February 2024

Inverted Flat Roofs
WATERPROOFING AND OTHER KEY
CONSIDERATIONS
As Rob Firman, Technical and Specification Manager at Polyfoam XPS, points out, inverted
flat roofs can be waterproofed with bituminous membranes, single ply membranes, or
liquid-applied membranes, but he says the most common solution is liquid-applied
waterproofing known as hot melt. In this article, Rob explains why it is so popular, and
explains how the choice of waterproofing depends on the structural roof deck.
During construction of an
inverted roof, the structural
deck is waterproofed prior to
installation of the thermal insulation
and roof finish – which also acts as
ballast. This protects the roof structure and
building from rain sooner, but also subjects the
waterproofing layer to more foot traffic.
The type of waterproofing selected should
therefore reflect the conditions it will be subjected
to during both installation and use.
What is the most common structural deck
for inverted roof waterproofing?
The overwhelming majority of inverted roofs are
Left: Rob Firman, Technical and
Specification Manager at Polyfoam XPS.
concrete decks (or metal liners with
a concrete topping). There are a
variety of reasons for this being the case:
• The roof structure must be capable of bearing
the weight of the ballast.
• Many inverted roofs are designed to bear the
weight of foot traffic, mechanical plant, or green
and blue roof systems. The roof structure must
also be capable of bearing these loads.
• As a non-combustible material, there is less
risk associated with concrete and a hot-applied
waterproofing material like hot melt.
Timber decks provide a flat substrate for the
waterproofing, but are much less commonly
specified than concrete. Metal decks, meanwhile,
cannot be used by themselves in an inverted roof
because of their profiled shape hindering the
waterproofing installation.
Properties of different inverted roof
waterproofing options
A variety of factors contribute to the choice of
system to waterproof the specified structural
deck. Method of attachment to the deck, speed of
installation, and overall system performance
must all be taken into account.
If the waterproofing product requires mechanical
40 TC FEBRUARY 2024