April 2018

Dormer Windows
GET YOUR DORMER DETAILS RIGHT
The Technical Team at Marley Eternit explain how to get the key details for valleys and side
abutments correct and ensure your dormer window installation is right first time.
Dormer windows are becoming increasingly
popular as a way of increasing usable
space in the loft, as well as to create
more visually interesting roof designs. There are
many different types of dormer, ranging from the
most common gable fronted, through to hip
dormers and the more specialist eyebrow dormer.
For roofing contractors, each dormer will require
its own particular detailing, depending on roofing
material, location and how it is framed. However,
most dormers will include common roof details
such as valleys, side abutments, eaves, ridges or
cloak verges.
Here, we focus specifically on design details for
valleys and side abutments, using clay plain tiles
as an example.
Valleys
Most dormers create valleys on either side; this
means a higher volume of water runs through
them as they are collecting the rainwater from the
two roof slopes. Therefore flashing and
weathering is very important.
The following design steps should be followed:
1 - The width of an open valley trough should be
determined in accordance with BS 5534, which
takes into account the volume of rainwater at
specified flow rates draining from plan areas and
roof pitches.
2 - When designing the valley construction, it is
important to set the timber boards for valley
linings and troughs on noggings between the
rafters, so that the face is flush with the top of
the adjacent rafters.
3 - The width of the boards needs to be wider than
the determined valley width, to accommodate the
ends of the slating or tiling battens and any tilting
fillets used for metal valley linings.
Below: Detailing – Curved clay valleys
265 x 165mm
Marley Eternit Clay Plain Tile
H60/120
Marley Eternit Curved Valley Tiles
H60/630
Tiling Batten
H60/245
Underlay batten
H60/240
Timber valley board, 4mm ply lining
H71/630
G20/311/770
4 - Valleys for clay plain tile dormers can be
created using lead, a GRP dry valley system or
purpose made clay valley tiles (for pitches 30-50
degrees) – see detailing diagrams above.
Side abutments
Some dormer windows will also feature side
abutments. An abutment occurs where the edge
of the roof slope meets a wall that rises above it.
Normally, on a side abutment, this is at right
angles to the eaves or ridge.
1 - It is best practice to ensure that rainwater
runs away from a side abutment, but where this
is not possible, an inclined valley should be
created which is designed to cater for the water
flow. These should be formed with a suitable
flashing, usually lead soakers, which can provide
the required weather protection and accommodate
any thermal movement that may occur.
2 - Cavity trays and dpcs in the masonry wall
should be designed so that they allow any
flashings to be inserted below them, thereby
maintaining their weatherproofing function.
3 - Side abutments using flat tiles and slates are
particularly vulnerable to wind driven rain, and
should therefore be designed with secret gutters
and / or soakers and cover flashings, which
provide suitable protection.
Further design detailing for ridges, eaves and
cloak verges is available at
www.marleyeternit.co.uk
Below: Detailing – GRP Valley
265 x 165mm
Marley Eternit Clay Plain Tile
H60/120
265 x 248mm
Marley Eternit Clay Plain Tile-anda-half
tile. H60/120
Tile-and-a-half-tile cut to angle
of the valley
Tiling batten. H60/245
Underlay
G20/311/770
Marley Eternit GRP dry valley
trough, low profile. Code
30422). H65/606
Timber valley board, 4mm ply lining
H71/606
G20/311/770
Below: Design detailing – side abutment with clay plain tiles
265 x 165mm
Marley Eternit Clay Plain Tile
H60/120
Code 4 stepped lead cover flashing
H71/310
Code 3 lead soakers
Tiling batten. H60/245
265 x 248mm
Marley Eternit Clay Plain Tile & Half Tile
Underlay. H60/240
General advice on tiling dormers
When it comes to flashing and weathering, tiled
dormer roofs should be treated in a similar way to
the main roof work. However, single lap tiling is
less suitable for small covered areas and for a
dormer, it is generally preferable to adopt an
alternative form of covering.
If dormer cheeks are tile hung, close cut vertical
tiles to rake of roof over a flashing fixed to side of
dormer and dress well into the adjacent tiles.
A special note on eyebrow dormers
Eyebrow dormers feature a curved roof and
usually require highly skilled roofing expertise. As
the tiles often reduce in pitch, they sometimes
need a functional sub roof with the tiles just used
for aesthetic purposes.
With this type of dormer, the transition between
eyebrow and the rest of the roof should be
regarded as a valley and precautions should be
taken (see above) to prevent water leaks.
Contact Marley Eternit’s Technical Team
01283 722588
www.marleyeternit.co.uk
@MarleyEternit
32 TC APRIL 2018