January 2019

Perfectly Pitched
MORTAR IN MODERN ROOFING
CONSTRUCTION
Roofing consultant and pitched roofing expert John Mercer – writing on behalf of Edilians –
talks all things mortar.
Ihave written many articles advocating the use
of dry fix products such as ridge, hip and
verge systems, with all the benefits they bring
to modern roof construction. However, we must
not forget that mortar has been used in roofing for
a very long time. Middle Eastern builders used a
material similar to concrete around 1300 BC in
coating their buildings with burnt crushed
limestone, which set hard as it reacted to gasses
in the atmosphere. Cement, as we know it today,
was patented in 1824 by Joseph Aspdin – he
called it Portland cement because it resembled
Portland stone.
With the right mix of sand aggregate and cement,
mortar can produce a very strong
bond between building
components. But the reason
mortar fails in roofing is its
inability to cope with differential
movement between components;
even a well installed mortar-bedded
roof junction, using the correct mortar mix, can
fail very quickly if any slight movement in the
building structure takes place.
Since 2014, BS 5534 recommends that all
mortar-bedded roof components, such as ridge
tiles, hip tiles and verges, be mechanically
secured to the roof structure using fixings such as
“The reason mortar fails in roofing is its
inability to cope with differential movement
between components”
Left: John Mercer, Pitched Roofing Technical
Consultant.
nails, screws or clips. Of course,
in many cases designers, roofers
and clients decide to use a dry fix
system and omit mortar altogether.
However, there are still many roofs where
mortar bedding is desirable for aesthetic and
traditional reasons. For example, Edilians has
some traditional clay tiles in its range, such as its
Phalempin plain tile and classic clay pantile
ranges in many popular colours, that look equally
good with mortar as well as with dry fix systems.
A great example of combining traditional mortar
bedding with mechanical fixing is the Edilians
710 clay angle ridge system; ridge and hip tiles
can be installed by mortar edge-bedding
combined with a hidden screw and clip fixing into
a ridge or hip timber. Equally, Edilians 710 ridges
can be used without mortar, with a ridge or hip
roll.
What makes a good mortar mix?
The strength and durability of mortar is
dependent on the type of sand used and even the
cleanliness of the water. Sharp sand, also
referred to as ‘rough’ or ‘coarse’ sand, tends to
give a stronger mix than soft sand; i.e. the type
used by bricklayers. The disadvantage of sharp
sand is that it is more difficult to work with and to
form smooth, neat finishes. Soft sand, often
called ‘building sand’, is easier to work with, but
it does not achieve the same strength as sharp
sand. For these reasons, BS 5534 now
recommends a mortar mix using a blend of sharp
and soft sands. In a typical three-parts sand to
one-part cement mortar mix, the sharp sand
32 TC JANUARY 2019