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The designers of roof tiles/slates that are
manufactured using a mould or similar process can create features that
modify and control the flow of water and wind between the tiles/slates,
which will enhance the overall performance of the roof covering. Two
such features are weather-bars and interlocks. Generally speaking,
weather bars are horizontal and interlocks are vertical, but, as is
often the case, this is not a hard and fast rule and they can be
diagonal. So what is the difference?
Weather-bars are raised or depressed features on the surface of the
tile, in the head lap or side lap portion of the tile/slate, on either
the upper or lower surface. The position and shape of the features will
define their function.
Weather-bars that are moulded into the underside
of the leading edge of an interlocking tile perform two functions.
Firstly, they produce horizontal points of contact with the head of the
tile below. The greater the surface contact, the easier water travels up
the surface of the tile by capillarity between the upper and lower tile
Secondly, the voids between the weather-bars reduce the
wind force blowing up between the tiles. By forcing the wind to blow
through a very small gap between the tiles (less than 1mm) into a large
void (up to 10mm high) the wind speed and force is reduced. If done two
or three times, this can reduce the wind speed down to the point where
it is unable to blow rain up the surface of the tile/slate within the
However, unless the weather bars run the full width of
the tile there is the risk that the wind driven rain will drive in under
areas like the interlock where there is insufficient depth to mould in a
large void behind the weather-bar. It is much easier to form features in
the underside of a tile, as this is created by the mould/pallet.
Depressions in the top surface of a tile/slate will help to attract
capillary water on the surface and may delay the onset of water tracking
down a nail hole. However, this will never be as effective at keeping
out water as an upstand on the surface, as this acts as a small wall.
Locating one upstand weather bar below the nail hole,
and one at the head of the tile, provides the maximum protection to stop
water penetrating the head lap. A weather-bar across the head of the
side interlock of a tile/slate prevents water blown up the side
interlock from draining over the head of the tile.
Unfortunately with tiles manufactured using an extruded
process, horizontal or diagonal features on the top surface are
difficult to achieve, as a smooth profile needs to be formed up the
length of the unit as it passes through the extrusion stage of
manufacture. Any depressions or upstand ribs have to be formed as a
With moulded and pressed tiles, such as resin slates,
it is easy to incorporate all of the features in the top and underside
surface in one operation.
If the weather-bar ribs and depressions on the upper and lower surface
of the tiles/slates are designed to locate one with the other, they are
termed interlocks. Introducing interlocks with upstand ribs close to
each other will restrict the amount of shunt between the tiles.
The side interlock shunt of concrete interlocks is
maximum 6mm from full open to full close, while the variable head-lap
for a concrete interlocking tile can be up to 50mm.
While there are some clay interlocking tiles
that have variable headlamps, most designs have a fixed head lap and
tend to have weather-bars that interlock. This makes the tile designs
very efficient at keeping out water, but they have to cope with the
variations in the clay from being fired in a kiln, which is not an issue
with concrete tiles or resin slate.
Vertical side interlocks also act as gutters to drain
water away, unlike horizontal weather-bars. The relative height of the
interlock with the general surface of the tile will determine the
anticipated vulnerability and water capacity of the side interlock.
The interlock of a flat concrete interlocking tile will
inevitably be below the general surface of the tiles, while a similar
design with clay pantiles will result in the interlock being at the same
level as the remainder of the tile, while some high profile tiles have
the interlock appreciably above the general surface of the tile. The
higher the interlock position, the better the performance.
Some interlocks have two channels and two ribs that
interlock, while others only have one with a larger void between them,
increasing the overall capacity of the drainage channel. The design of
the side interlock channel is critical, as the side lap distance is only
30-40mm while the head lap can be between 50mm and 125mm. It is probable
that the first mode of failure of a tile system will be through the side
interlock, before failure through the head lap.
Weather-bars moulded into the surface of a tile can greatly enhance the
weather resistance performance of a clay, concrete, or resin slate roof
covering. However, the method of manufacturing the tile will restrict
where the weather-bars can be installed.
Interlocks are weather-bars that locate with features
on an adjacent tile to make them interlock. Quite often, tiles of the
same profile from different manufacturers will not interlock and are
therefore not compatible.
Similarly, plastic ventilation tiles that are
sold as fitting a range of tiles may not perform as well as the tiles
they fit with, as the design of the weather-bars will be a compromise to
allow them to be fitted to a range of manufacturers’ products and
therefore will not be as tight a fit as they should be.
- Never mix tiles from different
- Never stretch the horizontal
coverage or head lap of a tile, as it may result in weatherbars
riding up on each other, causing a local stress point and breakage.
- Tiles should always lay in the same
plane to ensure the weather-bars on the underside of the tiles are
in close proximity to the tile below, allowing it to do its job
- Depressions in the top surface of a
tile are not as effective as an upstand rib undertaking the same
function in the same locality
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774