The statement that you see on drawings and
specifications to the effect that ‘lead flashing to lap tiles by 150mm
minimum’ is, as statements go, correct but very misleading, as it means
different things to different people and does not provide any guidelines.
Capillary reaction
The capillary between two smooth and parallel surfaces can suck water
up a vertical distance of 75mm regardless of the type of material. To
overcome this mode of leakage the roof flashing needs to lap no less than
150mm at any true surface pitch above 30°.
The Lead Sheet Association states in its
manual: ‘for secret gutters and pitched valley gutters the laps should
conform to a vertical weathering height of not less than 75mm’. To
comply with this recommendation the resulting simple laps for pitches
below 30° will increase from 150mm to 200mm at 22°, 250mm at 17.5°,
290mm at 15° and 390mm at 11°.
Effective head lap
Whilst the requirement for laps in lead relate to secret valleys and
pitched valleys, where the concentration of water flows are at their
highest, the need to achieve the same laps over roof tiles and other
sections of metal flashing is equally as important. Whilst all laps with
tiles should be measured from the head of the tile, the true effective
head lap with interlocking tiles is not to the head of the tile but the
nail hole. For plain tiles it is from the head of the tile on the course
of tiles below, which is a distance of 65mm above the leading edge of the
tile.
True pitch
The true pitch a roof tile is always less than the rafter pitch by
between 3 and 10°, depending upon the hanging length, the thickness and
the head lap of the tiles. A simple rule of thumb is a plain tile is 10°
less than the rafter pitch, a standard concrete interlocking tile is 5°
less, a thin leading edge interlocking tile is 3° less, and inclined
valleys are about 5 ° less than the rafter pitch.
Top edge abutment
At a top edge abutment the head of the tile will always be a short
distance from the face of the wall. The amount will vary with rafter pitch
and profile height above the batten. The gap between the wall and the head
of the tile needs to be added to the head lap, upstand and turn in, when
calculating the width of lead sheet needed to fabricate the flashing.
A plain tile roof at 35° rafter pitch will have a true
tile pitch of 30°. With the 150mm lap measured down from the head of the
tile would result in the effective head lap being less than the gauge of
the tiles.
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