Damp,  Tips & HowTo

Can removing cement ‘waterproofing’ actually dry out your home?

Cement render is often applied in an attempt to ‘waterproof’ or tank a wall. On this house the back walls have been rendered both externally and internally in places. As you will see, there is convincing evidence that this has actually been trapping moisture, one way or another.

To give context, this is the room in question…

Before (14/08/2015) :

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During (01/08/2017) – having removed a mixture of lime plaster, gypsum & cement :

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After unblocking fireplace, replacing damaged bricks & slumping arch, applying 3 coats of limewash to walls & repointing chimney breast with lime putty mortar (09/08/2017) :

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You can see exactly how the limewash reveals the problem areas much more clearly, whilst also ‘coping’ with the damp (ie, not blistering etc). The before picture shows a wall that has been repeatedly ‘sealed’ with impervious tanking plasters, extremely hard cement renders, paints, vinyl wallpapers…. and yet all these measures have done is temporarily mask the symptoms, forcing the moisture further up the wall in a quest to evaporate.

At some point, the moisture has found a way through, salts have crystallised; causing the layers to bubble and crumble… thus the symptoms have reappeared with a vengeance. It would fit the commonly quoted measure of madness to keep repeating this process and hoping for different results, yet this is exactly what happens with far too many of these properties – mine being a perfect example.

The limewash allows us to visualise damp areas and possible cause/effect much more clearly whilst also stabilising the dust to make the environment a bit more pleasant after removing the plaster, allowing the walls to ‘breathe’ and dry out.

The wall being discussed :

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You can see the area at the top of the room on the external wall had the original lime plaster – the bricks are in good condition. The lower half had extremely solid cement render welded on to the soft Victorian bricks. You can see the pattern this has been applied – presumably reflecting the original pattern of damp the person applying it was trying to ‘hold back’.

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The limewash reveals patterns of current manifestations of damp, acting as a visual ‘map’ of problem areas and aiding in the process of diagnosis. It is an economical material & requires little skill to apply.

We have found through our experience in the kitchen that the limewash seems to quickly carbonate (or go white at least) not only where it is dry, but also where there appears to be salt deposits. Additionally bricks that still have cement on them will often carbonate before other sections… even when the brick underneath is wet, it seems that the surface is dry enough to carbonate due to the impervious cement.

The house has this pattern all around the back downstairs, with very damp areas at the base of the walls, followed by sections that are white, then with a slightly damp section above.

On the the 3rd September I removed a good chunk of the cement render under the window after accidentally dislodging part of the windowsill cement ‘repairs’ when pulling off the TV aerial cable. I had a notion that it may be contributing to damp ingress via the fixings which punch holes through the render. Whilst there is only a small chance this is contributing anything significant, there is no harm in removing them since we don’t currently have a television :

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The external wall in question….
August 2015 :

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03/09/2017 – you can see the original permeable bath stone / oolitic limestone windowsill emerging and the blown cement render coming off in satisfying chunks under the window:

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04/09/2017 …the morning after my initial cement removing rampage! You can see that the mortar is very dark, almost completely black in colour. It looks like mud on your fingers, but soon dries out going much paler – which indicates that it is definitely damp, rather than just being dark in colour :

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05/09/2017 – Ian helped remove some more – this was much more slow going since it was better adhered… it is a good vent for anger at first (whacking a wall with a chisel and lump hammer) but then the frustration at whoever thought it was a good idea to put this evil material on originally becomes too much to keep going!

Most of the brick faces have sadly been blown by this decision to render with cement. It may be mainly cosmetic, we will have to observe over winter whether the freeze/thaw cycle causes excessive spalling of the bricks due to their more vulnerable inner faces being exposed. I will then have a choice of whether just to repoint with lime, limewash, or lime render to provide more protection whilst retaining breathability (or more accurately – vapour permeability)

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A slug demonstrating a good potential point of water ingress & hiding spot – a gap between the garden wall (that my neighbour’s lean-to rests upon) and the wall of our houses. There is cement render all around acting like a nice bucket to hold in any water that gets in! :

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A substantial crack in the windowsill, carried through to the cement that had been added on top, meant that water landing may have run along & been channeled into the crack, then unable to escape, would travel down into the wall. As it was originally designed, the oolitic limestone being permeable will allow the rain to sink in slightly where it lands, then evaporate back out. Any deluge that causes actual runoff would be protected by a ‘drip line’ … a groove under the windowsill that ensures the water falls away from the wall rather than seeping down it.

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I also removed render from the garden wall (it was mostly blown anyway so ‘wanted’ to come off)

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…and we used an angle grinder to cut a 20cm section of the concrete floor away from the base of the wall. You can see the foundation stones starting to dry out. After this has dried out a bit more, we will backfill with some gravel to provide an evaporation zone at the edge of the wall & ensuring no damp soil is held against the base of the wall :

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Now to the improvement I have observed, after having removed the external cement render. Bear in mind that this section is not even an external wall – it actually backs onto the kitchen – so the moisture is moving sideways from/to under the windowsill.

21/08/2017 – almost 3 weeks after applying limewash, just after we had been away from the house for a week (hence had not been opening windows, but also had not been generating moisture ourselves with cooking/showering etc) – this pretty much reflects how the wall looked prior to removing the external render :

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06/09/2017 …a couple of days after removing the cement render. This wall has been (more or less) in stasis for around a month looking like the photo above – look at the various sections of bricks that are now carbonating/drying out by going white :

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I was aware that external cement render was bad for solid walled houses, but I hadn’t appreciated quite how quickly it’s removal could make improvements to manifestations of damp on the internal wall. That moisture – wherever it is coming from – is desperate to evaporate! Fingers crossed that much more dries out too.

Meanwhile on the other side of the window…
09/08/2017 – soon after applying limewash :

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21/08/2017 – after returning from being away for a week. You can see the damp almost seems to have spread out or diffused in our absence. Some of the carbonated areas have become damp again, for example at the base of the wall just above the floorboards. There is a ‘tidemark’ spreading sideways that is quite clear at the bottom right of the photo :

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06/09/2017 – a couple of days after removing the external cement render :

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Meanwhile on the outside…

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You can quite clearly see a delineation between the darker wet areas of mortar and the upper sections that have now dried out.

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“Dear humans. Please stop putting cement on your houses. You aren’t waterproofing, you are making us wetter. You are crumbling our faces off & disintegrating our joints. Plus it really, really hurts when it is removed. Avoid the false economy. Be kind to us, use lime. Yours sincerely, Your Walls.”

Further information/videos

Peter Ward from Heritage House demonstrates and explains why cement render is so damaging to old houses on both his website and his Youtube channel.

This video on his channel shows the removal of cement render from a stone and lime mortar house. It is amazing how quickly it begins to dry out.

Another video of his demonstrates a similar dampness of mortar as with my house after removing the cement render – it is like mud on the fingers and yet quickly drys to dust upon exposure to air.


    • bristolterracerestoration

      Absolutely! It really illustrates how important it is to consider the house as a whole – holistically if you like – since everything is interrelated in often surprising ways. By being sensitive to small changes, using the various senses, measuring & documenting (in this case via photography), contemplating the physical/chemical material properties & putting some thought into the more abstract processes related to moisture; we can start to build up a bigger picture of the whole to aid in troubleshooting, rather than simply covering up symptoms as has been done previously.

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