do that, or warn the client of this inherent risk. In this case, both the contractor, and the homeowner, were both reasonable people, and this wasn’t the cause of any major dispute, but it’s easy to see how it could be.

9 responses to “Damp in New Extension”

  1. Abby avatar

    Interesting read, thanks!

    Presumably, especially with new builds, absolutely no one lets a house sit for 6 months before decorating a newly plastered wall. What are they doing to not fall into the same trap, or are they all just getting lucky?

    1. Joe Malone avatar
      Joe Malone

      Abby, constructors are moving away from ‘wet’ building trades, so we rarely, if ever, see hard plaster systems in new build. In general, everything is dry lined, with a 2-3mm skim coat of finishing plaster, hence, the 6 months drying time is not required. The blog refers purely to hard plaster systems, that might have a 10-15mm bond coat, and then a 2-3mm skim coat to finish.

      Regards. Joe Malone

      1. Abby avatar

        Ah that makes sense. I know nothing about construction but am fascinated by it all, thanks for your reply! Love reading the blog.

        1. Joe Malone avatar
          Joe Malone

          Thanks very much Abby. I’m hoping to get more time on it next year. I’ve been a bit lax this year. Joe

  2. George Gaduzo avatar
    George Gaduzo

    Good example of why thorough investigation is required to enable correct diagnosis.

  3. Robbie Robinson avatar
    Robbie Robinson

    As a decorator i do come up against this sometimes and customers look at me for
    Sound advice.
    Very helpful.

    1. Joe Malone avatar
      Joe Malone

      Thanks very much Robbie.


  4. Rod Butcher avatar
    Rod Butcher

    Hi Joe, interesting case but would like to know the remediation required. Difficult to remove the paint so hack off the Plaster in the defective area and replaster ?

    1. Joe Malone avatar
      Joe Malone

      Hi Rod,

      No, I’d never recommend removing perfectly sound plaster plaster, and in this case, the damage had already been done, but most of the drying had occurred, with the paint blistering away from the wall, as moisture migrated to the surface. Sometimes a stain block may be required, where damp staining has occurred, prior to redecoration, but in this case, I’d simply rub down the wall with a very fine sandpaper, and repaint. There are no guarantee’s with this sort of problem, but I’ll normally make judgement calls based on how long the plaster has been applied.

      Regards. Joe

Leave a Reply

Your email address will not be published. Required fields are marked *