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The external elevations as shown on an
architect’s drawing can look very clean cut with lines meeting exactly
at intersections, giving the appearance of precision and style. But in
reality things are never that precise. For instance at the intersection
point between two hip lines and a ridge, the ridge is rarely at the same
level. The same goes for slates or tiles installed in close proximity to
a soffit or fascia board, especially around a dormer. The draughtsman
can draw it but often it can be difficult to impossible to construct, as
there is no room to swing a hammer, or twist a screwdriver. In these
situations what should you do? Use common sense.
On a dormer the side abutment construction starts off quite
conventionally and as it gets closer to the valley the construction gets
more and more difficult. If there is no soffit then it is possible to
fix the top three courses of slates or tiles to the battens that finish
on the last rafter under the valley, through the rafter feet of the
dormer, which is not as easy as it sounds. Once the soffit board is
installed, that option is removed making the installation of the top
slates or tiles almost impossible.
With tiles that have nibs they are often not
nailed, but with slates this is not an option. Even where there is a
soffit, and always where there is no soffit, the point at which the
soffit and roof covering meet are ideal locations for birds to nest, and
it is often impossible to reach in to install a lead flashing. If there
are open rafters then the diminish space can disappear up under the
valley into the roof void giving rats and mice a route into your loft.
Once the diminishing space drops to a height of about
300mm it would be better to stop slating or tiling and to box out the
corner with a vertical return, out to the line of the dormer fascia
board, and install a triangular infill up to the fascia board, or tilt
With open rafters the boxing in should coincide with a
rafter position to make fixing the timber work easier. This moves the
side abutment close to the valley out to the fascia board line, making
it easier to fix the slates or tiles, and making the flashing at the
base of the valley easier to integrate into the slates or tiles. The
boxing in can either be clad in the same material as the dormer, or if
you are feeling rich, lead sheet, and incorporating the side and top
edge abutment flashings.
Where there is a horizontal soffit board at the eaves it is possible to
take the vertical tiles or slates up to the soffit and finish with a
lead cover flashing over the nail fixings. In some instances the tiles
and slates can be fixed before the soffit is installed and the nail
fixings hidden by the soffit board, but this causes problems for fixing
the back edge of the soffit.
With open rafters finishing the tiles or slates up
between the rafters is a nightmare as the rafter centres never coincide
with the module of the tiles or slates, and short lengths of batten are
fixed to noggins, as there is nothing to fix them to above the wall
plate. If the tiles or slates can be installed before the fascia board
is installed then things are easier, but once the fascia board is
installed nailing or screwing the top three courses of tiles or slates
is almost impossible.
The better solution with open rafters is to
install the tiles or slates as high as possible and to fix timber boards
between the rafters, on noggins fixed to the sides of the rafters, to
lap over the heads of the vertical tiles or slates and cut in under the
rafters if they come that low. The tops of the timber boards should
finish level with the top of the rafters. If low level ventilation is
needed it is possible to fix a ventilation strip along the top of the
timber board before it is installed, provided the grill does not
protrude above the upper level of the rafters. This detail avoids the
need for a lead flashing to cover the top course nail fixings.
A similar situation can occur where vertical tiles or slates meet the
gable ladder of the roof. Sometimes there is a wide overhang giving
plenty of room to install the additional nail fixings for the cut tiles
and slate. But often with a narrow barge board the tiles will need to be
cut around the gable ladder just behind the last rafter.
In this instance the best solution is to remove
the barge board, form a raking or Winchester cut, whichever is
appropriate, at the line of the gable ladder, and reinstall a wider
barge board to cover the last 50mm of the raking cut. If the gable
ladder is close but not tight, a soffit board screwed to the underside
of the gable ladder will be needed to stop birds and bats gaining entry
to the roof void between the vertical tiles and the bargeboard.
Diminishing spaces at edges and junctions are a challenge to construct,
and will require some carpentry work if it is not possible to undertake
the slating and tiling before the fascia, soffit and barge boards are
installed. Even when they are installed, fixing the ends of the soffit
boards and lead flashings can be difficult unless the junction detail is
identified and integrated at an early stage of the roofing process.
- To fix slates or tiles you need
enough room to swing a hammer or twist a screwdriver.
- Diminishing spaces up under soffits
are ideal places for birds to nest so they should be eliminated by
being designed out.
- Cutting tiles or slates in around
rafter feet where there are open rafters is difficult, and should be
designed out by using vertical boards between the rafters
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774