June 2022

Perfectly Pitched
LOW PITCH ROOFS
Pitched Roofing Consultant John Mercer discusses the
key considerations when constructing low pitch roofs.
Of the many enquiries I receive, one of the
most popular topics relates to low pitch
roof construction. Experience has taught
us that the ideal range of roof pitches for roof tiles
and slates is anywhere between 30 to 50º. During
rainfall or wind-driven rain spells, water is shed
from one double lapped tile or slate course to the
next one below, the tiles and slates having sufficient
lap and pitch to prevent any sideways or upwards
‘creep’ of the water through capillary action from
penetrating the roof covering. Single lapped tiles
perform the same function by virtue of their lap
over the course below and side laps which can be
either simple overlaps or more sophisticated
interlocks with water bars and troughs.
Although the trend in recent years is to build
homes with steeper pitch roofs to provide living
spaces within the roof, there are many tiles and
slated roofs constructed at low pitches to reduce
construction costs, for example, or reduce a
building’s impact on the horizon, or very
commonly, to enable the building of a home
extension to fit below the upstairs windows.
Roof tile manufacturers have developed and
continue to develop roof tiles and associated
systems that are suitable for use at low pitches.
However, under certain weather conditions, water
may be driven through the slating or tiling and must
be captured by the underlay system and drained
away to the gutter. Certainly, at low roof pitches,
the risk of water ingress increases through or
around the tiling, simply because everything is
working closer to its limits. For example, though
the individual components have been
thoroughly tested and perform fine at the
stated minimum roof pitch, constructing
the roof with the same accuracy as
that used under test conditions can
be difficult. It only takes a small
gap or two in the tiling where it
passes over a flashing and the risk of water
ingress is increased.
BS 5534 states that the underlay layer should
provide a continuous under-roof barrier to water,
snow, and dust, and should be capable of
draining any moisture that might be deposited
onto the surface of the underlay to the roof
drainage system. Although BS 5534 does not
provide specific design details on how to
construct a waterproof underlay system, it does
recommend that consideration be given to sealing
any penetrations through the underlay, such as
nail holes, with suitable tapes or sealants.
The design recommendations provided in
BS 5534 are an acknowledgement that there will
be occasions when rainwater finds its way
through the tiling and onto the underlay, so the
roof should be regarded as a system, rather than
simply a collection of individual components.
It is important to follow the installation
recommendations given in BS 5534 and BS 8000-6
for the underlay. Underlay laid unsupported, directly
over the rafters or counterbattens, must be laid with
a sufficient drape to enable water to run freely into
the underlay troughs and under the tile battens
towards the gutters. If the underlay is laid too tight,
without a drape, water on the underlay will be
trapped behind the tile battens and will eventually
find its way through the batten nail holes in the
underlay and leak into the roof structure.
On low pitched roofs, rainwater is more likely to
be driven through the tiling during heavy winddriven
rain events, though roof design
battens
Counterbattens
should consider other factors
too. For example,
manufacturers usually
set a maximum rafter
length for their tiles when
used at the minimum
recommended roof pitch. It is
Nail tape
Tile
Summary
• Observe the tile manufacturer’s
recommendations on minimum roof pitch
and maximum rafter length.
• Avoid draining water from higher roof
slopes onto a low-pitched extension roof.
• Ensure the underlay is detailed correctly at
junctions such as abutments and properly
sealed around penetrations such as pipes and
roof windows. Use adhesive tapes where
necessary.
• Consider the use of counterbattens with nail
tape between the counterbattens and underlay
to seal the nail holes through the underlay.
important to observe any such restrictions by, for
example, raising the roof pitch if possible, or
dividing the roof into shorter sections with a
gutter part way down the slope. On single storey
extensions, care should be taken to avoid
draining a higher roof onto the low pitch extension
roof via a valley or through a gutter downpipe.
Even dormer windows can pose a risk if the
rainwater from the dormer roof is drained directly
onto the low pitch roof.
It is equally important to detail all junctions
correctly when installing the underlay, with the
correct turn ups at abutments and suitably
sealed around penetrations such as pipes and
roof windows.
There are no special measures or variations to
sub-roof designs that relate to roof pitch in
BS 5534. However, there are methods that can be
adopted to ensure the underlay is watertight. For
example, consider using counterbattens between
the underlay and tile battens, with a bituminous
nail tape fitted between each counterbatten and
the underlay (see image left). The counterbattens
will ensure that there is a path for water to reach
the gutters without the need for a drape, and the
nail tape will seal the batten and counterbatten
nail holes through the underlay.
Contact John Mercer
www.johnmercerconsultant.co.uk
18 TC JUNE 2022