Discharge Capacity

The back and side curbs of roof penetrations must be regarded as internal gutters and, therefore, must be designed with the same parameters as gutters.
As the catchment area above any penetration concentrates the runoff, the design should allow for turbulence at the discharge exit and the extra capacity required. The extra capacity required depends on the catchment area, the profile, and rainfall and can require the discharge to be spread over two or more pans.
Corrugate and symmetrical trapezoidal profiles do not have the rainfall carrying capacity of those with wider pans or deeper profiles. To determine if one pan or corrugate valley discharge is adequate for the rainfall, the catchment area behind the penetration should be calculated as per table 6.1.6.

Similarly, asymmetrical trapezoidal sections may require the discharge to be spread over more than one pan.

Where it is necessary to increase the penetration outlet to more than one pan of a trapezoidal profile, the side curbs can be tapered from the pan to the height of the rib as shown in Penetration Discharge.

Maximum discharge capacity for penetrations discharging equally per side in a 100 mm/hr Design Rainfall Intensity (DRI) area.
Max. catchment
Symmetrical trapezoidal
5 & 6 rib trapezoidal or trough

5 m²
10 m²
20 m²
This table is based on equal discharge to each side of the penetration.If the catchment area per side is unequal or exceeds that shown in the table the discharge must be spread over two or more crests each side. The maximum catchment area shown in the table must also be adjusted proportionately if the design rainfall intensity is greater than 100 mm/hr.
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The discharge capacity is limited by the capacity of one side of the penetration flashing (half the total catchment). For Example, the maximum length of run for a penetration 1m wide using corrugate is 10m.

Example 1

Five-rib trapezoidal with a 1 m wide penetration 15 m below the apex in a 100 mm/hr DRI area.

WidthLengthDRICatchment per side
1151007.5 m²
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Result—OK to Use


Example 2

Corrugate with a 1.2 m penetration 10 m below the apex in a 140 mm/hr DRI.

WidthLengthDRICatchment per side
1.2101408.4 m²
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Result—Exceeds table. Discharge over two crests as per drawing 6.1.6B

If the catchment area is greater, the discharge is spread over two or more pans as shown in Penetration Discharge.


Penetration Discharge

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The pitch of the side curb of the penetration flashing shown in Penetration Discharge depends on the pitch of the roof and the height of the rib. However, it can be reduced to 1.5° as allowed for a cricket back curb. 


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