Page 6 - MGMA MRM issue 02

Make the
Calculated
Choice
Low
The UK climate is becoming warmer, wetter and
windier and extreme weather is now an all-year-
round probability in the UK, ranging from flash
floods one day to heat waves the next.
Climate change is one of the key drivers for
improving roof drainage and water management.
Developing new standards, regulations and
testing methods to ensure products and
solutions align to the worst-case scenarios of
climate models is essential.
Rainwater systems are integral to protecting
our buildings and channelling water from roof to
ground. Gutters, outlets and downpipes must
demonstrate in-situ strength and structural
capability when handling increasing volumes of
rainfall and wind loadings.
Rainfall intensity design is the critical first step
in any roof drainage design. After blockage by
debris, poorly selected rainfall intensity is the
leading cause of water ingress into buildings
from gutter systems. Unless the design process
is understood, the design can be significantly
under what is required and can result in regular
flooding; or the design can be significantly over
what is required, and excessive materials used
and/or a siltation risk can be generated.
The key factor to note about design rainfall in the
UK is that it is based on summer thunderstorm
conditions, thus it is of very short duration (two
minutes) and is heaviest in the driest parts of
the country, where the worst thunderstorms are
seen. Design rainfall in the north of Scotland
will be less than half the value in London and the
south east, which may surprise those not familiar
with the process.
The UK is unusual in that varying rainfall
intensities are used based on geographical
location and risk to building contents, unlike
many areas of Europe where a single blanket
rainfall intensity is used for the whole country and
any building type.
6 7
Very Low
Medium
High
Distribution of rainfall levels across the UK
UK design practice is based on BS EN12056-
3:2000 Gravity drainage systems inside buildings.
Roof drainage, layout and calculation which
assesses the risk of overflow to the building, and
then assigns one of four risk categories:
Category 1: External gutters only - reaches
maximum capacity in a one-year
storm
Category 2: Normal inboard gutters - reaches
maximum capacity in a storm 1.5 x
the building life
Category 3: Inboard gutters in important
buildings - reaches maximum
capacity in a storm 4.5 x the building
life
Category 4: Nuclear power stations and archive
buildings - this event should never
be exceeded.
However, when working with these categories,
some issues are raised. How is building life
determined, is it the component life or the life the
building that must be protected against ingress
from water? In this context the wording in the
standard is not particularly helpful, it probably
would be better expressed as protection of
contents life.
A building may have a time period allowed for a rainfall intensity in l/ (s.m2). There are graphs
the replacement/refurbishment of sheeting and
rooflights of only 20 years, but in most cases,
it would not be acceptable to flood the building
contents once every 20 years. Conversely, a
building where the construction materials have
a very long expected lifespan, but where the
contents of the building have no real value,
may have a protection life much shorter than a
theoretical building component’s life.
The only logical approach is to determine how
often it would be acceptable for the building
to flood and then to use that value. Too short
and the occupiers will be disadvantaged; too
long and the system will be uneconomic. For a
building of normal use, 60 years at Category 2
protection is a suitable level of protection.
It could be argued that Category 4 is the best
design level to use in all cases, as it will never
flood. This is technically correct, however there
must always be a balance between protection
and cost.
Category 4 is likely to give figures 2.5 times
higher than a rainfall intensity derived from a
realistic protection life, which on some buildings
may make it impossible to drain, on others very
difficult, and on all, more costly. There is a more
serious issue with the use of Category 4 when
applied to siphonic drainage, as there is a risk
the systems will clog up before they ever really
function siphonically.
Siphonic systems are laid level and rely on
regular flushes of high velocity flow to stay clear.
If the systems are designed to Category 4, then
it may be 600 years or more between flushes
through, by which time they will have clogged up.
Once a building protection life has been
obtained, the graphs in the national annex of
BS EN12056-3:2000 can be used to determine
Damage caused by inadequate roof drainage
for one year, five years, 50 years and 500 years
in the standard, and also a more complex
procedure which allows rainfall intensity to be
calculated for any return period. Most calculation
software will use this more complex procedure.
The graphs are not very detailed, so some
interpolation will be needed, both to locate the
site and then work out the rainfall intensity.
The rainfall intensity can then be multiplied by
a catchment area to get a flow rate in l/s, which
can be compared to manufactured capacities, or
used for more complex detailed design. Selecting
the correct rainfall intensity is critical to the
design process, and effort expended at this early
stage, will help ensure the correct design later on
in the process.
The MGMA has produced an eaves gutter design
flow chart GD18 Eaves gutter design flow which
enables the user to perform calculations to BS
EN12056-3:2000. By entering simple information
relating to building dimensions, geographical
location and proposed gutter style, the flow chart
will provide guidance on gutter specification.
The flow chart is available for download on the
MGMA web site.
It is important that rainwater manufacturers offer
the relevant support and technical advice to their
specifier and installer customers so that metal
rainwater gutters are correctly specified across
all projects. MGMA members can advise on
design to BS EN12056-3:2000; further advice
and guidance is available from any MGMA
member company whose details can be found on
the MGMA website at www.mgma.co.uk
Selecting the correct rainfall is essential