May 2019

An Inspector Calls
DON’T BE THE FALL GUY WHEN IT
COMES TO FLAT ROOFING
In our regular monthly column – ‘An Inspector Calls’ – Total Contractor has teamed up with
the experts at BMI UK & Ireland to help you avoid the common pitfalls that can often cost
you both time and money, and ultimately help you achieve roofing success.
This month, the Inspector looks at how
updates in BS 6229:2018 can stop you
being the fall guy when it comes to flat
roofing.
One of the most eye-catching amendments in the
revision of BS 6229:2018 was the reference to
falls. This is now much clearer, stating that “all
flat roof surfaces (including gutter beds) should
be designed with a fall of 1:40 to ensure finished
drainage falls of 1:80 are achieved. This should
take account of construction tolerances,
permitted deviations and deflection under load,
and account for deflections/settlement.”
In other words, any design should allow for all
factors that could reduce or hinder the drainage,
eliminating the risk of ponding on roofs. Without
these considerations, it is highly likely standing
water will occur. Although improvements in
membrane technology and performance have
increased significantly, standing water may still
result in additional and unnecessary stresses in
the membrane, particularly in the winter when
that standing water freezes. Furthermore, in the
event of a defect being present in the area of the
ponding, greater water ingress will occur when
compared to a well-drained roof.
Accelerate the ageing process
Standing water is typically defined as water that
remains on the roof for longer than 48 hours, and
whilst this is not an immediate threat, it can
accelerate the natural ageing process and have a
detrimental effect on the membrane’s lifespan.
It’s possible that standing water can still occur
even when the structural deck has a fall within it,
as compressed insulation,
blocked or faulty drainage
and damaged membranes
can all contribute
towards this issue too.
Regular maintenance
can therefore reduce
this risk, alongside
reducing the number and
weight of items permanently
stored on the roof. Another
contributing factor could be the
sequencing of membrane laps and edge details
sitting proud of the finished floor level,
consequently reducing the water’s ability to
navigate past the resulting water-check.
The reference to falls is all the more pertinent
given that, these days, there are certain thirdparty
certified waterproofing and insulating
Above: Ponding can shorten the life of a roof. Below:
Standing water due to lack of fall.
systems that have gained
approval for use with zero
falls. Hot melt systems
in particular are
popular for this area
of work. For these
systems, zero falls
are acceptable, but
negative falls are not, so
should be corrected. It is
not really acceptable in this
day and age for any contractor to
install roof decks with large depressions, back
falls and non-draining areas.
To ensure a zero fall finished surface i.e. one that
is totally flat, a design fall of 1:80 should be
used, along with a detailed structural analysis to
account for construction tolerances, settlement
and deflection under load.
Remedial action
If sites have negative falls, thereby increasing the
likelihood of ponding, then remedial action has to
be taken before the roof system is applied. This
could be by laying a localized screed to falls and
firings, fitting tapered insulation or fitting
additional rainwater outlets at the lowest points.
As a result, the roofing contractor should expect a
flat, properly drained surface on which to lay the
specified system and the finished roof should not
suffer from ponding or inadequate drainage.
Contact BMI National Training Centre
01285 863545
www.redland.co.uk/training
@_Redland / @Icopal_UK
16 TC MAY 2019