October 2018

Perfectly Pitched
if we take the premise that bitumen underlay
self-seals around the nail holes, then the simple
answer is to re-create that phenomenon. The way
we do that is to fit bituminous nail tape between
each batten and underlay over the rafter. Nail
tape, as the name suggests, comes in the form of
a continuous roll of self-adhesive tape.
However, it is not practical to simply run nail tape
continuously up the underlay over the rafters, as
the roofer must stand on the rafters. Therefore, a
way to protect the nail tape is to install
counterbattens over the nail tape, underlay and
rafters, prior to installing the tiling battens. That
way, the nail tape can adhere to the
counterbattens and underlay and will create a
good seal around the nail holes.
This brings us to the second issue; i.e. lack of
underlay drape. Counter-battens not only protect
the nail tape, they also obviate the need for the
drape as they lift the tile battens clear of the
underlay, providing an efficient path for water to
run under the battens and towards the gutters. In
this application, the counterbattens need only be
10 to 15mm deep to satisfy the BS 5534
requirement for a space between the underlay
and tile battens to enable water drainage.
Detailing junctions
At the eaves, it is important to fully support the
underlay to prevent it sagging behind the fascia
which can lead to water ponding and eventual
leakage. The use of proprietary underlay support
trays is the ideal solution as these not only fully
support the underlay to maintain a fall towards
the gutter, they also provide a durable and UVresistant
drip edge into the gutter.
At abutments such as walls, chimneys and roof
window upstands, it is important to turn the
underlay up the abutment sufficiently and
consider taping the underlay to the wall or
upstand using a suitable durable adhesive tape.
For penetrations through the underlay, such as
pipes and ventilation terminals, cut the underlay
in an ‘X’ and turn it up the pipe or terminal. Fit a
diverter in the underlay directly above the
penetration so that any water on the underlay is
directed away from the penetration as it runs
down the roof slope.
Conclusions
What every contractor wants is to do a job as
efficiently and cost-effectively as possible, be
paid and to not need to go back to rectify
complaints. Following these simple
recommendations will go a long way towards
ensuring that each job is a success, first time.
Contact Imerys Roof Tiles / John Mercer
www.imerys-roof-tiles.com
@imerys
@johnmercer3
Below: Imerys Beauvoise Slate installation.
“What every contractor
wants is to do a job as
efficiently and costeffectively
as possible”
36 TC OCTOBER 2018