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A good pitched roof will, over its lifetime,
be subjected to many thousands of gallons of rainwater, and never let
any of it pass onto the building below. But if you were to take the same
roof and turn it upside down and lower it into the sea, it would sink.
This may sound obvious, but it demonstrates that a
pitched roof is full of holes and gaps, through which water can flow but
water does not come in, as the roof covering is lapped in such a way as
to direct the water through and away from the gaps and holes. But, while
a roof is full of gaps and holes, what size of holes and gaps are
acceptable and unacceptable?
Between every slate and tile on a roof there are gaps,
either at the side or headlap. These gaps may be small and measured in
thousandths of a millimetre or large and measured in tens of
millimetres. Some are needed and others are not. Those that are needed
relate to water, wind and insect control; those that are unwanted are
generally caused by poor workmanship or defective materials.
The whole reason for having a roof is to keep rain out, and this is done
by channelling water away from the edges and the top of each tile. With
modern, single-lap tiles, this is achieved using interlocks on the
surface and underside of the tile, where they meet.
The side interlocks are small gutters that collect the
rainwater and channel it away before it can spill off the edge of the
tile. For the gutter to work, it has to have a cross-section that allows
the maximum volume of water to flow down the channel, while at the same
time preventing water from seeping sideways by capillarity. As most
domestic roof slopes are no longer than 9m, most side interlock channels
are designed to cope with the maximum water run-off at the lowest pitch,
for the conditions typical once every 50 years anywhere in the UK.
The interlocks also need to keep out wind-driven rain and
debris such as pine needles and leaves.
Side interlocks generally have matching pairs of ribs
that lock one into the other, but the best arrangement is where one big
channel with a deep side and no corresponding upper rib allows the
maximum water capacity from top to bottom. At the bottom, the water
should have an unimpeded exit out onto the lower tile. For the sake of
appearance, we have tiles with thin leading edges with cut-back
interlocks that discharge the water into the headlap of the lower tile
and expect it to seep out through the headlap gaps. This works well
under average rainfall, but if the underside of the interlock rests on
the lower tile, water can be sucked back under the interlock and over
the head of the lower tile by capillarity, unless there is a device to
stop the capillarity (normally a groove that creates a gap).
Where two surfaces are within 1mm of each other, the
ability of water to travel up or across the tile between the surfaces is
made easier. But, by increasing the gap to 2mm or more, the water flow
is stopped. Large gaps can help to get water out of the side interlocks
and away before it can build up, especially if there is silt or debris
in the interlock channels, which can be a problem, especially at low
At the headlap, weather bars are designed into
the underside of the tile to prevent capillarity, and to slow down the
rain being driven in by the wind. Wind that passes through a narrow gap
and then enters the large void between the weather bars will slow down.
If this is done two or three times, the speed of the wind can be reduced
significantly. The amount of speed reduction is dependent on the minimum
gap size between the tiles and the maximum height of the weather bar
Therefore, to achieve the maximum wind speed
reduction, the weather bars need to be close-fitting onto the lower
tile, and the voids between the weather bars need to be as deep and as
wide as possible.
If a tile is not laid correctly, kicking up around a
roof window, or sprocketed at the eaves, the close fit will be
compromised and therefore the weather bars will not perform efficiently,
if at all.
To provide sufficient space for both the side interlock channels
and the headlap weather bars, the tile needs to be approximately 25mm
thick.With clay interlocking tiles, the tolerances are harder to
maintain, so the width and depth of the channels and weather bars are
greater than for concrete tiles.
Ideally, the gaps along the outer edges where the
tiles abut another tile should be less than 1mm to achieve the maximum
performance. For tiles with a thin body depth, compromises have to be
made regarding the weather bar and interlock design. With resin slate,
this can be achieved through a high degree of accuracy and design.
Traditional pantiles that have no interlocks and weather bars have to
rely on a steep (30°) rafter pitch to prevent wind-driven rain entering
the roof, and the large gaps to prevent capillarity.
If the gaps in the roof covering are larger than 4mm,
there is a high risk that large insects will get in between the tiles
and nest in the batten cavity. If the gaps are larger still, there is
the risk that bats, mice, birds, or squirrels will get into the roof. It
is for this reason that all dry-fix and ventilation components are
designed to have no gaps larger than 4mm. Traditionally, mortar has been
used to fill in all the known gaps around a roof, but there are places
where the gaps have always existed, and these should be restricted to
With high-profile pantiles and double Roman tiles, there has
always been a gap between the corrugation and the fascia board, through
which birds and rats can enter the roof from the gutters, and these
should be filled in with an eaves comb, or filler unit. The traditional
method was to lay a course of plain tiles and mortar the pantile onto
the plain tile undercloak.
When there were no eaves clips, this was the best
method available, but the fitting of eaves clips clashes with the plain
tile undercloak and should not be used together. Where more than one
course of plain tiles is used at the eaves the roof should be steeper
than 35°, otherwise the plain tiles will leak.
The second part of this article will deal with slates and
plain tiles and how gaps between double-lap products have a different
effect to single-lap tiles.
- Interlocking tiles should all lay in
the same plane, with no tiles kicking up at the perimeters.
- There should be no gaps wider than
4mm between the tiles or the surrounding perimeter construction or
penetrations such as roof windows.
- Tiles with cut back interlocks look
neater, but can result in debris collecting and blocking the water
path from the side interlock.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774