What are the best solutions
for installing door thresholds?
Why does creating door thresholds within a new flat roofing scheme often mean headaches for contractors?
Stephen Cleminson, technical director at SIG Design & Technology (D&T), suggests some solutions to make
thresholds work perfectly
Flat roofing continues to be a
popular choice with architects
for several reasons. They are big
fans of the aesthetics, they are
very cost effective, and can provide extra
While these schemes may be simple in
conceptual and visual terms, specifiers
should never underestimate the level
of detail required to achieve a ‘fit for
purpose’ solution that provides longterm
protection from the elements.
Adding to the complexity of the
design process is the choice of different
materials and systems available – and
the interfaces with elements when
constructing the roof.
There’s a lot to bear in mind when
designing and installing a roofing system,
and one element that contractors often find
challenging are door threshold interfaces.
This element has certainly been
overlooked at times – in some cases,
doors have been installed before the
waterproofing has even been started (not
good sequencing, leaving the roofer with
an almost impossible task)!
The holistic approach
As with any flat roofing scheme, SIG
Design & Technology (D&T) strongly
advocates a holistic approach. A scheme
should be designed “from the top down,”
and every element’s relationship with
the other should be carefully considered
before the work starts.
Interfaces such as door thresholds are
a fundamental element in your checklist
for creating that perfect roof. As you
identify your preferred waterproofing
solutions, look and think carefully about
how your roofing solution will interface
with the threshold material, and where
this is located.
Where schemes either succeed – or fail
– it is often down to forward planning
(or lack of it) and agreeing a logical
sequence of work for which elements are
How the interfaces are treated will
relate to available upstand heights,
insulation type and whether you have
a warm, cold or inverted roof. You
must ensure you will end up with the
right details to get the guarantee for
your client, so list all the interfaces
involved including doors.
Thresholds – key
At SIG D&T, we come across
many different approaches to
tackling thresholds. Some work
well, but quite a few solutions have
proven to be a roofing nightmare.
It’s common for us to discover
schemes where the doors have been
installed although the waterproofing has
not yet started.
This means the contractor is then
left with the requirement to dress
waterproofing under a door threshold
which has already been installed,
making life very difficult in terms of
limited access. Thresholds are key pinch
points in the scheme.
Roofers always need the best possible
access to provide the intricate detailed
required so the waterproofing works.
Another issue is the use of EPDM
FOUR KEY TIPS ON INSTALLING
• Sequencing is key on a new project. Install the
• Do make sure the finished roof is protected from
following trades or consider just completing the
upstands prior to door and frame installation,
then complete the main area and the link to the
• Falls must run away from the pinch point
• Ensure adequate drainage. This means a minimum of
two outlets and potential overflows
BS 6229 is currently under review and this
key area is proposed to be included in the
revised document with detailing very
similar to that identified by the NHBC
and also within the SPRA design guide.
rubber sealing gaskets on door
thresholds. While these may seem like a
logical choice to some, these gaskets may
be incompatible with single ply or some
Out of your depth?
Specifiers should look closely at the
threshold requirements in a roofing
system. Consider the whole scheme and
the height available for the depth of your
As well as the doors, you should take
a look at the structural grid, finished
floor levels, and location and size of
abutments and restrictions such as
planning restrictions and any windows
Investment of time when making the
decisions early on will save money in the
long term. Particularly considering the
impact cost of a flat roof not performing.
Go to http://www.singleply.co.uk/flatroof
download the flat roof specifier checklist
for your roofing project.
This provides advice on flat roof
interfaces with elements such as door
SIG Design &
66 www.rcimag.co.uk July 2018