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Part 2 of verge construction deals with the
under-cloak, fixings, and mortar-bedding of the tiles or slates.
The under-cloak is a permanent support for the mortar bedding. Without
mortar there would be no reason to have an under-cloak.
Traditionally steep pitch slate roofs have never
been mortar bedded and therefore an undercloak is not needed, but the
height of the barge board is increased to finish flush with the top of
the slate battens. With plain tile roofs a plain tile is often used to
form the under-cloak by removing the nibs and wedging it under the ends
of the battens laid face down, with the nail holes at the lowest point.
With clay tiles the straighter ones should be sorted for this purpose.
Plain tile under-cloaks should never be used with
tiles or slates with a gauge greater than 250mm, as it is essential that
the inner edge of the tile is wedged under at least one batten end.
Because plain tiles will have some form of camber, in either direction,
water can track back towards the wall or barge board; therefore they
should never be used on rafter pitches below 35°.
An undercloak of fibre cement board or natural
slates, when slid into position under the tile battens, should not cause
the end of the tile batten to kick up, unlike with plain tiles. Where
they are installed on a gable rafter V cuts may be required at batten
positions to allow the batten end to be nailed to the rafter, yet still
be wedged under the batten. There is a risk that if the batten is nailed
through the fibre cement board or slate it will break it.
All under-cloaks should be laid horizontally, or
with a slight fall away from the building to discourage water tracking
back to the wall or barge board. All butt joints should be as tight as
possible to prevent small birds, bats or large insects from entering the
roof system, especially at the apex. At the eaves, between the first
batten and the top of the fascia/tilt fillet, the line of the
under-cloak should change to a shallower pitch to allow the underlay to
exit the roof over the fascia board. This will require a small wedge of
timber to be inserted above the barge board to support it and avoid a
All perimeter tiles and slates must be mechanically fixed. This
means that if the tiles are nailed, such as with plain tiles and slates,
the edge tile must be fully nailed.
For tiles with one nail hole, that means one nail.
Those with two nail holes, this means two nails; with one exception. At
a plain tile verge the tile-and-a-half can be twice-nailed, but the tile
can only be once-nailed as the second nail hole misses the end of the
Interlocking tiles that need to be clipped, and those
with half tiles where the single nail hole misses the end of the batten,
must be clipped using verge clips. Each verge clip must be nailed or
attached to the batten in compliance with the manufacturer’s
recommendation. This needs to be done to a predetermined string line
before mortar is placed. Pushing the clip into the mortar after the
tiles have been installed is not acceptable.
The mortar that is used for bedding verges should be a 1:3 cement:sand
mortar mix. The sand should not be soft sand, as used in bricklaying,
but sharp sand – not to be confused with screeding sand. Sharp sand has
an even proportion of a range of aggregate sizes from the finest up to
5mm, while screeding sand is all 3mm-5mm aggregate.
The under-cloak and verge tiles should be
damp, so that when the mortar is placed the under-cloak and tiles do not
suck all the moisture out of the mortar before it has had an opportunity
to complete its chemical reaction. The verge tiles should be pushed into
the mortar to make it spread out on the underside of the tile and
compact into the surface of the tile.
The mortar should not be too wet that it slumps,
or too dry that it is too rigid. The tiles should be nailed as soon as
possible and the surface trowelled up to remove any excess and leave as
a smooth surface.
Laying the tiles and pushing the mortar into the
gap between the tile and under-cloak is not acceptable, as unchecked the
mortar will push through into the batten cavity and not bond onto the
underside of the tile or undercloak. Mortar that comes into contact with
the batten ends will allow an alkaline solution to soak into the end
grain of the timber. The timber will swell and remain wet for long
periods. This will both crack the mortar and eventually cause the timber
to rot away, preventing the nail or clip fixings from being secure.
With some profiled interlocking tiles the verge edge of the tiles can be
high above the under-cloak leaving a large space for the mortar bedding
to fill. Traditionally plain tile are cut to 50mm widths for use as
dentil slips to reinforce and thin out the mortar. In some instances two
courses of dentil slips may be needed and if used, the joints should be
staggered to give it strength.
There should always be a bed of wet mortar between each
dentil sip, the under-cloak. and the tile – ideally 10mm-15mm thick.
This needs to be placed before the verge tile is laid and nailed into
Once a mortar-bedded verge has been finished, it should be left for as
long as possible without being disturbed to allow the mortar to cure.
Sun and heat will prevent the mortar from curing by drying it out, so it
should be kept damp if possible. Frosty weather can freeze the moisture
in the mortar before it has had time to cure, causing it to expand and
A mortar-bedded verge that is done correctly should
last as long as the roof tiles or slates.
- The under-cloak should have a fall
away from the wall to prevent water running back to the wall and
leaving a stain, or soaking in.
- Never use a plain tile undercloak with
- All mortar-bedding should be placed on
the under-cloak before the verge tile or slate is placed and fixed
- Interlocking tiles, with a half tile
on the verge, should be clipped where the nail misses the batten end.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774