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Cement render entraps moisture against a solid masonry wall

August 27, 2012

Sketch showing damp pathway through the solid masonry wall.

I inspected a property recently and thought you might be interested in the dampness caused by cement render entrapping moisture against the masonry wall.

Internally, there was evidence of moderate levels of dampness but nothing serious. However, prior to my arrival a remedial damp company claimed the dampness was rising damp and therefore expensive remedial work was required which included chemically injecting the walls and blar, blar, blar…you know the rest!!

However, when inspecting the wall from the outside it was clear that there were 3 more obvious reasons why the wall was damp:

1. The external ground levels have been raised. This shortens the distance between the surface rain water on the ground and the dry masonry above the damp proof course.

2. The external render has been taken down to the ground. This bridges the damp proof course and allows surface water to become drawn up the narrow gap between the render and the brickwork.

3. The cement mortar render is hollow. This allows surface water to migrate up the narrow cavity formed when the render and the masonry wall de-bonded.

The damp pathway is now complete!

When it rains any surface water is sucked straight up the narrow gap between the render and the masonry wall, bridging the DPC and making the internal surface of the bedroom wall damp. See my sketch above.

Removal of the hollow render revealed the dampness behind.

During my inspection I got permission to remove the hollow render which I did with the blue shovel. The render just fell off in large sections and revealed the band of dark masonry at the base of the wall which confirms the bricks were damp.

The photo was taken immediately after removing the render. The sun was shining that afternoon and since the wall was south-west facing the dark damp band of brickwork, visible at the base of the wall and which had reached 5 brick courses above the line of the DPC, dried within 10 minutes!

I recommended to the owner that the ground levels should be lowered to at least 150mm below the damp proof course and the rest of the render removed and the wall repointed with a lime based mortar.

This approach to dampness is in full compliance with BS6576: 2005 part 4. This however is in stark contrast to the remedial damp company who confirmed rising damp before establishing whether there were any other reasons why the wall was damp. I therefore saved the home owner a large amount of money from unnecessary remedial work.

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From → 3. Dampness

17 Comments
  1. Hi, your posts are very informative thanks.
    What is the purpose of rendered plinths on 1930’s properties as these bridge DPC’s don’t they (I presume they are cavity walls though)? and have you encountered problems with these in the past?

    • The rendered plinths at the base of the wall do indeed bridge the DPC and this is why it is vital to check the thickness and condition of the rendered plinth as a possible damp pathway.

      The purpose of the render plinth is to protect the base of the wall against rain splash thus keeping it dry. However, if the render is too thin the render will crack and moisture will become drawn into the render through the numerous tiny capillaries. Once the moisture has entered the render it will be trapped and unable to escape.

      It is vital that the ground levels are low, surface water drainage is good and the plinth thick enough not to crack, otherwise the render will act as a wick sucking moisture into the wall.

      My most recent post illustrated very nicely what happens when everything is wrong!

      • jonathan reid permalink

        Hello… I removed my rendered plinth a couple of months ago and lowered the ground level outside separating the concrete path from the wall of my victorian property. I have a huge amount of damp now… the walls are literally dripping wet and black with mould. As I took the render off it took with it the outer skin of the brick. This brick is now like a sponge soaking up all the rain water and dripping down the inside wall. My plan to prevent a cement based render causing damp problems by removing the DPC bridge has clearly backfired…. I now need to put the plinth back… but what with? Cement, lime or a mixture of the two? Also, is it a problem that I’m just bridging the DPC all over again? or… should I leave the render above the DPC and leave the crumbly unprotected brick below without render? so many questions…

      • If the wall above the DPC is rendered then create an expanded metal lath drip just above the DPC. This will protect the wall from rain penetration and the water will run off the render and onto the drip formed by the expanded metal lath.

        The wall below the DPC can be left in exposed brickwork if in good condition. The most important thing is to keep the ground level as low as possible, make sure surface water drainage is effective and make absolutely sure the gutters don’t leak.

        I hope this helps.

  2. meurig permalink

    Hi, we have just bought a ground floor flat in North London with moderate damp problems and have received a lot of conflicting advice. It is possible to engage you in a professional context to help diagnose our problems? I’m afraid I could find no contact details on your blog. I can be contacted via damp1@meurig.com

  3. Adam permalink

    Hi, I’m in a similar situation to the design above but I’m not in the position to lower the external ground level. Would a bell cast shape render on the bottom of the brick prevent the water rising or would you recommend just lime mortar grouting (not my preferred option as the bricks aren’t in a great state)?

    • Yes, the bell cast rendered drip would create a break in the damp pathway and prevent moisture climbing up the wall (between the brick and render) by capillary action.

      Preventing the ground from becoming wet and forming puddles is another good strategy. This can be achieved by efficient surface water drainage or creating a narrow trench at the base of the wall 10cm wide and filling it with pea-shingle.

      Lime mortar is great for allowing the bricks to dry out, as it has a more permeable open structure than cement mortar. But as you say if they’re a bit rough then covering them over with render is one option.

      You could always get a specialist brickworker to tidy the bricks up instead of covering them with render. I got Mathias Restoration to do some repairs to my house and the effect was amazing.

  4. Hi there, we have exactly the same problem. We’ve just had our house re-rendered and a new plinth put on (by a family friend) Having read your post it’s clear our plinth is too high (above the dpc) and this is causing damp in our hallway. Outside the water does puddle up against the wall so would digging a trench solve this completely or would we have to do more? We’ve read on various sites to remove the bottom section of the plinth, even up to the dpc which would look awful! Our bricks are old stock and in a bad way so ideally we don’t want to ruin the aesthetics of our newly rendered property but clearly we need to do something. If the plinth is above the dpc and we dug a trench would the water from the ground still soak up the bricks, into the plinth and subsequently above our DPC again? Would really appreciate your advice on this as you obviously know what you’re talking about! Thank you!

    • Thanks for the message.

      It is perfectly okay to have a rendered plinth at the base of the wall but the render applied to the wall tends to be too thin. This results in the render cracking and de-bonding from the bricks behind. The water then is drawn up the gap between the render and the bricks by capillary attraction.

      So make sure the rendered plinth is quite thick (25mm) and most importantly ensure that the surface water drainage is really good. This will prevent puddling of rainwater at the base of the plinth which provides the water that can become drawn into the wall causing internal dampness.

      A flower bed at the base of the wall is a good idea because any rain falling onto the ground will quickly drain away.

      I hope this helps

      Kind regards

  5. Howard Clarke permalink

    Hi, I too am getting conflicting opinions from various traders with quotes varying between hundreds of pounds to thirteen thousand pounds. The render is cracked with water also being being drawn in from the pathway giving us damp cold interior walls both upstairs and downstairs. We had our patio lowered, L shaped air bricks and a drain fitted at the rear and the whole ground floor fitted with a new wooden floor as this had risen to the point you could fall over! We were told last year this would solve our problems. Now the wooden floor is starting to rise again and now we are being told its because of the rendering on our largest side wall is drawing in water in, we are at our wits end! This is the first blog I have read that totally describes what is happening to our house. Unfortunately I can’t find your contact details anywhere could you please email me on the email address I’ve provided on registration. Or can I send in some photos?

  6. Daniel permalink

    Hello

    Great blog! Ive trawled the internet for weeks looking for proper, unbiased advice on my damp problem. I live in a ground floor flat which has damp, internally visible on nearly all of the external walls. I have had several Damp specialists out and they have all given conflicting advice but they all suggest, surprisingly, that I have a chemical damp proof course installed. Through some research Ive found that all the ground levels are the same externally as they are internally and that the external render goes straight to ground level and there are no plinths. There’s no doubt that there is damp present internally but what I want to do is get to the bottom of why it’s present. Getting a chemical dpc installed may stop the damp coming through but it would still be trying to come through. seems to me like I need to treat the cause, not just deal with the consequences. What are your thoughts? lower the ground levels? install a drainage channel for surface water? hack off render and re-render to above dpc and use a plinth? or get a damp specialist to re-plaster with all their fancy water repellent chemical plaster? would you be interested in letting me contact you directly for some professional advice?

  7. Amir permalink

    Hi

    My property has the three symptoms you described, render taken to bottom no plinth and ground level raised, I’m installing a French drain around the property which will be 150mm lower than damp proof course and then shingle/slate on top.

    Will I still need to take render up and put in a plinth?

    • Thanks for the email.

      The ideal approach is to create the french drain and then create a drip just above the damp proof course. Water running down the face of the render will be swept down to the drip and hopefully fall safely into the drain.

      Please reply if you didn’t quite understand.

      Regards

      • Amir permalink

        Many thanks for your reply, does this mean a bell shaped drip?, extending the render outwards? Apologies for not understanding!
        Cheers amir

      • Yes, it does mean a bell-shaped drip. A good builder/plasterer will know exactly what is required. Good luck.

      • Amir permalink

        Hi, so I got the works done and the following happened; French drain installed, and trench filled with pea shingle, he also painted the bricks at the foundation level with a coat of bitumen and said he couldn’t find damp proof course, and it must be higher. He has guaranteed work for 10 yrs and said the bell shape drip is not required as the pebble dash render hangs over the newly bitumen painted brick work. The trench and shingle was however filled back up to the same level as the pebble dash render/paving. Does all this sound plausible? I suppose I’m looking for peace of mind that I have taken required preventative damp proof action, and will not be susceptible to it, given to date I haven’t had any issues!

  8. Andrew permalink

    Hi there,

    Like a poster above, I too would love to engage you to survey my property. Your posts have provided me with the clearest and most helpful information I have been able to find anywhere on the web. I live in North London and am very keen to establish contact.

    I presume you can find my email address which I have given in order to post. If not please reply and I will send it to you.

    Best wishes.

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