Project1_Layout 1 07/05/2013 GOOFS ON THE ROOF
Pitch: The angle of your roof. It is one of,
if not the most important measurements
when designing, specifying and
installing your roof. Everything we do on
the roof is generally designed to achieve
two major outcomes. We want to keep the tiles on
and the water out. That is our goal. We want to
do it safely and, while we want the end product to
look good, keeping the tiles on and water out is
what we are there to achieve.
Let’s consider the effect of pitch on these two
outcomes. Firstly, and most simply, keeping the
tiles on. At a low pitch, the weight of the tile will
help keep it in place, whereas when the pitch is
increased, the deadweight resistance is decreased
as the pull of gravity encourages it to slide down
the roof, and the tile is forced to hang from its
nibs supported on the batten. Most tiles will hang
at a 90° pitch (i.e. vertical) but stronger fixings
maybe required to hold those tiles in place.
Now let us consider keeping the water out.
Tiles and slates are laid in a way that controls the
flow of water down the roof and prevents water
from leaking through the junctions between the
tiles. Many roof tiles have interlocks formed on
their left edge, which are essentially water gutters
designed to aid this process.
High or low pitch
The pitch of the roof will control the speed at
which the water travels down its surface. At a high
pitch, the water will travel quickly and care must
be taken to ensure that the guttering at the eaves
can deal with higher volumes of water.
At a low pitch, the water will move more
slowly, giving it time to move sideways via
capillary action across the tile, perhaps aided by
the wind. It can flood interlocks and seep between
tiles or slates, commonly on longer rafter lengths
particularly with flat interlocking tiles.
In extreme cases wind-driven rain can be
blown back up under the headlap of the tile or
slate causing excessive water ingress beneath the
Special care must be taken when dealing with
double-lap products such as plain tile and natural
slates. These materials have no water channel
formed into their surface. They do not interlock.
Instead, they are laid in a staggered formation we
call broken bond, with alternate courses shifted
over by half a tiles width. This means that water
falling between the tiles lands in the centre of
the tile beneath. To ensure weathertightness, the
tiles must be laid double-lap meaning that one tile
covers the next two tiles beneath, rather than just
one tile over one, or single-lap, which is how an
interlocking tile is laid.
The double-lapping and staggering of tiles
and slates is effective, but only to a point. At
lower pitches (below 35° for plain tile and 25° for
natural slate) the water will flood the outer roof
covering as it is moving too slowly down the roof.
092 JUNE 2018 RCIMAG.COM
In the latest of our Goofs on the Roof series, Mat Woodyatt, technical training manager at Redland, explores the importance of roof pitch
Every roof covering will operate at a minimum
and maximum pitch and should never be used
outside those recommendations. We want to keep
the tiles on and the water out, and while we might
like the look of our natural slate extension laid at
a 15o pitch, we will not like the cost of the repairs
when the slates leak and the roof is in need of a
There are many tiles out there designed to
An incorrect pitch angle
work at a low pitch, some as low as 12.5°, so
you do have options. Always check your roof
pitch and always follow your manufacturer’s
recommendations for their tiles or slates.
www.redland.co.uk/training Mat Woodyatt, technical training manager at Redland