September 2019

Common Mistakes
PITCHED ROOFING INSTALLATION:
COMMONLY MADE MISTAKES
We asked Pitched Roofing Consultant John Mercer to discuss some of the common
mistakes contractors can make when installing pitched roofing products and
materials...
Firstly, I want to start on a positive note;
there have been many advances in pitched
roofing during my career, in both materials
and installation techniques. These have been
driven by several factors, including revisions to
British and European design and installation
Standards, with these, in turn, being motivated
by organisations such as the National Federation
of Roofing Contractors (NFRC) and National
House Building Council (NHBC) as well as
building material producers.
However, new products and new practices can
sometimes introduce new challenges. There was,
for many years, a lack of awareness amongst
contractors of the requirements of BS 5534,
particularly relating to roof tile fixings. However,
persistent campaigns within the industry, coupled
with roof tile manufacturers’ development of
software to make calculating fixing specifications
extremely quick and simple, has dramatically
raised awareness in the last five years.
Remember, a fixing specification should be
calculated for every roofing project.
Underlays
The introduction of ‘breathable’ underlays
brought with it several complications. Firstly,
even the word ‘breathable’ is misleading. Early
marketing campaigns and third-party certification
of some of these products led people to believe
that roof space ventilation was no longer
required, with confusion surrounding details such
as air and vapour control layers in ceilings. Even
the way lightweight underlays are installed can
cause problems. These were originally designed
to be installed over rigid sarking or insulation, but
here in the UK we traditionally lay underlay
“The important thing is for all industry sectors to
work together to develop, improve and overcome
the issues we encounter”
unsupported over rafters – not in Scotland, I hear
you say. It is more difficult to lay lightweight
underlay with a drape and the underlay does not
naturally seal around the batten nail holes.
Therefore, the combination of no drape and no
seal at the nail holes can result in water
penetration, whether it be from rainwater driven
through the roof tiling or condensation within the
batten cavity. To overcome these issues,
consider using a proprietary nail tape to seal the
nail holes and counter-battens in place of
underlay drape. It is important that the installer
obtains the correct installation advice from the
product manufacturer and is familiar with the
recommendations given in BS 5250 on roof
space ventilation.
Low pitch roofs
This leads me to another common installation
problem; that of low pitch roofs. For many years,
there has been commercial pressure on roof tile
manufacturers to reduce the minimum
recommended roof pitches for some of their
products. Manufacturers have responded through
vigorous wind-driven rain testing and
improvements to their tiles. This, in turn, has
encouraged designers and contractors to
construct buildings with lower roof pitches.
Low pitch roof-related problems often manifest
themselves on small, domestic, single-storey
extensions. Tiles, which perform adequately when
laid perfectly in a wind tunnel, may not perform
so well when they are laid around, for example,
roof windows, where the flashings cause the tiles
to lift and not seat as well as they should. A
common mistake is to shed water onto a lowlevel
extension roof from a higher roof through a
downpipe or valley. This volume of localised
water can overwhelm the roof tiles, causing
leakage. Similarly, where a roof has a low pitch
combined with an overly-long rafter length, the
tiles in the courses close to the eaves can be
flooded due to the volume of water from higher up
the roof slope.
It is important to avoid shedding water onto lower
roofs and to follow the roof tile manufacturer’s
advice on minimum roof pitch/maximum rafter
length. Seal all penetrations, laps, and junctions
in the underlay layer.
In summary, yes, of course, roofers make
mistakes, but I hope I have demonstrated here
that the important thing is for all industry sectors
to work together to develop, improve and
overcome the issues we encounter through good
design, improved products and correct
installation.
Contact John Mercer
@johnmercer3
56 TC SEPTEMBER 2019