ROOFING & CLADDING
You don’t need to be a geographer or a
Considering wind loadings, and how they vary around the country, is a crucial factor when designing a roof so
it will not be damaged. David Patrick, marketing and technical director, BMI UK & Ireland, looks at the detail.
the building, the pressure directly above the roof is
decreased on the leeward side of a duo-pitch roof.
At the same time, complex air flows are set up
in the batten space in the roof which results in
increased internal air pressure on the underside of
the tiles on the leeward side of the roof that increases
the pressure differential across the tiles resulting in
an upward force; in effect, a suction or uplift force
from above. The stronger the wind the greater the
differential and therefore the greater the uplift force.
As a result, it is critical to make sure the roof
design takes the various factors described below into
account; and consider the use of dry-fix products
on perimeters, particularly in exposed locations.
You should also exploit the free-of-charge technical
services provided by tile manufacturers, who will
calculate one in 50-year design wind uplift loads for
your roofing project; and provide you with a British
Standard-compliant fixing schedule for the tiles.
Before we get to the factors that affect wind uplift,
OCTOBER 8 | BUILDING PRODUCTS
meteorologist to know that you might
encounter different wind speeds and
wind loadings in different parts of the
country. Yet the picture is more complex than
simply: northern and exposed equals gales, and
southern and sheltered equals gentle breezes. There
are many factors which affect the wind loadings on
roofs – and these can vary quite dramatically, even
within the same location.
Again, you don’t need to be a weather expert to
appreciate changes in weather patterns over the past
few years. The UK is exposed to a severe Atlantic
weather pattern and we are seeing an increasing
number of severe weather events – including more
100mph+ winds. But it doesn’t take a hurricane,
especially in the face of inadequate roof fixings, to
cause wind damage and costly repairs.
The damage caused by wind is generally not, as
many might imagine, from the downforce of wind,
but from wind uplift. Wind uplift occurs when
While you might encounter different wind speeds and wind there are two important standards to bear in mind.
loadings in different parts of the country, there are many factors
the air pressure below the roof structure is greater
which affect the wind loadings on roofs – and these can vary
than the air pressure above. As wind flows over quite dramatically, even within the same location
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