Civil Engineering Reference
In-Depth Information
materials crack, moisture gets in, filters
downwards, and can't get out again. This is
one of the main causes of damp in modern
houses.
concrete foundations, then we have created
a waterproof barrier that allows moisture to
collect at the base of the wall just where we
don't want it and prevents it from filtering
downwards and safely out of the building.
Modern buildings use a tray dpc that
directs any moisture at the base towards
the outside wall not the inside, as part of a
cavity wall system.
Cement is used in buildings as a rigid,
waterproof and strong material. Unfortu-
nately, all these properties can contribute to
damp problems. Firstly, its rigidity means
that as the ground (and therefore the
building) moves imperceptibly over time,
the cement cracks because it is not flexible.
Then, rain gets into a building through these
cracks, filters down behind the stone or
render finish, and builds up at the base of
the wall because it cannot get out again as
the cement mortar joints are waterproof.
Also, the strength of cement means that it is
often stronger than the brick or stone it is
holding together and so erosion of the
weaker material happens at the interface
between the two, causing the brick or stone
to flake away and the cement pointing to
remain intact. This creates cracks through
which damp can enter, causing problems
over long periods of time.
The metal wall ties that are used to hold
the two wall skins together in cavity-wall
construction also hold any cement mortar
that is dropped into the cavity, a common
occurrence when bricklayers are working
fast and not caring too much about quality.
This mortar then becomes a bridge for
moisture to pass along to the inside of a
building, and causes problems years later.
Designing foundations
When thinking about designing foundations,
the first thing to decide is whether the
ground itself can provide us with a natural
foundation. If it can, then there's absolutely
no need to dig it away and create an artificial
one. It has become the norm not to think
about what the actual ground conditions are
on a given building site, but to go for the
lowest common denominator, a solution
that will work in pretty much every situation,
regardless of what the earth is doing for us,
and dig 450mm- (18“)-deep trenches and fill
them full of wet concrete. In many cases,
this is not necessary and thus creates an
unjustified environmental impact that can
also be expensive! But it does mean you
don't have to think or have interesting
discussions with your engineer or building
inspector.
The problems caused by inappropriate
use of materials and design take years
to become apparent,
so it can be hard to
accept that the damp at the bottom of your
150-year-old stone-and-lime kitchen wall is
caused by the cement pointing the previous
owner did 30 years ago coupled with the
new gypsum skim you plastered the walls
with two years ago.
Using porous materials with a damp-proof
course (dpc) on top shouldn't be a problem
as long as there are no holes in the dpc -
which is easier to ensure with slate than
plastic. But if the dpc extends the whole
width of the wall, e.g. on top of poured
