Rising Damp in 3 Year Old Extension

This week I attended a property in London where the client was suffering from quite significant decorative spoiling, caused by rising damp above the skirting boards in the rear extension. She had only recently purchased the property and given that the rear extension was only around three years old, and works were signed off by her local building control department, understandably, she was a little worried by this.


The extension was of cavity walled construction, with a lightweight concrete block inner and outer leaf of masonry, and an 80mm cavity, fully filled with Rockwool batts. One initial observation showed that the sidewall of the extension was unfinished concrete blockwork, where even the mortar joints had not been pointed up. External ground levels were also significantly too high.

Exposed concrete blockwork
Exposed and highly porous concrete blocks left exposed to the elements. The wall had not even been pointed up.

Moisture Readings

Moisture readings to the hard plaster system were only slightly elevated, but high enough to warrant further testing for moisture at depth using calcium carbide.

High damp readings
Protimeter MMS2 shows slightly elevated readings above the 20% trigger point, where testing for moisture at depth is required.

We carried out testing for moisture at depth using calcium carbide and found a very high reading of over 20% total moisture content at wall base, and 12% at the next highest level. A rising damp moisture profile, showing that moisture is being sourced from the ground. Of course, you have to contextualise these readings in light of the material being tested, and these highly porous concrete blocks will soak up moisture very quickly. Had these been brick walls, then moisture readings would have been significantly reduced.

Calcium carbide testing
Saturated concrete block walls with rising damp moisture profile

How could rising damp affect relatively new construction?

Since we know that ‘Part C’ (Resistance to Moisture) of the building regulations requires the damp proof membrane (DPM) in the floor to form a continuous barrier with the damp proof course (DPC) in the wall, then clearly, if those guideline had been followed, then the property would not be suffering from rising damp. Invasive work was now required to locate the damp proof course in the wall, to firstly ascertain if a DPC was present, and if so, was it linked to the DPM in the floor, and was it possibly bridged?

We started to remove plaster from the wall base and thankfully, located the DPC in the wall, which proved to be bridged by the internal plasterwork. This also showed that the floor DPM was not linked to the DPC, a clear breach of building regulation requirements.

Bridged damp proof course
Bridged damp proof course not linked to DPM in floor.

Bridged Wall Cavity?

External observations showed high ground levels and with the render extended down to ground floor level, then clearly any DPC present in the outer leaf of masonry had to be bridged.

Bridged DPC
External render will bridge the physical DPC

This issue should not present a significant issue with regards to moisture transferring to the inner leaf of masonry, so long as the wall cavity is clear of debris, since the cavity will form a ‘moisture break.’

However, with the high moisture content recorded to the inner leaf of masonry, we suspected there was a problem with the cavity and opened the cavity up for inspection. As suspected, we found significant amounts of debris, bridging the cavity at wall base. The debris serves to transfer moisture across from the outer leaf of masonry, to the inner leaf of masonry.

Debris in wall cavity
Debris in wall cavity

Incidentally, the wall cavity was inspected with a borescope, but full fill insulation prevented a proper view of the wall cavity, which is why I decided to open up the cavity at the door reveal base.

Should we call in the damp proofers?

Had the client called in the damp proofers, it is almost certain that they would have diagnosed rising damp, using a hand held electronic moisture meter, recommended that the walls be injected with a retrofit damp proof course, and re-plastered the internal walls with a waterproof tanking plaster or render. The water proof tanking, may have provided a dry wall surface for a while, but would no doubt have failed in the not too distant future, since the underlying problem had not been addressed.

It will be almost impossible to remedy the lack of bond or connection between the DPM in the floor and the DPC in the wall but addressing the issues that can be easily dealt with should remedy this problem. Key actions to address this problem will include:

  1. Remove the skirting boards and remove all plaster from the internal wall base to fully expose, and un-bridge the physical damp proof course.
  2. Opening up the wall cavity to remove all debris, which is transferring moisture from the outer leaf of masonry to the inner leaf of masonry.
  3. Ideally, reduce external finished floor levels, so that they are a minimum of 150mm below internal finished floor levels.
  4. Remove external render from the wall base, ensuring that the render is a minimum of 150mm clear of external finished floor levels.

None of this is specialist work, and can be carried out by any reasonably competent builder., as is often the case when it comes to remediation work for damp.

8 responses to “Rising Damp in 3 Year Old Extension”

  1. George Gaduzo avatar
    George Gaduzo

    Shocking work.
    Would be interested to know if it had been surveyed prior to purchase. Would it not have been advisable to have the flank wall rendered?
    Also, if hack off external render to avoid bridging, what’s exposed? If formed a bell cast drip at DPC and then leaving a gap rendered below the DPC using waterproof cement based render that would address the aesthetics.

    1. Joe Malone avatar
      Joe Malone

      Hi George,

      Yes, the property was subjected to a thorough pre-purchase survey by a RICS qualified surveyor. It may not have been damp at this time, but surprisingly, he never spotted the unfinished concrete blockwork. You’re right to question what may lie beneath the external render at wall base. In fact it’s concrete blockwork, and I’ve already has a discussion with the client about replacing this with two courses of blue engineering bricks at wall base.

      Incidentally, the flank wall can’t be rendered, most of it is directly abutted by a neighbouring building. On the upside, the wall isn’t particularly exposed.

      Regards. Joe

  2. Anthony avatar

    1.What would it look life if the DPC and DPM were bonded? What is the physical DPC they used?

    2. Seamless indoor-outdoor spaces with no height difference between interior/exterior finished floors are being specified more frequently now for their aesthetics. Do you think that this design choice is reconcilable with preventing moisture ingress, or are these two concepts fundamentally incompatible?

    3. To install engineering bricks would you remove the lowest course of the blocks one by one and replace immediately with the brick course, then move into the next block and do the same until all blocks replaced by the new DPC of engineering bricks?

    1. Joe Malone avatar
      Joe Malone

      Hi Anthony,

      The DPC and DPM should form a continuous barrier, therefore, you would see the DPM lapped up the wall, and turned into the same bed joint, as occupied by the DPC. The fact that the masonry can be seen below the DPC tells you that this connection has not taken place.

      Regards. Joe

      1. Aaron avatar

        Thank you, I’m doing an extension and my builder has already built up the wall and put the DPC, however they have not lap the DPM inside the wall with DPC, building control said it’s fine to put the DPM alongside the cavity wall and over the level of DPC for 150mm, is that right?

        1. Joe Malone avatar
          Joe Malone

          Hi Aaron,

          No, that’s not correct. The DPM should have been lapped over and bonded to the DPC in the wall forming a continuous barrier. Part C (Resistance to Moisture)- Section 4.7 (c) clearly states that
          ‘damp-proof membrane above or below the
          concrete, and continuous with the dampproof courses in walls, piers and the like. If
          the ground could contain water soluble
          sulphates, or there is any risk that sulphate or
          other deleterious matter could contaminate
          the hardcore, the membrane should be
          placed at the base of the concrete slab.’

          Kind Regards. Joe Malone

  3. Block Laying North Shore avatar
    Block Laying North Shore

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  4. Paul avatar

    Hi Joe,
    In the end how did you actually link the DPM to the DPC please?

    I have a similar situation, but my DPM is too short and in some places and wont reach above the DPC! I’ve already dug out a six inch trench (out of the concrete) around the room so as to give myself more work space. My plan is to tape a new section of DPM onto the existing one so as to in affect extend in further up the wall, giving me the option to then grind a slot next to the DPC. Do you think this is the right thing to do?

    By digging out this trench I’ve discovered that the existing DPM is quite badly damaged, as it wasn’t laid on sand but straight onto hardcore. I’m going to repair the exposed damage using tape. Or shall I just dig the whole lot up and start again?

    Many thanks

    PS. I’m not a builder just a DIYer!

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