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It is possible to design a pitched roof
without any inclined valleys, yet due to dormer windows or complicated
building shapes, many pitched roofs do contain them. Therefore, their
design must ensure that they can cope with the amount of water flow that
they will encounter during the life of the roof.
An open valley is a gutter between two roof slopes that are
not parallel to each other, and is inclined from a ridge or top edge
abutment to an eaves or other low level feature. They are mainly
used between roof slopes covered with interlocking tiles/slates or with
double lap plain tiles/double lap slates where the rafter pitches and
batten gauge of the two adjacent roof slopes are different and cannot be
coursed around from one roof slope to the other.
In 1990, Brunel University conducted research into the effects of
rainwater on inclined valleys, to determine the minimum open channel
widths. The results were published by the Lead Sheet Association and
were subsequently incorporated into BS5534: The code of practice for
slating and tiling.
The research found that the ability of the open
valley channel to collect water and drain it away safely will depend on
the volume of rainfall, the true pitch of the valley, and the area of
the roof served by the inclined valley.
The higher the rainfall, the shallower the true
valley pitch, and the greater the roof area served by the valley, the
wider the open channel will need to be (and in turn, this will determine
the width of the material that forms the valley liner).
The capacity of inclined valley gutters, like internal, parapet wall and
back gutters, has to cope with the anticipated worst-case conditions in
For inclined valleys on permanent buildings, the
design rainfall rate is considered to be 225mm/hr. However, a severe
deluge of rain is unlikely to last one hour, so the intensity during a
short burst can be far higher than the figures suggest.
Maximum rainfall occurs when there is no wind,
therefore the highest recorded deluge conditions tend to be on the
eastern side of the UK, rather than wind-driven rain that is more common
in the west. The more wind there is, the lower the volume of rain.
Eventually, when the wind reaches a high enough speed, there will be no
Gutters that are fixed to the
perimeter of a roof are designed to cope with a lower rainfall rate
than those for inclined and internal gutters, because once an
external gutter reaches its maximum capacity, it will overflow
outside of the building fabric, with little or no long term effect;
unlike an internal or inclined valley, where water could find its way
down through the building fabric into the habitable rooms.
Water running off each roof pitch into the open
valley channel should flow at the same rate, equalising into the centre
of the channel. If the pitch on one side is steeper than the other, or
the volume of water flowing off one roof slope is greater than the
other, the faster/greater volume will wash over the centre line of the
valley. If the volume/flow is great enough, the water running off one
slope can wash in under the roof tiles/slates on the other.
True valley pitch
All inclined valleys have a true pitch that is less than the rafter
pitch. If the two roof slopes have the same rafter pitch, are at right
angles to each other, and the valley line bisects the angle, the
difference is approximately 5°. If the plan angle between the two roof
slopes is greater than 90°, the difference will be less than 5°. If the
plan angle is less than 90°, the difference will be more than 5°.
The steeper the true pitch of the valley,
the quicker the water will drain out of the open channel and the greater
volume it can collect. The shallower the true valley pitch, the wider it
will need to be to drain away the same capacity of water.
At the head of a valley, very little water will drain into the valley
channel. The further down the valley, the greater the area of roof that
will drain into the valley to add to the water already in the valley
channel. The greatest volume of water will therefore be at the base of
the valley, where it discharges into the eaves gutter, or back onto the
roof. The longer the valley, the greater the area of roof that will
drain into the valley.
As rain falls vertically during a deluge, the area of rain
falling on a roof slope is measured as the horizontal area rather than
the slope area of the roof. This makes calculating the roof area for
maximum rainfall from a roof plan easy.
As rain falls vertically during a deluge, the
area of rain falling on a roof slope is measured as the horizontal area
rather than the slope area of the roof. This makes calculating the roof
area for maximum rainfall from a roof plan easy. For simplicity, the
British Standard recommendations have two categories: up to 25m2
and between 25m2 and 100m2. If high-level roofs
drain down onto low-level roofs then they should be added into the
calculation. It is the total roof area above the valley outlet that is
required. At present, there are no recommendations for roof areas
greater than 100m2. For this situation, the cross-sectional
area could be increased in line with the additional roof area draining
into the valley, or additional roof areas should be drained away
separately to reduce the water volume draining into the valley.
Regardless of the material used to line the
valley gutter, the open channel width should be the same.
- Avoid draining rainwater off high-level roofs onto low-level
roofs, especially if they drain back into an inclined valley.
there are vertical walls, the water running off them must be
- For roof areas over 100m2 there are no recommendations,
therefore either the cross-sectional area should be raised
proportionally, or measures should be taken to reduce the roof area
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774