May 2021

Perfectly Pitched
CONSULTANT CASE STUDY:
EAVES DETAILING IN FOCUS
Technical Roofing Consultant John Mercer teamed up with roofing expert Chris Thomas
for a survey of a traditional handmade clay plain tile roof, which had suffered from water
ingress through the fascia and bargeboards. As John explains, as is often the case, the
issue was with the detailing at the eaves and subsequent troughing in the underlay…
Irecently inspected a traditional handmade
clay plain tile roof where water has been
running through the fascia and bargeboards,
causing damage to the timberwork.
I was accompanied during the inspection by Chris
Thomas, whom I persuaded to come out of
retirement temporarily to assist me. Chris has a
vast wealth of knowledge on roofing, and he wrote
an article many years ago on eaves detailing, so it
was great to have him along.
One of the most common problems I
come across when carrying out
roof surveys is the detailing at
eaves, and in particular the
lack of support for the
underlay behind the fascia
board or eaves tilt fillet. This
causes a trough in the underlay
directly behind the fascia, which can
trap and collect water.
“It is important that the underlay be laid to
ensure that any water in the batten cavity can be
drained safely into the gutters”
Left: John Mercer, Technical Roofing Consultant.
The water invariably leaks
through the underlay as it
finds its way through any cuts
and nail holes.
The importance of the
underlay’s function as a watertight
layer is often overlooked. The underlay
is the last line of defence if water is driven
through the tiling in extreme weather conditions,
and with the use of vapour and air permeable
membranes there is the likelihood of
condensation forming within the batten cavity.
Therefore, it is important that the underlay be laid
to ensure that any water in the batten cavity can
be drained safely into the gutters.
Manufacturer’s details always show the underlay
fully supported at eaves, whether it be by a
plywood board or proprietary plastic underlay
support tray, though it is surprising how often
these are omitted.
Above: Roofing expert Chris Thomas, who accompanied John on this survey, checks the roof pitch at the eaves.
It is not good practice to continue the general
underlay into the gutter as it will degrade through
exposure to sunlight, therefore the common
solution is to install proprietary plastic eaves
support trays which not only support the underlay
behind the fascia at the correct fall, but they also
extend into the gutter and are UV-stable. The
alternative solution is to fit a timber support and
lay a minimum 300mm wide strip of 5U
bituminous underlay at eaves, which being
resistant to UV degradation can extend into the
gutter.
20 TC MAY 2021