May 2019

Perfectly Pitched
Zonal Limitations
• Ceiling must be well sealed
• Ridge height must not be greater than 15
metres
• Roof pitch must be between 12.5º and 75º
• Site altitude must not be greater than 100m
• There should be no significant topography
“If there is any doubt
about the suitability of
an underlay for any
particular site or
project, obtain the
actual underlay uplift
resistance value
figure”
installers must be mindful of the list of criteria
that apply to underlay Zonal tables. These are as
follows:
• The ceiling must be ‘well sealed’, as defined in
BS 9250 (referred to as ‘continuous’ in BS 5534)
• The ridge height must not be greater than 15
metres
• The roof pitch must be between 12.5º and 75º
• The site altitude must not be greater than 100
metres
• There should be no significant site topography
(as defined in BS 5534)
Wind uplift calculations
Where a project falls outside any of these
parameters, the predicted wind load may exceed
the maximum declared load resistance in the
Zonal Classification table for the location.
In these cases, it is a requirement of BS 5534
that a wind uplift calculation is performed to
determine what strength the underlay needs to
be.
Some concerns have been raised recently that
these exclusions are not always considered
Above: Wind zones map.
when choosing an underlay. Many roofing
projects fall outside these parameters, possibly
because the ceiling isn’t well-sealed or perhaps
the ridge height is higher than 15 metres.
Similarly, there are many areas in the UK that
are much higher than the site altitude restriction
of 100 metres.
In view of these concerns, I ran some test wind
uplift calculations for actual development sites
close to where I live. The results were alarming. I
live in an area where the height above sea level is
much greater than 100m and many locations can
be described as having significant topography due
to the hills and valleys in the area.
For two sites, I found that the predicted wind
load far exceeded the maximum load resistance
given in the Zonal Classification Table for the
area. This means that, certainly for these
locations, it is not possible to use an underlay
that only achieves the uplift resistance given in
the Zonal Classification Tables for the location.
Therefore, it would be necessary to use an
underlay that has a higher uplift resistance.
Conclusion
In summary, it is important to always check if a
project is likely to exceed any of the Zonal
Classification Table’s conditions. If there is any
doubt about the suitability of an underlay for any
particular site or project, obtain the actual
underlay uplift resistance value figure.
Alternatively, use an underlay that is classified for
unrestricted use in all UK wind zones, such as the
range of Tyvek Supro underlay systems.
“For two sites, I found that the predicted wind load
far exceeded the maximum load resistance given
in the Zonal Classification Table for the area”
Contact EDILIANS / John Mercer
www.imerys-roof-tiles.com
@imerys
@johnmercer3
34 TC MAY 2019