dampness in buildings

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EXTERNAL RENDERING & DAMPNESS IN BUILDINGS
NOVEMBER 28, 2011 BY RMASURV
RMA Surveyors have attended many building surveys and found that an internal damp problem can be directly
attributed to an external render specification.
The subject properties are predominantly solid wall construction, although cavity walls can and do suffer equally
from poor rendering. The problems I found are all similar and occur due to a lack of understanding as to the
purpose of an external render coat. In many cases I have found that external render coats are applied in an
attempt to keep water out. The line of logic employed seems to be that the more impervious the external layers
the less likelihood there is that water will penetrate. This approach will usually do more damage to a building
than good. A render coat is a wearing layer that protects the wall behind from the elements, whilst allowing the
wall to evaporate moisture.
External walls should be designed to get wet. They should also be designed to get dry. Often I find that external
renders are solid. By which I mean they have been applied using a mortar that is high in cement content. This is
done on the basis that a strong cement mortar will keep water out. It is true that by specifying a strong cement
mortar the gaps between the sand aggregate are filled and this initially prevents the passage of water into the
structure. It also stops water getting out.
How does water get in? You’ve applied a nice solid coat that stops the passage of water, therefore dry walls
right? In the short term maybe, in the longer term almost certainly not.

2. Cracking in a strong cement mortar
A fact of construction is that all buildings are in a constant state of movement. Gravity, entropy, friction, inertia,
tension, torsion, expansion, contraction, thermal movement and all the other universal laws of physics apply and
result in movement.
Different building materials move at different rates. The more flexible a material the more is able to
accommodate movement. This usual result is the harder less impervious material begins to crack, which allows
water to penetrate into the structure.
For walls to effectively dry out there must be plenty of surface area in contact with the air for moisture to
evaporate from. A wall covered with an impervious layer of hard cement mortar will not allow water to reach the
surface. Water that has entered into the structure of the wall is now trapped behind the render; it has no way of
getting out.
The years pass. The rain pours. The wall gets wetter. Damp begins to appear inside the house.
In this scenario people sometimes turn to damp proof contractors for an answer. Damp proof contractors can be
very good. They can understand the nature of dampness in buildings and give excellent advice. They are also
businesses who have a wider interest than just providing a damp proofing solution. The advice they give may
not always be in the best interest of the building.
I have been to many properties where damp proofing work has been carried out but damp is still a problem. That
is another subject for another post.

3. Many problems with modern rendering come down to the prolific use of cement. Render coats were usually
made from lime not cement. Lime is more porous and flexible. Cement has been widely used as it is very
strong, durable and is fast to set. When I specify a render I always incorporate some lime to increase workability
and avoid shrinkage related cracking. Unfortunately many who apply render do not understand the properties of
the materials they are applying. Trades and professionals employ a strong cement to improve workability during
installation and to act as a weatherproof barrier. Hard cement renders often form cracks that look unsightly as
well as reduce the overall weatherproofing and thermal performance of the wall. They can also detach from the
masonry they are adhered to becoming loose and hollow sounding, when tapped.
In practice it is a weaker render mortar that will perform better than a stronger mortar in providing weather
protection. Although a weaker render is more porous and will allow some moisture to penetrate, that moisture is
more able to evaporate.
Cracking can also occur where the surface of the render has been over trowelled, where cement content is
denser, as it has been drawn to the surface, as it dries out crazed cracking forms on the surface.
Render meets ground level
Other common rendering defects are manifest. For example render is often taken down to ground level, covering
over any damp proof course in the process. This effectively creates a bridge for water to travel from the ground
up the wall. Render should stop just above DPC level and the DPC level should be 150mm above the ground.
If you have dampness in your property or are concerned by the quality of workmanship please contact us.

4. FILED UNDER: BUILDING SURVEYING TAGGED WITH: BUILDING CONTRACTOR, BUILDING DEFECTS, BUILDING
PATHOLOGY, BUILDING SPECIFICATION, BUILDING SURVEYOR, CEMENT RENDER, DAMPNESS IN BUILDINGS,
HUNGERFORD, LIME RENDER, NEWBURY, PARTY STRUCTURE, PARTY WALL ACT, PROJECT MANAGER, RENDER,
SURVEYING SERVICES, SURVEYS, UK SURVEYING COMPANY
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