Common roofing mistakes
During his career inspecting roofs and problem solving, John Mercer, a technical roofing consultant, has
come across many re-occurring faults or mistakes. Some have been design-related, whilst others have been
down to poor installation. However, they all have the potential to cause a roof failure, whether that be
rainwater ingress, wind damage or simply an unacceptable appearance, as John explains in this article
A good roof must start with good
design. Ideally, a roof should be
designed with the roof tiles in
mind, considering factors such
as roof pitch, detailing and rafter length.
Avoid designs where water is
discharged from a higher roof slope onto
small lower roof areas, or into another
junction such as a side abutment.
If practical, design the building to
break up the roof into smaller sections.
For example, allow the upper roof to
shed directly into a gutter, rather than
onto a lower roof. When building a
lean-to extension, do not be tempted
to drain the gutter of the main roof
onto the extension roof through
a down pipe. Extension roofs, by
their nature, may have quite a low
roof pitch, therefore, the tiling can
be overwhelmed by water being
discharged from the roof above in a
Overly long rafter lengths can be
a problem, particularly when using
single lap flat tiles at low pitches.
Tile manufacturers give maximum
recommended rafter lengths for their
products, with guidance on increasing
the roof pitch for long rafter lengths.
Roof tiles are subjected to rigorous
wind-driven rain testing in wind tunnels
and on roof rigs that usually simulate
a 6m or 7m rafter length. Therefore, if a
rafter length is longer than this, there is
a risk of water ingress through the tiling,
particularly in the tile courses near
It is often assumed that rainwater
should never penetrate the roof tiles.
This is generally true for, perhaps, 99%
of the time, but there is always the risk,
particularly during a heavy storm,
that rain may be driven through the
tiles. Condensation may also develop
occasionally in the batten cavity (the
space between the tiles and underlay).
Therefore, underlay must act as a
second line of defence, as well as making
the building weathertight before the tiles
A common mistake is to lay the
underlay too tight, i.e. without a drape.
Any water in the batten cavity then
becomes trapped behind the battens
and cannot flow towards the gutters.
Where this happens, there is a risk that
water will penetrate through the batten
nail holes. Where the underlay is laid
unsupported over the rafters, it must
have enough drape to allow water
to drain under the battens, up
to a maximum drape of 15mm.
Alternatively, consider the use of
nail tape and counterbattens.
Nail tape seals between
the underlay and battens and
counterbattens encourage air flow within
the batten cavity and provide a gap for
any water to flow under the tile battens.
The underlay must also be installed
to be weathertight at junctions such as
side abutments, valleys and around roof
windows and chimneys, by turning
the underlay up against abutments and
lapping over valley sides.
Another detail prone to incorrect
installation is the eaves. Firstly, it is
common to see the first tiling course set at
the wrong angle. If using double lapped
plain tiles, then it is permissible to have a
‘sprocketed’ or ‘bellcast’ eaves provided
that the tiles within the sprocketed
section are laid at or above their
minimum recommended roof pitch.
For single lap tiles, BS 5534
recommends that the tiles are not
sprocketed at eaves and should be set
at the same angle as the general tiling
above. Single lap roof tiles rely on their
headlaps to be set correctly, therefore,
a sprocket will run the risk of leakage
through the headlaps.
It is common to find that the underlay
is not supported behind the fascia or tilt
fillet, creating a ‘trough’ in the underlay
in which water can collect. If water pools
behind the fascia, it will find its way
through any cuts or holes and will rot the
underlay. Therefore, the underlay must
be properly supported at the eaves, with
adequate fall, behind the fascia or tilt fillet
using proprietary underlay support trays.
Valleys are one of the most likely
areas of a roof to fail because water is
directed into a valley from the adjacent
roof planes. Unfortunately, they are
also one of the most likely details to be
There is a common misconception
that the mortar in a valley is there to seal
it, but think of plain tile or slate valleys
that are installed without mortar. When
constructing a lead valley, it is important
to install tile battens to create upstands in
the lead each side of the valley. The lead
valley lining can then be dressed over the
upstands, with a clear gap between the
mortar and upstand, and its outer edges
finished in a welt, thus providing two
defences against water ingress.
The valley tiles must not be mortarbedded
directly onto the lead; lead
expands and contracts with changes
in temperature, therefore, mortar must
be laid onto an undercloak of slate or
fibre cement board to allow differential
movement between the lead and
Side abutments must be constructed to
suit the tile type they are being used
with. For example, individually soakers
should be used with double lapped plain
tiles, and cover flashings are fine for
profiled single lapped tiles.
But a common mistake is to use a
cover flashing solely with single lap
flat tiles. Without a tile profile, or some
form of upstand in the tile shape, water
can penetrate between a flat tile and the
cover flashing. Therefore, it is important
to install a lead-lined secret gutter or
Ideally, if using a secret gutter or
continuous GRP soaker, a step and
cover flashing should also be installed
to prevent debris getting into the water
channel and clogging it over time.
Lastly, but just as important, is the tile
fixing specification. There are still roofs
being installed without a wind load
calculation having been carried out. Tile
manufacturers can supply written fixing
specifications for a project. This service is
free and generally available as an instant
shows a side
70 www.rcimag.co.uk May 2020