Talking flat roofs: beware
of the misconceptions
Describing dierent types of flat roof may appear simple, especially for those who deem to be roofing experts,
but errors are still being made. Rob Firman, technical and specification manager at Polyfoam XPS, highlights
common misconceptions and oers advice on the types of flat roof construction
In flat roofing, as in all areas of
construction, the success of a project
depends on accuracy. Whether it’s
calculating U-values to determine the
correct insulation thickness, setting the
roof out to achieve effective drainage falls,
or ensuring minimum upstands around a
roof, attention to detail is paramount.
This also applies to how we talk
about flat roofs. It’s important for both
industry newcomers and experienced
practitioners to be confident and
consistent in using roofing terminology.
Otherwise the picture can become
muddy and that increases the risk of
mistakes being made.
Types of flat roof construction
One of the fundamental areas of
roofing where misconceptions
arise is how the type of roof is
described, using the phrases
‘warm roof’ and ‘cold roof’. The
same phrases are also used to
describe different pitched roof
constructions, though we are
concentrating on flat roofing here.
Then there is an inverted flat roof,
sometimes called an ‘upside down’
or ‘protected membrane’ roof. This is
still classed as a warm roof because
the description of flat roofs is related
to how they are insulated, and the
temperature at which the structural
deck is maintained.
What is a cold flat roof?
The type of installation you might find
on an old domestic extension is a cold
roof and consists of a waterproofed
timber deck on timber joists. Any
insulation is installed between the roof
joists, and sometimes underneath the
joists as a continuous layer.
A minimum 50mm ventilated
airspace between the underside of
the deck and the top of the insulation
is required, and it must be fully
ventilated. Effective cross ventilation
is a must and having an opening at
one end only is not acceptable. The air
movement carries any moisture vapour
away from the structure, reducing the
potential for condensation to occur on
the underside of the cold timber deck.
Cold roofs are an unpopular option
and generally advised against. In
working on a warm
or inverted flat roof,
consistency is key and
that applies across all
areas and to specifiers,
Scotland, they are regulated against
entirely. Where they do feature on
projects, the limited space available
usually requires insulation products
with a low thermal conductivity like
rigid PIR or phenolic foam boards.
What is a warm flat roof?
A warm flat roof is suitable for – and
specified on – buildings of all sizes,
from small domestic projects through to
large commercial or industrial schemes.
Insulation installed above the structural
deck keeps the deck at or around
the building’s internal temperature,
eliminating the risk of condensation.
Ventilation is not required, but a
suitable vapour control layer is, which
should be installed on the deck prior to
securing the insulation layer.
The thicknesses of insulation,
particularly to achieve the very lowest
U-values, can be substantial. However,
there are often fewer constraints on
height, so a wider range of insulation
materials can be used, subject to
compatibility with the waterproofing
Most flat roofs constructed today
are warm roofs, but this ‘conventional’
warm roof build-up puts stress on
the waterproofing layer. As well as
resisting rainfall, the waterproofing is
subject to ultraviolet (UV) radiation,
expansion and contraction from natural
temperature cycles, and extremes of
temperature like frost.
What is an inverted roof?
To combat those stresses, an inverted
flat roof sees the waterproofing
installed directly onto the structural
deck, followed by the insulation layer.
The ‘usual’ order of insulation and
then waterproofing is inverted or
turned upside down in order to protect
the waterproofing layer (which now
also acts as the vapour control layer).
Rather than securing the insulation
with fixings or adhesive, a ballast layer
of gravel, paving slabs or a green roof
covering protects against wind uplift
and UV radiation.
The inverted roof is still a form of
warm roof construction because the
structural deck remains at the internal
temperature of the building.
As the waterproofing is protected
by the insulation, greater performance
demands are placed on the insulation
layer. Only a few material types are
suitable for the application – including
Polyfoam extruded polystyrene (XPS)
boards. XPS insulation is unique in
terms of its tolerance of moisture and
freeze/thaw cycles, providing thermal
performance and strength for the
lifetime of the building.
Whether you’re working on a warm
or inverted flat roof, consistency is
key and that applies across all areas
and to specifiers, manufacturers and
contractors. Clear language, processes
and technical guidance can benefit
all parties and ensure projects are
delivered to the highest standard.
46 www.rcimag.co.uk January 2019