wavin OSMA Rainwater - Page 8

Design Procedures
Wavin Osma Rainwater
50 www.wavin.co.uk
E
D
Figure2:Wallabuttingroof
Determining Gutter
Flow Capacity
Calculating Effective Roof Area:
Wall Abutting Roof
Wallsaboveabuttingroofswilldrainontotheroofbelow.
Forasinglewallthee�ectivecatchmentareaistakento
behalftheareaoftheelevation.
Method(seeFigure2)
1. Measuresinmetres:
D.Lengthofwallaboveroof
E. Heightofwallaboveroof
2. Usetheformula:E�ectiveWallArea(inm)=(DxE)
––
2
3. AddtheE�ectiveWallArea(ifappropriate)tothe
E�ectiveRoofAreatobedrained.
Factors to Consider
Gutter fall
Gutterslaidtoafallaremoreecientthangutterslaid
level.Afallof1:600(25mmin15m)isrecommended.
ThehighcapacitycharacteristicsofWavinOsmagutter
systemsmeanthatperformanceisnotcompromisedby
beinglaidlevel.
However,layingtheguttertoafallwillincreaseow
capacityandtheareaofroofthatcanbedrained–
particularlyifthedownpipeispositionedcentrally,Silting
willalsobeprevented.
Number of outlet(s)
Themoreoutletsalongagutter,andtheshorterthe
distancerainwaterhastotravel,themoree�ectivethe
system.
Example:
A
B
C
LayoutBismoreecientthanA,andCismoreecient
thanB.
Location of outlet(s)
Outletslocatedneargutteranglesalsoreducesystem
eciency.
Example:
Whereagutterlengthcontains1ormoregutterangles
(greaterthan10º)thenmultiplytheMaximumE�ective
RoofAreasinTable1byareductionfactorof0.85.
Flow Rates (litres/sec)
Ourchangeableclimatemeansitisimpossibletoruleout
oodingandoverow.Generally,itissatisfactorytodesign
toarainfallintensityof75mmperhourwhereoverowwill
notcausedamagewithinthebuilding.
Forvalleyandparapetguttersorwhereabuildingorits
contentsrequireadditionalmeasuresofprotection,a
higherrainfallintensityshouldbeused.See
BSEN12056-3:2000.
Table3showsthemaximumcapacityforeachsystem,
gutterlaidlevelortoafallof1:600.Establishedusingthe
testmethodprescribedinEN12056–3:2000(seenext
page).
Tocalculateow:
• For75mmperhourrainfallintensity,calculate:E�ective
RoofArea(m)x0.0208=litrespersecond
• Forotherrainfallintensitylevels,calculateasfollows:
E�ectiveRoofArea(m)xrainfallintensity(mmper
hour)÷3600=litrespersecond