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Previously we have looked at open inclined valleys and GRP valley
troughs. Now it is the turn of dry valleys to be discussed.
In essence a dry valley is an inclined valley
that has no mortar bedding. This could apply to plain valley tiles,
swept, laced, or mitred valleys either with plain tiles or double lap
slate. But for the purposes of our discussion we will restrict it to
inclined open valleys and proprietary systems for use with interlocking
The theory regarding rainfall rate, true valley pitch and
roof area, as previously discussed, will all apply to dry valleys, just
as it did for mortar-bedded open valleys.
Double lap slate and plain tiles
There are many occasions when an inclined valley is formed between roof
coverings that are either different, or are at different rafter pitches
requiring the use of an open valley. Traditionally, where double lap
slates/tiles require an open valley, a lead or GRP valley trough is
formed with no mortar bedding, as it is possible to cut the edge
slates/tiles from 1.5 wide units, so preventing small unsupported cuts.
With most natural and FC slates the material
thickness is between 4mm and 8mm, therefore the theoretical gapping
between the underside of the slates and the top of the tilt fillet
should be the same. With plain tiles the gapping will be between 12mm
and 15mm, which is large enough to let in not only large insects but
also small rodents.
The larger the gapping size the greater the volume of
water that could leak through, if and when the valley reaches its full
capacity, and is blocked by leaves or other debris. Having no mortar
bedding the overall capacity (cross sectional area) of the valley is
between 55% and 60% greater than a mortar bedded valley, providing a
large safety margin before maximum capacity is reached.
There are two proprietary dry valley systems for use with interlocking
tiles. The first uses a parabolic shaped trough with high profile ribs
down each side, and second uses a central up-stand rib design.
Parabolic trough: The parabolic shape of the
trough encourages the water to flow down the middle, and has a notional
cross section slightly higher than for a 125mm-wide mortar-bedded
valley. During deluge conditions any overspill will be captured in the
small drainage trough between the ribs and drained away.
To prevent birds, rodents and large insects entering
the batten cavity, a comb filler is installed down the outer edges and
the fingers flex to follow the shape of the underside of the tiles. The
parameters for use are: minimum rafter pitch 22.5º; maximum rafter pitch
90º; maximum valley length 8m (or 25m2 of horizontal roof
area); and an open valley width between the cut edges of the tiles of
This means that it is not suitable for rafter lengths in
excess of 5m, or for very shallow rafter pitches. The edge tiles must
all be head nailed, but the tiles on the right-hand side are not
clipped, and the tiles on the left of the valley can be clipped. This
leaves the cut-edge tiles very vulnerable to wind damage during extreme
There does not appear to be any recommendations
regarding the maximum wind uplift resistance of this arrangement.
Central up-stand rib trough: The central
up-stand rib divides the valley into two distinct drainage channels,
left and right of the central rib. The tiles on either side are cut to
finish against the central rib, which is not as easy as it sounds as the
rib is not at right angles to the surface of the tile, and a bevel cut
would be needed; this is most noticeable with high profile tiles.
Water drains off the ends of the tiles and down between
the ends of the tiles and the central rib, into the drainage trough
Under deluge conditions, the volume of water flowing down the tiles
is often greater than can flow through the gap between the ends of the
tiles and the central rib causing the water to back up, or overspill the
top surface of the tiles, especially with high profile tiles.
The cross sectional area of the two drainage channels
is approx 45% less than the equivalent mortar-bedded open valley. During
deluge conditions the excess capacity can overspill into a draining
trough between the two outer ribs, but these are relatively small and
therefore their capacity is also small.
In the pre-purchase literature there are no in-use parameters
regarding minimum or maximum rafter pitch, maximum valley length, or
maximum roof drainage area. There are also no recommendations in the
pre-purchase literature regarding the fixing of the cut edge tiles.
However, in reality the central rib prevents the tiles from rotating
upwards provided they are close mitred to the central rib. SS clips are
available to lock interlocks together and little bridges available to
support small cuts on the left-hand side of the valley.
In use, debris on the roof from construction washes
into the valley, and is almost impossible to clean out without
dismantling the valley. The valley can block up with leaves, twigs and
other natural material very easily, and the resulting blockage can cause
water to spill off the sides of the valley trough onto the underlay.
It has also been found that frost damage is higher with
the cut edges of clay tiles with this design of valley than with a
conventional mortar-bedded valley.
Most dry valleys work within the limitations of the installation and use
instructions provided, but make no allowance for the build-up of debris
and the consequential effects of the blockage.
At this point they rely upon the underlay down the
valley as a second line of defence. In the case of the central up-stand
rip trough, to clear out the debris that collects below the tiles
requires the edge tiles to be removed. With the parabolic trough, and
the conventional dry valley for slates, this can be done without any
problem. Dry valley for plain tiles, with no means of keeping out large
insects and small rodents, are not recommended.
- It is essential to secure all small
cut pieces of interlocking tiles. Will the system allow for all the
variations that may occur?
- The compatibility of the dry valley
with the tiles/slates used, and the roof size and pitch, needs to be
carefully considered before use.
- With no mortar bedding, the cut edge
tiles/slates are more vulnerable to wind uplift damage, than for a
- All gaps should be kept to below
16mmto keep out birds, and below 4mmto keep out large insects.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774