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Cement mortar renders – defective from day one!

October 2, 2010

Cement mortar render is used by builders for a plethora of different applications including filling gaps, covering-up masonry and to provide a “waterproof bridge” between two building elements (e.g . a wall and roof tiles).

Cement mortar render on parapet wall

However, I have two problems with cement renders used on older buildings…

The first problem occurs soon after the ingredients of cement, sand and water are mixed together. The hardening cement paste will shrink and then crack.  The shrinking will begin when the volume of water in the mix starts to become “consumed” during the curing process.

The second problem occurs when the cement render has fully cured. It becomes impermeable and will entrap moisture.

Some believe that this is great news. If the render is impermeable the building will remain dry, it’ll keep out the rain. – WRONG!

The best way to think of a cement mortar render is like a lobster pot. The moisture can get in, but it can’t get out.

But how does it get in? 

The moisture gets in initially through the micro-cracks which were formed when the cement mortar started to shrink during the curing process. Further cracking will occur due to thermal stress.

Okay, why can’t the moisture just evaporate away?

Cement mortar renders are applied in varying thicknesses but seldom less that 5mm. Once the moisture enters a crack it is sucked into a labarinth of fissures and voids by capillary attraction. Once the moisture has penetrated through the cement mortar render it will be sucked into the substrate, usually soft brickwork.

The moisture has penetrated the parapet wall

The moisture can’t just evaporate away for three reasons, the first is that the render blocks out solar radiation, the second is that cement render is highly impermeable and thirdly, the mositure is being sucked ever deeper into the brick wall.

Why is the moisture going into the wall?

Well, here’s the funny bit…

In a centrally heated house the inner face of an external solid brick wall will be quite warm and dry. The drying effect of the central heating will cause the solid brick wall to “wick” across its width. Wicking is the process by which moisture is drawn into the structure as moisture on the warmer inner face of the wall evaporates out of the wall and into the room.

The process is a bit like an old parafin lamp. The flame is the central heating, the wick is the brick wall and the parafin is the moisture.

I’m waiting for the funny bit, I’m not laughing yet!

Cement mortar rendered plinth

Well if the scenario outlined above was at the base of an external wall on the ground floor, when the owners of the house come to remortgage or sell, a surveyor will do a damp check at the base of the wall and probably get high moisture meter readings. Queue Mr Remedial Damp Salesman with £ signs in his eyes just itching to inject that wall.

The funny thing is that to stop the wicking all you have to do is hack the cement render off the wall and let the sun dry the bricks out!!


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