PITCHED ROOFING
Frequently asked
questions answered
John Mercer, consultant at EDILIANS, the new brand of IMERYS Roof Tiles, answers some of the frequently
asked technical questions the company receives from roofing contractors
Can I use tiles below the
manufacturer’s minimum
recommended roof pitch?
This is always a thorny issue and the
short answer from any manufacturer
is usually no. Using tiles below their
minimum recommended roof pitch
increases the risk of water ingress
through the tiling, which in turn, can
lead to long-term damage of the roof
structure. There is also the issue of
aesthetics; roof tiles, in particular plain
tiles, simply do not look as good at low
pitches.
A typical example is a single storey,
low pitch extension to an existing
two-storey house with plain tiles on
the main roof. Rather than installing
plain tiles on the extension below their
minimum recommended pitch, it is
better to choose an alternative covering
such as the EDILIANS Beauvoise or
Double HP 20 tiles. Both look great at
low pitches as the angle of the view
of the roof from the ground gives
the perception of the tile tails being
closer together, mimicking a plain tile
appearance.
If the use of a tile below its
minimum roof pitch cannot be avoided,
then the sub-roof below the tiles
must be designed and constructed
to be waterproof and durable, to the
satisfaction of the local Building Control
department.
How should I twice fix
perimeter tiles?
BS 5534 recommends that all perimeter
tiles on a roof be twice fixed. This
is a prescriptive recommendation,
regardless of the predicted wind loads
on the building.
Generally, roof tile mechanical
fixings are nail or screw at the tile head,
and a clip at the tail. Although tiles
should be set out to avoid cut tiles at
verges where possible, BS 5534 allows
the use of suitable adhesive as a second
fix in conjunction with a nail, screw or
clip where it is not possible to use two
mechanical fixings.
Increasingly, dry verge systems
are installed in preference to mortar
bedding to reduce future maintenance.
Dry verge systems must now be tested
to assess their wind uplift resistance,
vertically and horizontally, in
accordance with the test methods in BS
8612; the British Standard for Dry Fixed
systems. Consequently, a dry verge
system can be regarded as a secondary
roof tile fixing, so long as it provides
enough resistance to predicted wind
loads for the particular project.
Why do my tiles break when I
walk on them?
Roof tiles and slates are not
designed to be walked on. They
must be strong enough to withstand
normal transportation, handling
and installation, but this does not
necessarily account for supporting
the weight of a person. In any case,
for health and safety reasons, roofers
should be working in such a way that
avoids walking on the tiles, or by
providing adequate protection and safe
walkways if access over laid tiling is
unavoidable.
Having said that, roof tile
manufacturers do everything they
can within the confines of weight and
material strength when designing tiles
to make sure the tiles are as robust as
possible. By avoiding ’point contacts’
between tiles and ensuring a load is
borne fully in the strongest part of
tiles; for example, directly over the tile
battens and spread across tile head laps,
the risk of damage can be minimised.
A further problem caused by foot
traffic is that broken tiles may only
become apparent sometime after the
tiles were laid, leading to complaints
that the tiles are breaking on the roof.
I have yet to come across correctly
installed roof tiles that simply break on
a roof without some kind of load being
applied to them. The usual reason why
the tiles appear to break after being
installed for a period is that they were
cracked or broken before or during
installation. Subsequent weather, such
as frost and wind, will then open the
cracks and cause sections of the tiles to
dislodge.
Do I have to mechanically
fix ridge and hip tiles on my
heritage project?
BS 5534 recommends that all ridge
and hip tiles be mechanically fixed,
and there are many benefits in doing
so, particularly for structural and
safety reasons, and to reduce future
maintenance. In the latest edition of
BS 5534, the scope has been amended
to make clear that the standard
applies to the design, performance
and installation of not only newbuild
pitched roofs and vertical cladding, but
that it also applies to re-roofing work,
including repairs to existing slate or
tiled roofs.
With regards to historically or
architecturally important buildings,
the scope states that some elements
of BS 5534 may not be appropriate
where traditional and/or reclaimed
materials are used. For example, a dry
ridge system or visible mortar-bedded
ridge tile mechanical fixings. In these
cases, advice should be sought from
the local planning authority, and
appropriate conservation organisation
and all parties should agree on the final
specification.
w www.imerys-roof-tiles.com
88 www.rcimag.co.uk November 2018
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