For many years
terracotta and concrete tiles have been laid with wet valleys, that
is, the edges of tiles adjacent to valleys have been bedded in mortar.
This has led to a number of problems, including:
. Restriction
of drainage behind the bedding;
. Mortar cracking
due to structural and thermal movement;
. Moisture
entering the roof space and rotting or corroding building materials;
. Development
of ant nests behind the mortar bedding;
. Corrosion
of valley gutters; and
. Unsightly
finished valleys with clear widths of up to 175mm.
Dry valleys
have been universally used on state roofs and steel decking. For
many years they have also been used successfully on tiled roofs
in other States of Australia. Now dry valleys (and dry barges) are
being introduced into Victoria.
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The relatively narrow clear space of dry valley
improves appearance. Dry valleys also offer
better drainage and reduced callbacks. |
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What is a Dry Valley?
In a wet valley
the edges of the tiles adjacent to the valley are bedded in mortar.
Many people are under the impression that the mortar bond the tiles
to the valley gutter and prevents movement. However all structures,
from aeroplane wings to houses, must flex and these movements often
cause the mortar bed to crack and possibly dislodge.
A dry valley
eliminates mortar and allows drainage over the full width of the
valley gutter, including the area directly under the tiles.
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What are the Advantages of a Dry Valley?
. No maintenance: Creates a long-term, maintenance-free finish.
. Improved
drainage: The gutter is effectively wider and there is no mortar
to reduce or block the flow.
. Fewer outflows: Improved drainage reduces the chance of overflows that can lead
to rotting or corrosion of underlying materials.
. Improved
appearance: Because of this improved drainage a narrower section
of the gutter can be exposed thereby
improving appearance.
. No cracked
mortar: There is no cracked mortar to trap leaves and twigs, provide
a home for ants, or dislodge and roll
into the spouting.
. No callbacks: Callbacks to repoint cracked mortar are eliminated.
. No corrosion: Corrosion caused by the reaction between mortar and valley gutter
is eliminated.
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Is a Dry Valley Harder to Build?
A dry valley
is basically identical to a wet valley and the same materials are
used, except mortar is not required. There are fewer steps in constructing
a dry valley, leading to additional savings.
As before,
the valley gutter is placed on valley boards. Weathering rolls must
not be flattened! They are vital because the full width of the valley
gutter is used for drainage. Using valley boards about 8mm thinner
than the battens eliminates any need to flatten the weathering roll.
The tiles will sit flat and maintain a constant profile across the
roof.
Because there
is no mortar to disguise a rough edge, care must be taken when cutting
tiles to be placed in the valley.
Hand cutting
is satisfactory providing care is taken to ensure straight lines.
Small cuts are held in a place by construction adhesive or galvanised
clips.
The clear width
of the valley between the tiles should be about 100mm. The narrow
opening improves appearance and protects the roof space from wind-driven
rain.
What About Dry Barges?
Wet barges
have similar problems to wet valleys. Cracking mortar is unsightly
and can lead to water entering the structure and rotting timber.
A dry barge uses a simple soaker tray and barge capping that pretects
the barge board and acts as a gutter under the tile edge.
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This article sourced from the Victorian Roofing Tile Association Inc. |
