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On almost every new slate roof you will find
at least one ventilation slate that is connected to a soil pipe or
extractor fan, or that is installed to control condensation. Due to
plastic designs, the majority are now flush with the surface of the
roof, making them more inconspicuous.
Vent slates come in two generic types: those that lap
under the adjacent slates, and those that do not. Generally, those that
have hoods or mushroom caps on the top surface do not have side
flanges/channels, while most of those that have flush-fitting grills do
The flush-fitting grills require the bulk of the vent to be located
below the surface of the slate, and therefore the duct tends to take up
most, if not all, of the width of the raised central section of the
vent. The flanges/channels down either side of this section serve two
purposes: they help to hold down the slate at the leading edge, and they
provide additional side lap to the slate below, which has been cut to
allow the vent duct to pass through it.
The thickness of the flanges/ channels causes the adjacent
slates to kick up by their thickness, which is about 2mm on average.
Also, the flanges/channels are 25mm-50mm wide, so pass under the nail
hole position of the adjacent slate, forcing the nail fixing to be
driven through the flanges/channels, or a new nail hole to be punched
further from the edge of the adjacent slate. The raising of the adjacent
slate also reduces the penetration of the fixing nail by the same
amount. With fibre cement slates, the copper disc rivet fixing in the
adjacent slate will either cause the slate to bend slightly, or there
will be less of the rivet pin bent over the top surface – neither of
which is desirable.
Many of the vent slates with flanges/channels are
designed for use with natural slates. With FC slates there is a need to
provide a copper disc rivet fixing to hold the leading edge of the
slates in the course above. Because the rivet needs to pass under the
two adjacent edges of the slates below, this is not always possible, as
the flange/channel being moulded into the vent slate removes one of the
Opening up the side gap to accommodate the copper
disc will affect the coursing of the slates, and leave the rivet
unstable and able to rotate and pull out from under the adjacent slate.
Cutting off half the copper disc rivet eliminates one of these
requirements but not the other, so is also not acceptable. Notching the
rivet into the side of the adjacent slate and punching a new hole in the
tail of the slate above is a slightly better option, but weakens the
edge of both slates.
The best option is to use a vent slate
specifically manufactured for FC slate that has a slot built into it for
locating the copper disc rivet. Having flanges/channels down each side
makes it impossible to cut the vent slate to fit any other size of slate
than that which it has been manufactured for.
Vent slates with no flanges/channels down each side tend to have hoods
or mushroom caps on the top surface and a smaller duct through the
underside, making the side lap distance greater.
Butting up to the adjacent slates stops them kicking
up, provided that the overall thickness of the vent slate is the same as
– or less than – the adjacent slates. Also, there should be no
interference with the adjacent slate nail fixing, and the copper disc
rivet for the slates above should be accommodated without modification.
With no side flanges/channels, the leading edge of the vent slate should
be held down with a copper disc rivet or a slate hook, to prevent wind
uplift (regardless of the type of slate).
The majority of vent slates are designed to suit both 600mm x 300mm and
500mm x 250mm slates. While some natural slates in the UK are supplied
in these sizes, most are likely to be one of the 28 imperial sizes,
making some vent slates difficult or impossible to install with natural
slates. They would therefore be more suitable for use with FC slates,
provided the thickness is correct.
Colour and texture
Plastics, when new, can be produced to closely match the natural or FC
slate colour and texture, but they will never be the same, as the
materials will weather differently over time. Often, if they start off
different, the colour and texture may blend in better after a few years.
The precise finish, colour and texture of the plastic vent slates –
relative to the adjacent slates – is almost impossible to predict, so
colour and texture variations should be expected. Placing vent slates in
inconspicuous locations is therefore always a good idea.
All good vent slates will come with a set of fixing instructions, which
should be adhered to. They should always be laid to the correct half
bond with the surrounding slates. This may be difficult with some
designs where the exposed surface of the vent extends up over two
If the duct on the underside coincides with the
rafter or counter-batten position, the vent slate may have to be
installed in the course above. This may not always be possible where the
rafter centres are at below 450mm. If this is the case, a vent slate
with a narrow duct could be used, or the coursing of the slates on the
roof can be adjusted to ensure the vent slate is positioned at the mid
point between two rafters. For a very small variation, it may be
possible to trim the edges of the slate or the vent slate (if it does
not have side flanges/ channels), so long as it does not affect the
minimum side lap or the structure of the vent slate.
Where the slates on the course above and below
are cut to accommodate the duct on the underside, and the grill on the
top surface, the cuts should be as close to duct and grill as possible
without lapping over them and reducing the ventilation capacity.
Where the grills on the top surface are raised,
lapping onto the grill will cause the cut slates to kick up, and may
affect the nail or copper disc rivet fixing. Vent slates should never be
installed close to the perimeters of any roof slope. The closer they are
to the centre of the roof slope the better.
- Vent slates with side flanges/
channels need a disc rivet slot when used with FC slate.
- Use a vent slate with a narrow duct
where the rafter centres are less than 450mm.
- The leading edge of the vent slate
needs to be held down to prevent wind suction damage.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774