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Having explained the design principles for
an open inclined valley in part 1, part 2 will deal with the issues of
installing the valley.
Support boards should be used under all open valleys formed with metal
and GRP valley liners. They should be thick enough to support the weight
of a man standing on the valley and be set between the rafters, or
counter battens, on noggins. They should be wide enough to extend beyond
the edge of the valley lining material, and under the ends of the
battens by at least 50mm, to allow the batten ends to be nailed to the
The type of underlay used will determine whether it can be laid under
the valley or not. On the main roof slopes, it should not directly lap
under the valley, as water will be able to drain under the valley liner.
By lapping the underlay over the welt on the edges of the valley
construction, most of the water on the underlay will drain down between
the welt and the batten ends, and exit at the eaves.
Underlay The type of underlay used will determine
whether it can be laid under the valley or not. On the main roof slopes,
it should not directly lap under the valley, as water will be able to
drain under the valley liner. By lapping the underlay over the welt on
the edges of the valley construction, most of the water on the underlay
will drain down between the welt and the batten ends, and exit at the
eaves. With a lead valley, special fleece materials should be laid under
the valley to allow the lead to expand and contract without abrasion or
adhesion during hot summer temperatures. Bitumen underlay should never
be laid under a lead valley, as it will act like an adhesive between the
lead and the support boards. With other types of underlay, you should
consult the manufacturer.
Three lines of defence
A good open valley design should have three lines of defence: the mortar
bedding to define the sides of the open channel; the tilt fillets
raised sections that discourage water from tracking sideways; and the
welt, which stops any water that may reach the edge of the valley from
dripping off the edge.
Each of these features should be separated
to work effectively. Therefore, mortar should never come in contact with
the tilt fillet or the welt, as water will wick through between the
mortar and the feature. The welt should not be incorporated into the
tilt fillet, as the underlay should finish between the two and never
extend over the tilt fillet into the bedding mortar, as again it can
create a wick path.
The edge of the open channel to the outer edge of
the welt should be about 200mm. Therefore, the total valley width should
be twice that distance under the tiles/slates, plus the open channel
width. The metal liner will be wider to dress over the tilt fillet and
form a welt.
The tilt fillet should be positioned approximately 75mm from the edge of
the open valley channel line to ensure there is sufficient space for the
mortar bedding. If no mortar is being used, it will be the first line of
defence and can be closer to the cut edge of the slates.
The tilt fillet should not be higher than the
tile battens or it will cause the roof covering to kick up. If the
support boards are between the rafters, there will be room for a batten
cut on the diagonal. Without a tilt fillet especially on a slate roof
with no mortar bedding the valley will leak during heavy rainfall.
Most open valleys are formed using ductile metals such as lead, zinc or
copper, although GRP valley troughs are also used.
Lead is the most popular metal for forming open
valleys due to its long life expectancy and its ability to be bossed or
cut and welded to form the flashings at the head and base of a valley.
Generally, copper valleys are more expensive
and zinc valleys are cheaper, but the principles of forming the valley
should be the same. The maximum sheet lengths will vary with the
materials and the thicknesses.
The welt is the edge of the metal liner turned back on itself, with a
4mm gap between. Lead, which is unable to support its own weight, often
collapses and closes the gap, so it is better to turn the lead up to
form a vertical wall level with the top of the battens/tilt fillet.
The mortar bedding should form the edges of the open channel. This is
essential with most interlocking tiles to locate the small cut pieces of
tile, especially on the left-hand side of the valley, where without
mortar they would not lie at the right angle and may not be fixed to a
With double-lap slates, there may be no need to use
mortar bedding. With plain tiles, the thickness of the tiles can allow
insects, birds and rodents to enter the batten cavity if there is no
When mortar bedding is used, it should be laid on a
slip plane normally a piece of undercloak which is laid on the metal
valley liner. The mortar is placed on the slip plane before the cut
edges of the tiles are placed. The reason for not placing the mortar
directly on the metal valley liner is to allow the liner to expand and
contract freely, so that it does not fracture along the side of the open
Mortar should not be used between the laps of the tiles
as this will restrict the self-draining effect of the headlap or the
side interlocks. The mortar mix should be one part cement to three parts
sand, placed during dry weather only.
Where a valley reaches an eave, the true pitch of the valley should be
maintained. If the valley forms a horizontal shelf where it passes over
the fascia board, the capacity of the valley will be decreased, as in
heavy rainfall water can slow and back up. It is better to cut down the
fascia board for the width of the open channel and the tilt fillets, to
allow the true valley pitch to run through. The tiles/slates on either
side of the valley should also lie correctly.
Where two open valleys meet at a ridge, such as above a dormer, the
valleys should run through and be covered with a saddle. The ridge tiles
should be cut in line with the edges of the open channel and should not
cross the valley, as this will restrict the water flow off the main roof
slope, creating a wick path. Where the head of a valley meets a hip or
other feature, again a lead saddle should be installed, and the hips and
ridge tiles should be mitred and fixed over the saddle.
- Do not allow bedding mortar to come
into contact with tilt fillet.
- With metal valleys, always place
mortar on a slip plane to allow the metal liner to expand and
- The underlay should never lap over
the tilt fillet, or make contact with the bedding mortar.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774