Technical note

Recent and future advances in roof drainage designand performanceS Arthur BEng (Hons) PhD and GB Wright MEng PhD

School of the Built Environment, Heriot-Watt University, Edinburgh, UK

The past 10 years have witnessed significant changes in the way roof drainagesystems are understood and designed. In particular, there has been a step-change in the confidence with which siphonic roof drainage systems may bespecified and expected to perform. These changes have occurred whilst urbandrainage design in general has been revolutionized by wider acceptance ofSustainable Urban Drainage Systems and greater public concern regardingpluvial flooding within the context of climate change. This text considers, indetail, both how roof drainage systems are designed and how they should beexpected to perform. Particular attention is drawn to weaknesses in accepteddesign methods. Consideration is also given to ‘innovative’ roof drainage relatedapproaches such as green roofs and rainwater harvesting.Practical application: Over the past few years there have been many changes inhow roof drainage systems are specified and designed. On large buildings,technologies such as ‘siphonic roof drainage’ are now commonplace and there isan ever increasing demand for ‘green roofs’ to be specified due to their potentialto ‘green’ developments. Based on ongoing research, this paper details howthese different types of roof drainage solutions can be efficiently designed andwhat levels of performance can be expected.

1 Introduction

Over the past decade urban drainage systemshave moved towards what are now commonlyknown as ‘Sustainable Urban Drainage Sys-tems’ (SUDS) or ‘Best Management Practice’(BMP). Fundamental to the implementationof these systems is addressing both runoffquantity and quality at a local level in amanner which may also have the potential tooffer amenity benefits to stakeholders. This has

led to a change in the way new developmentsnow look and interact within catchments.However, despite the availability of such toolsto reduce, attenuate and treat urban runoff,substantial areas of the urban environment arestill 100% impermeable and drain rapidly;namely roof surfaces. Normally, roof drainagesystems do not always receive the attentionthey deserve in the area of design, constructionand maintenance. Although the cost of asystem is usually only a small proportion of abuilding’s total cost, it can be far outweighedby the costs of the damage and disruptionresulting from a failure of the system to providethe degree of protection required.

Address for correspondence: Scott Arthur, School of the BuiltEnvironment, Heriot-Watt University, Edinburgh EH14 4AS,UK. E-mail: s.arthur@hw.ac.uk

Building Serv. Eng. Res. Technol. 26,4 (2005) pp. 337�/348

# The Chartered Institution of Building Services Engineers 2005 10.1191/0143624405bt127tn

There are basically two different types ofroof drainage system, namely conventionaland siphonic (see Figure 1). Conventionalsystems operate at atmospheric pressure, andthe driving head is thus limited to the gutterflow depths. Consequently, conventional roofdrainage systems normally require a consider-able number of relatively large diameter ver-tical downpipes, all of which have to connectinto some form of underground collectionnetwork before discharging to the surfacewater drain. In contrast, siphonic roof drai-nage systems are designed to run full-bore(turbulent gutter conditions mean that therewill always be a small percentage of entrainedair within the system, typically 5%), resultingin sub-atmospheric system pressures, higherdriving heads and higher system flowvelocities. Hence, siphonic systems normallyrequire far fewer downpipes, and the depres-surized conditions also mean that much of thecollection pipework can be routed at highlevel, thus reducing the extent of any under-ground pipework.

Both types of drainage system comprisethree basic interacting components:

. the roof surface;

. the rainwater collection gutters (includingoutlets);

. the system pipework.

Each of these components has the ability tosubstantially alter the runoff hydrograph as itis routed through the system. This text will

focus on the role and performance of each ofthese components. As the principles of sipho-nic drainage are generally less well under-stood, and certainly less well documented,particular emphasis will be placed on theperformance of siphonic roof drainage sys-tems in this text.

2 Roof surface

The design of the roof surface is usually withinthe remit of the architect rather than thedrainage designer. Notionally, there are threetypes of roof surface:

2.1 Flat roofsFlat roofs are normally associated with

domestic properties in climates with low rain-fall, and with industrial buildings in developedcountries. Such roofs are seldom truly ‘flat’,but simply fall below the minimum gradientassociated with sloped roofs in the jurisdictionunder consideration; for example, in the UK aflat roof is one where the gradient is less than108.1 Minimum gradients are usually specifiedto avoid any unwanted ponding (BS 6229:2003specifies a 1 in 80 minimum gradient), and tohelp prevent the development of any adversegradient due to differential settlement.2

Although flat roofs can be problematic ifnot maintained properly, they are often pre-ferred as they reduce the amount of deadspace within the building and they attenuateflows more than sloped surfaces.

Drivingheads

b. Siphonic system

Submerged outlet conditions

a. Conventional system

Free surface outlet conditions

Annularflow inverticaldownpipes Extensive

undergroundnetwork

Full-bore flow in allpipework

Minimal underground network

Figure 1 Schematics of a typical conventional and siphonic roof drainage system (at normal design condition)

338 Recent and future advances in roof drainage

2.2 Sloped roofsMost residential and many commercial

properties have sloped roofs. Such roofs aregenerally favoured as their ability to drainnaturally means that there is less risk ofleakage. In temperate climates, their specifica-tion also means that snow loading is less of anissue. Once a rainfall event is underway, therate at which the runoff flows across a roof is afunction of roof slope and roughness. Whererainfall data is available, runoff rates fromroof surfaces may be readily assessed usingkinematic wave theory.3

2.3 Green/brown roofs (sloped or flat)Arguably the oldest type of permanent roof

is a green roof. These involve the planting ofroof areas to attenuate and/or dissipate rain-fall, and can take the form of a roof topgarden with trees and shrubs (termed inten-sive), or a lightweight carpet of growth mediaand flora (termed extensive). The latter tech-nology is already employed widely (e.g., theRolls-Royce plant at Goodwood, purportedlyEurope’s largest ‘green roof’). Many of theseapplications tend to focus on the aestheticbenefits such systems offer to high profiledevelopments, and are often installed in aneffort to ‘green’ a development and thus helpsecure planning consent in sensitive areas.4

However, as well as being aesthetically pleas-ing and hydraulically beneficial, green roofsmay also offer thermal insulation,5 reduce theheat island effect (the phenomenon wherebyabsorption of solar radiation by urban sur-faces causes a marked increase in ambient airtemperature),6 provide acoustic damping andextend the service life of the roof mem-brane.7�10

Green roof systems are used extensively inGermany and to a lesser extent in NorthAmerica, but again their specification isprimarily due to a desire for a reducedaesthetic impact associated with a particulardevelopment. Germany probably has the mostexperience to date, a direct result of their usein the 1800s as a low fire risk alternative

to tarred roofs in deprived urban areas.11

Currently, German research is focused pre-dominantly on planting issues, and there isonly a limited understanding of how thesystems may be used to mitigate the impactof urban runoff. One research project, whichran from 1987 to 1989 in Neubrandenburg,8

found that an installed green roof with 70 mmof substrate could reduce annual runoff froma roof by 60�/80%. Work in Vancouver(Canada), based on an uncalibrated computermodel, suggests that for catchments where theroof area comprises 70% of the total surface,installing an extensive system could reducetotal runoff to approximately 60% over 12months.12 The same model was also used toassess specific synthetic rainfall events; theseresults indicated that the catchment experienceincreased runoff during longer rainfall events.

Neither of the above studies detail howgreen roofs could be expected to performduring a particular rainfall event, or whereefficiencies may be gained in the design ofcollection pipework. Limited testing in theUSA,13 where green roofs are often irrigated,has indicated that runoff can be reduced by65% during a single event. The most author-itative design guidance for green roofs in theUSA is produced by The New Jersey Depart-ment of Environmental Protection.14 This isfocused on lightweight structures and givesguidance on how to ensure ‘rapid draining’where the rainfall return period exceeds 2years.

Rainfall return periods are normally setwithin the context of failure probability andconsequence. Conventional systems areusually designed assuming 100% runoff for a2-minute duration storm; the 2-minute dura-tion is selected, as it is the typical time ofconcentration for conventional systems.Although advice is given in codes for settinghigher runoff rates, there is little guidance onsetting runoff rates below 100%. These ob-servations mean that inadequacies are encoun-tered if conventional codes are used to designgreen roofs:

S Arthur and GB Wright 339

. Runoff coefficients should be expected tobe below that used for conventional roofs;100% is used by BS EN 12056-3:200015 and98.7% was recorded by Pratt and Parker.16

. Peak runoff rates will be reduced; evenwhere there is no infiltration, the surfaceroughness will have a significant impact.

. Time of concentration would be expected tobe greater than 2-min; particularly relevantwhen designing collection pipework forlarge roof areas i.e., public sector, commer-cial and industrial properties.

. As with other elements of urban drainagedesign, it is not efficient for a complexsystem such as a green roof to be matchedto a single rainfall event. It is probable thatthe duration of runoff hydrographs will beorders of magnitude longer than comparedwith conventional systems, and runoff in-teractions between independent rainfallevents are probable; this may make a time-series approach more appropriate.

3 Rainwater collection gutters

The basic requirement for rainwater collectiongutters is that they have sufficient flow capa-city to accommodate flows from the designstorm.17 Although it is common practice toinstall gutters at a slight gradient to preventponding, the nature of the construction in-dustry and the process of settlement meansthat it is normal to assume gutters laid atslack gradients are actually flat; for example,BS EN 12056-3:2000 stipulates that gutters atgradients less than 0.3% shall be treated asbeing flat. In a level gutter, the water surfaceprofile will slope towards the outlet, and itis the difference in hydrostatic pressurealong the gutter that gives the incoming waterthe required momentum to flow towards theoutlet.18

3.1 Gutter outlet depthsKey to ensuring whether or not collection

gutters have sufficient capacity are the condi-

tions that occur at the gutter outlets. As wellas affecting the flow rates entering the drai-nage system pipework, the outlet depths alsoaffect the upstream gutter depths (via thebackwater surface profile). Hence, althoughthe depth at a gutter outlet may not cause anyparticular problems, the greater depths occur-ring at the upstream end of the gutter mayresult in overtopping.

Extensive experimental studies in the 1980sdetermined that the flow conditions in thevicinity of a gutter outlet in a conventionalroof drainage system could be categorized asbeing either ‘weir’ type or ‘orifice’ type,depending on the depth of water relative tothe size of the outlet.19 At depths below thatequivalent to half of the outlet diameter, theflow conditions are ‘weir’ type and outletconditions are calculated using an appropriatesharp-edged weir equation.18 At higher flowdepths, the flow effectively ‘chokes’ and theflow regime changes to ‘orifice’ type, with theoutlet conditions being calculated by anappropriate sharp-edged orifice equation.18

Although conventional roof drainage systemsare usually designed to ensure free dischargeat gutter outlets, design restrictions may meanthat the outlets cannot discharge freely; insuch circumstances, additional gutter capacity(storage) will normally be required to accom-modate the resulting higher flow depths.

In siphonic roof drainage systems, the out-lets are designed to become submerged inorder to allow full-bore flow conditions todevelop and be sustained; if this is the case thedetermination of outlet depth is complicatedas the gutter conditions are dependent uponthe downstream conditions (within the con-nected pipework) as well as the gutter inflows.Recent experimental work has also indicatedthat conventional roof drainage systems in-corporating ‘non-standard’ gutter sections,whose base width and height is significantlygreater than the diameter of the outlet, canresult in the development of full-bore flowconditions in the vertical downpipe and ulti-mately siphonic action;20 for a given gutter

340 Recent and future advances in roof drainage

section, the onset and extent of such condi-tions were observed to be dependent on thediameter of the downpipe. Similar phenomenahave also been observed in ‘standard’ guttersections (semicircular and elliptical); in thesecases limited siphonic action was observed tooccur for only a short distance below theoutlet.18

3.2 Flow division within guttersIn terms of flow division between multiple

outlets in a gutter under free discharge condi-tions, it can be seen from Figure 2a that theflow splits evenly in any given gutter section(between two outlets or between an end walland an outlet), irrespective of whether thegutter inflow is uniform or non-uniform.Figure 2b and 2c indicate the effect of outletplacement within a gutter; evenly spacedoutlets requiring far less gutter capacity (oroutlets) than those placed at the gutterextremities.

Where outlets are not freely discharging, theflow division between multiple outlets in agutter may not be as described, as theindividual gutter sections may ‘hydraulicallymerge’ to form one continuous channel and/ordownstream system conditions may becomesignificant. For example, the pipework in asiphonic system will run full-bore when oper-ating at or near its design point, and the flowdivision between outlets will be dependent onthe relative losses associated with each branchof the system.

3.3 Backwater profilesThe water surface profile in gutters can

only realistically be assessed by applying themomentum equation for channels with late-ral input. In many cases, the low velocitiesassociated with gutter flows mean thatgutter friction losses are minor and may beignored.18 If a gutter outlet is such thatit allows free discharge, and frictional effects

Q1 (uniform) Q2 (non-uniform) Q3 (uniform)

Q1 0.5 Q20.5 Q2 0.5 Q3

a. Flow division between multiple outlets in a gutter

0.25 Q 0.25 Q0.25 Q 0.25 Q

Q (uniform)

b. Flow division between evenly spaced outlets in a gutter

0.5 Q 0.5 Q

Q (uniform)

c. Flow division between outlets positioned at gutter extremities

Figure 2 Effect of outlet positioning on flow division in gutters

S Arthur and GB Wright 341

are neglected, the backwater profile may bedetermined by applying Equation (1) to de-termine the horizontal distance (DL) betweenany given upstream depth (h1) and down-stream depth (h2).

DL�gh2

h1

� �1 �

Q2T

gA3

��

So �Q2

A2mC2

��

dh (1)

Where Q�/flowrate (m3/s); T�/surface width(m); g�/gravitational constant (m/s2); A�/

flow area (m2), So�/bed slope (�/); m�/

hydraulic mean depth (m); C�/Chezy coeffi-cient (�/).

Equation (1) can be modified if frictionaleffects are considered to be significant (verylong gutter lengths or very high flow velo-cities), or if the gutter outlet is not freelydischarging.

3.4 Current design methodsThe foregoing discussion has highlighted

the key elements that should be consideredwhen designing a rainwater gutter. However,without recourse to some form of numericalmodelling, it is just not feasible to calculatethe backwater surface profiles, and hencegutter capacities, for roof drainage systems;this is particularly the case for large com-mercial or manufacturing developmentswhich may incorporate many kilometres ofdifferent types of guttering. Consequently,current gutter design methods for guttersinstalled in conventional drainage systemsare based primarily on empirical relation-ships19 and the assumption of free dischargeat the outlet. For example, BS EN 12056-3:2000 specifies that the design capacityof a ‘short’, level, semicircular gutter locatedon the eaves of a building (with outletscapable of allowing free discharge) is givenby (QL);

QL �0:9�QN �0:9�2:78�105 �A1:25E (2)

Where: QN �/notional gutter design capacity(l/s); AE �/Gutter cross-sectional area (mm2).

Clearly not all gutters can be designed byapplication of Equation (2). For example, BSEN 12056-3:2000 contains clauses to accountfor many eventualities, including:

. Location of gutter on building, which mayresult in varying consequences of failuree.g., eaves gutter, valley gutter, parapetgutter.

. Different shaped gutter sections.

. ‘Hydraulically long’ gutters (where fric-tional effects may be significant).

. Gutters laid at a significant gradient.

. Changes in gutter alignment (bends, offsets,etc).

. Additional system elements, such as strai-ners or rainwater hoppers.

. Restricted flow at outlets.

. Gutters installed on siphonic roof drainagesystems.

In addition to the type of clauses listed above,BS EN 12056-3:2000 also allows designers toutilize data obtained from experimental test-ing of a particular arrangement.

3.5 Numerical modelsA number of numerical models have been

developed that can accurately simulate theflow conditions in any type of gutter as aresult of either steady or unsteady roof runoff.An example of this is incorporated into the‘ROOFNET’ model recently developed aspart of a academic research project dealingwith the effect of climate change on urbandrainage.20 This model enables the user tospecify data describing the relevant aspectsof a particular installation, including: detailsof the prevailing rainfall conditions, details ofthe roof surfaces to be drained and detailsof the actual gutters. A kinematic wave modelis then used to route the rainfall over the roofsurfaces and into the gutters. A method of

342 Recent and future advances in roof drainage

characteristics based solution of the funda-mental equations of one-dimensional flow inopen channels is then used to route the runoffalong the gutters to the outlets,21 at whichpoint the flow enters the drainage systempipework. The model automatically deter-mines the flow conditions at the gutter outletsand, in addition to dealing with free dischargecases, can also simulate the effect of restrictedflow conditions and submerged outlet scenar-ios. Output includes depths, velocities andflow rates along the gutter, as well as thelocation and severity of any gutter overtop-ping events.

At present, models such as that describedare essentially research tools, in that they arenormally developed and utilized by universi-ties for specific research projects. However, it isenvisaged that such models may soon come tobe used as diagnostic design aids, particularlyto assist code development.

4 System pipework

The type and extent of pipework incorporatedinto a roof drainage system depends primarilyon whether the system is conventional orsiphonic.

4.1 Conventional rainwater systemsIn conventional roof drainage systems, the

above ground pipework generally consists ofvertical downpipes, connecting the gutter out-lets to some form of underground drainagenetwork, and offset pipes, used where thegutter overhang is significant. It should benoted that an offset pipe is defined as a pipewith an angle less than 108 to the horizontal.The capacity of the system as a whole isusually dependent upon the capacity of thegutter outlets rather than the capacity of thevertical downpipes.

The flow within vertical downpipes willnormally be free surface, with BS EN 12056-3:2000 specifying that downpipes run no morethan 33% full; this effectively installs redun-

dant capacity within the system. If the down-pipes are sufficiently long (normally greaterthan 5 m), annular flow conditions may occur.Similarly, the flow conditions within offsetpipes will also normally be free surface, withBS EN 12056-3:2000 specifying that offsetsrun no more than 70% full; indicating the needto install all offsets at a gradient. The designof the pipework can either be undertakenutilizing the design tables in BS EN 12056-3:2000, or by applying the Wyly-Eaton equa-tion for vertical downpipes22 and the Coleb-rook-White equation for offset pipes.23

4.2 Siphonic roof drainage systemsIn contrast to conventional systems, sipho-

nic installations depend upon the purgingof air from the system (priming) and thesubsequent establishment of full-bore flowconditions within the pipework connectingthe outlets in the roof gutters to the down-stream surface water sewer network (atground level).

Current design practice assumes that, for aspecified design storm, a siphonic system fillsand primes rapidly with 100% water.24 Thisassumption allows siphonic systems to bedesigned utilizing steady state hydraulic theory.The steady flow energy equation is normallyemployed,21 with the elevation difference be-tween the gutter outlets and the point ofdischarge being equated to the head losses inthe system. Although this approach neglectsthe small quantities of entrained air that alwaysenter a siphonic roof drainage system, it hasbeen reported to yield operational character-istics similar to those observed in laboratorytest rigs at the fully primed state,25,26

However, steady state design methods arenot applicable when a siphonic system isexposed to a rainfall event below the designcriteria, or an event with time varying rainfallintensity. In the former case, the flow maycontain substantial quantities of entrained airand exhibit pulsing or cyclical phases; a resultof greatly varying gutter water levels and anindication of truly unsteady, transient flow

S Arthur and GB Wright 343

conditions. Such problems are exacerbatedwhen the system incorporates more than oneoutlet connected to a single downpipe (multi-outlet system), as the breaking of full-boreconditions at one of the outlets (due to lowgutter depths and air entry) is transmittedthroughout the system and, irrespective of thegutter depths above the remaining outlet(s),results in cessation of fully siphonic condi-tions. As sub-design events are the norm, it isclear that current design methods may not besuitable for assessing the day-to-day perfor-mance characteristics of siphonic roof drai-nage systems. This is a major disadvantage, asit is during these events that the majority ofoperational problems tend to occur e.g., noiseand vibration.

Despite any demerits that current designmethods may have, many thousands of sys-tems have been installed worldwide with veryfew reported failures. Where failures haveoccurred, they have invariably been the resultof one or more of the following;

1) a lack of understanding of operationalcharacteristics;

2) poor material specification;3) installation defects;4) a poor maintenance programme.

In response to these perceived shortcomings, aseries of research projects have recently been

undertaken to augment the understanding ofsiphonic roof drainage systems, and to devel-op numerical models for use as diagnosticdesign aids.27,28 The remainder of this sectionwill present a selection of the salient pointsarising from this work.

In contrast to the assumption made withcurrent design methods, the priming of atypical siphonic system was actually found tobe as follows (refer to Figure 3):

1) Flow conditions throughout the systemare initially free surface (Phase 1).

2) Full-bore flow conditions form at somepoint within the horizontal pipework(Phase 1).

3) Full-bore flow conditions propagatedownstream, towards the vertical down-pipe, and upstream, towards the gutteroutlets (Phase 1).

4) Full-bore flow conditions reach the ver-tical downpipe, the downpipe starts to filland the system starts to depressurize(Phase 2).

5) Once the conditions throughout thedownpipe are full-bore, any remainingair pockets are purged from the system(Phase 2).

6) Full siphonic action occurs (Phase 3), andwill continue until the gutter depth(s) falls

0

75

150

0 10 20 30 40 50 60Time since start of simulated rainfall event (s )

Gut

ter

dept

h (m

m)

-1.8

-1.2

-0.6

0.0

0.6

Pre

ssur

e (m

H2O

)

Depth in gutter 1

Depth in gutter 2

Pressure P1

Gutter 1 Gutter 2

Schematic of test rig

Pressure P1

Phase 1 Phase 2 Phase 3

Air pockets leave system

Figure 3 Priming of a laboratory siphonic drainage test rig28

344 Recent and future advances in roof drainage

below the level at which air can enter thesystem.

The data shown in Figure 4a illustrates thetype of unsteady flow conditions that occurwhen a siphonic system is exposed to rainfallevents below the design point, and the gutterflow depths are insufficient to sustain fullsiphonic action. The data shown in Figure4b illustrates the type of unsteady flowconditions that occur when an installedsiphonic system is exposed to a ‘real’ rainfallevent, where the rainfall intensity varies withtime.

Figure 5 shows an example of the outputfrom one of the numerical models that haverecently been developed (SIPHONET). As canbe seen, the model can accurately simulate thepriming of a siphonic system (0s�/32s) as wellas steady siphonic conditions (32s�/62s). Thisdata also illustrates that the model cansimulate complex operating conditions, suchas the rise in system pressures when the depthin gutter 1 drops below that necessary for full-bore flow, hence allowing air to enter thesystem and break the siphon (at approxi-mately 62s)

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0 20 40 60 80Time since start of simulated rainfall event (s)

Pre

ssur

e ( m

H2O

)

Regime 1 (15 - 40% Qmax )

Regime 2 (40 - 60% Qmax )

Regime 3 (60 - 80% Qmax )

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

0 1000 2000 3000 4000 5000 6000 7000Time since start of rainfall event (s)

Pre

ssur

e (m

H2O

)

0

20

40

60

80

100

120

Rai

nfal

l int

ensi

ty (

mm

/hou

r)

Pressure P1 Rainfall instensity

b

a

Figure 4 (a) Measured system pressures for sub-design rainfall events within a laboratory siphonic drainage test rig.28

Note that this data refers to the pressure P1, as indicated in Figure 3; (b). Sub-design rainfall events within an installedsiphonic drainage system28

S Arthur and GB Wright 345

5 Conclusion

The text has illustrated how roof drainagesystems are a key, but often overlooked,element of urban drainage infrastructure. Ithas also been shown that their design is acomplex process, which relies heavily on gutteroutlet performance. The following conclusionsmay be drawn with respect to the operation ofroof drainage systems:

1) Their operation is dependent on three

interacting components: the roof surface,

the collection gutter and collection pipe-

work.2) Green or brown roofs provide an oppor-

tunity to reduce the flow from roof

surfaces, improve urban aesthetics and

increase urban biodiversity.3) Outlet conditions are key to understand-

ing how a system will perform.4) Siphonic roof drainage systems present a

more efficient way to drain large roof

surfaces. However, this must be consid-

ered within the context of possible higher

maintenance costs.5) The design of siphonic roof drainage

systems should include an allowance for

sub-design rainfall events and operationalproblems e.g., blocked outlets.

Although green roofs are an attractive alter-native, it is probable that conventional roofsurfaces will continue to dominate domesticinstallations. However, it is likely that greenroofs will experience a step-change in accep-tance by the commercial sector once morebecomes known about their performance andsustainability. Similarly, the efficiencies offeredby siphonic systems means that they willcontinue to play a significant role in draininglarge commercial buildings, particularly ifnumerical models are applied diagnosticallyto improve performance and reduce the occur-rence of costly system failures.

The biggest threat to roof drainage comesfrom climate change. Existing systems maynot simply become more prone to flooding;changes in rainfall patterns may result in longperiods of low precipitation, and self-cleans-ing velocities may be attained less frequentlyas a result. Furthermore, changes in windpatterns may also increase levels of rooftopdebris, and hence necessitate enhanced main-tenance programmes. As concern regardingclimate change grows, and the sustainabilityagenda widens, it is possible that harvesting

0

80

160

0 25 50 75 100

Time since start of simulated rainfall event (s )

Gut

ter

dept

h ( m

m)

-2.0

-1.2

-0.4

0.4

Pre

ssur

e ( m

H2O

)

Measured gutter 1 depth

Predicted gutter 1 depth

Measuredpressure P1

Predictedpressure P1

Figure 5 Measured and predicted system conditions within a laboratory siphonic drainage test rig: no inflow intogutter 1 between 62s and 82s.28 Note that this data refers to the system shown in Figure 3

346 Recent and future advances in roof drainage

roof runoff may become more widespread. Atpresent water consumption varies globallybetween 7 and 300 l/capita per day. In theUK, average consumption is 145 L/h per d,but only 1�/2 l may actually be consumed byhumans, whilst 30% may be used for WCflushing.29 Studies have shown that, whencoupled with storage, roof rainwater harvest-ing has the potential to contribute substan-tially to domestic water usage in bothdeveloping and developed countries.30,31

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348 Recent and future advances in roof drainage

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