October 2020

Perfectly Pitched
Continued from page 22
degrees and 290mm at 15 degrees. The welt
should be formed away from the tilting fillet, not
on top or the side of the tilting fillet, as it would
be ineffective and may cause the tiles to kick up.
The general roofing underlay must finish over the
edges of the valley boards so that any water in
the batten cavity will drain onto the valley; the
underlay must not extend under the valley,
otherwise water will run underneath the valley
construction and may leak into the structure.
“The ends of the
battens should be
supported and nailed
on the edge of the
valley support boards”
The tiling battens should be cut in line with the
valley, approximately 10mm short of the lead welt
to allow any moisture on the underlay to drain
down the eaves uninterrupted. The 10mm gap also
helps to prevent the ends of the battens puncturing
the underlay where it rises onto the welt. The ends
of the battens should be supported and nailed on
the edge of the valley support boards.
Mortar must be laid onto an undercloak of slate
or fibre-cement board rather than be bedded
directly onto the lead, otherwise the mortar will
crack and dislodge as the lead expands and
contracts. There should be a clear gap of at least
25mm between the mortar bed and valley
upstand to prevent water tracking over the
upstand. The mortar must not block tile interlocks
and should be bedded and pointed up in one
operation or be pointed up as soon as possible
afterwards to ensure the bedding and pointing
mortar set as one piece.
GRP valley troughs
GRP valley troughs are a great alternative to lead
lined valleys and are suitable for most situations
where the open valley is 100 to 150mm wide.
They are flexible enough to cope with valleys
where the roof slopes are within 15 degrees of
each other. For greater differences, check with the
manufacturer of the valley trough.
Generally, GRP valley troughs should be
supported on valley boards as described above
for lead valleys, though some manufacturers say
their valleys are sufficiently robust to enable them
to be installed directly over the rafters. These
valleys have pre-formed upstands to prevent the
ingress of water and, for mortar bedding, some
have integral sand strips to ensure a good
adhesion between the mortar and valley lining.
Preformed GRP valleys with central
upstands
An increasingly common alternative to traditional
open valleys is preformed types with a central
upstand. The roof construction is the same as
that for open GRP valleys, but instead of having
an open channel like traditional valleys, these
valleys have a central upstand and the tiling is
finished close to the upstand on each side. These
valleys have channels at each side of the upstand
to direct water towards the gutter without
discharging over its edges.
Valleys with central upstands are beneficial where
two roof slopes of unequal pitches meet at a valley.
With open valleys, there is a risk that water from
the steeper roof slope will run across and run over
the edges of the valley on the lesser pitch slope. A
central upstand will prevent this from happening.
“All perimeter tiles,
including cut valley
tiles, should be twice
fixed”
Tile cutting
Tiles cut to the valley rake should be cut in a
designated safe area on the scaffold, not in
position in the valley. Where possible, tile-andhalf
tiles, or double tiles should be used – if
available – to ensure the cut pieces are as large
as possible. All perimeter tiles, including cut
valley tiles, should be twice fixed. Some tile
manufacturers provide special clips to secure the
valley cuts. If possible, nail the head and clip the
tail of each tile. For small cut pieces, secure
using a cut tile clip and a suitable adhesive.
In summary:
• Always ensure the valley lining is adequately
supported on valley boards which are set at the
correct position in relation to the rafters and tile
battens.
• Remember that it is the upstands and welts in
a lead valley that prevent water ingress – never
rely on the mortar for water tightness.
• Make sure that the open valley width is
appropriate for the size and pitch of roof.
“Mortar must be laid onto an undercloak of slate
or fibre-cement board rather than be bedded
directly onto the lead, otherwise the mortar will
crack and dislodge as the lead expands and
contracts”
• Twice fix all cut tiles adjacent to the valley –
use special cut tile clips and adhesive where
standard nailing and clipping cannot be used.
Contact John Mercer
www.johnmercerconsultant.co.uk
@Johnmercer3
24 TC OCTOBER 2020