September 2022

Low Pitch Roofing
LONG-TERM SOLUTIONS RATHER
THAN SHORT-TERM FIXES
James Ford, Technical Sales Manager at Permavent, says ‘the Devil’s in the Detail’
when it comes to low pitch roofing projects, and in this article he explains how to avoid
the pitfalls when installing at a low pitch…
The concept of low-pitched roofing is
often challenging for contractors,
with many associating roof failure as
the result of unsuitable roof coverings
being installed on incorrect rafter pitches.
Our industry has diversified in recent years
however, with many new systems and
products now available to ensure the
designed roof and clients’ expectations are
achievable. A design such as a lean-to
extension, with a roof pitch hampered by a
first-floor window, can now be competently
waterproofed while maintaining the
appropriate aesthetics of the building as a
whole.
There are various pitfalls when installing at
low pitch and at Permavent Ltd we
always say, ‘The Devil’s in the
Detail.’
“The lesson
learned here
is to not
invite water
into the building
fabric – keep it above
the main roof covering”
Restrictions and failed remedies
Acknowledging that the pitch or perhaps exposure is
inappropriate for the roof tile or slate, designers and
installers in the past have made attempts to drain
the water away by other means. ‘Double-felting’ the
roofing underlay, bituminous sheeting and even flatroof
coverings below roof tiles and slates, were
thought to provide a complete waterproof barrier.
However, not only do these options often hamper
Left: James Ford is Technical Sales Manager at
Permavent. Above: Permavent offers a range of
solutions and can provide guidance for low pitch
roofing projects.
good roofing ventilation, persistent water
ingress and subsequent condensation will erode
roofing battens, fixings and supporting structures.
These are simply short-term fixes and not long-term
solutions. The lesson learned here is to not invite
water into the building fabric – keep it above the
main roof covering.
Downpipes
A solid and well installed roof is as only as good
as a solid design. It is important to avoid
constructions where water is discharged via a
downpipe directly onto lower elevations.
Particularly with low-pitched roofs, the tile or
slate can be compromised by discharging
Rafters
potentially high volumes of water in a
direct point load via a downpipe or shoe. It
is good practice to allow roof elevations to
drain independently, or in some scenarios
downpipes may need to be redirected.
Rooflights
It is always important to check a rooflight’s
minimum installed pitch and performance at
low-level. For example, many ‘off-the-shelf’
flashing kit units will provide a minimum
pitch of between 15 – 20 degrees, but
compatibility with roof tiles or slates should
be double checked. Below 15 degrees a
specialist window or ‘change of elevation’
flashing may need to be selected, subject to
the manufacturer’s recommendations.
“It is also important
to check with the
roofing manufacturer
their guidance on
rafter length”
It is also important to check with the roofing
manufacturer their guidance on rafter length.
Typically, wind-driven rain tests conducted
independently will work to a maximum rafter
length, dictated by the testing rig. If, for example,
a manufacturer declares a maximum rafter length
of 4m at, let’s say, a 15 degree pitch, it is
important to seek further guidance from them if
the rafters exceed this.
Eaves
It is common to experience roof failure at the eaves
28 TC SEPTEMBER 2022