GUTTER & DOWNPIPE DESIGN MADE EASY

24 Nov 2020

From the 5th November 2020 the new changes to the NZBC Acceptable Solutions and Verification Methods have been able to be used for compliance. In this case with Surface Water, the changes to NZBC E1/AS1 and the addition of NZBC E1/AS2, unusually provide a windfall that makes life a whole lot easier for a designer. This windfall relates to the NZ Metal Roofing Manufacturers Association Inc. (NZMRM), Code of Practice (CoP), roof drainage design 5.7 Capacity Calculator.

Changes to E1/AS1 and the addition of E1/AS2

E1/AS1 Appendix A has had the Rainfall Intensity maps deleted and very extensive Rainfall Intensity tables added providing locations all over New Zealand and their Annual Exceedance Probability (AEP). These tables have been based on National Institute of Water and Atmospheric Research (NIWA) rainfall data. This rainfall data forms the bases of calculations for roof and rainwater goods design for a particular site.

The new E1/AS2 allows AS/NZS 3500.3: 2018 Stormwater Drainage to be used as an Acceptable Solution (with some modifications). AS/NZS 3500.3, among other things, allows more complex and accurate calculations for roof and rainwater goods design, also using NIWA rainfall data. The calculation can get a little complex, however an easier solution already exists.

NZMRM Capacity Calculator

The Capacity Calculator has existed for some time, it was developed by NZMRM in response to what they believed was an Acceptable Solution that severally under-rated requirements for roof and rainwater goods design. NZMRM developed the Calculator to be consistent with AS/NZS 3500.3 not the old Acceptable Solution.

The new changes adding NIWA rainfall data to E1/AS1 and the addition of E1/AS2 means there is a close alignment with AS/NZS 3500.3. This means the NZMRM Calculator can produce an outcome that corresponds with or exceeds the Acceptable Solutions.

The calculator allows you to easily calculate or test:

gutter sizes for residential and commercial projects, both internal and external gutters (& spouting) and the resulting maximum roof area, etc.
downpipe sizes for residential and commercial projects, for both internal and external gutters (& spouting), round or rectangular downpipes and the resulting maximum roof area, etc.
valley gutter sizes for various roof pitches and the resulting maximum roof area, etc.

Note; for all the above elements the calculator applies various "short term multiplication factors". This allows for 1or 2 minute rainfall intensities which can be as much as 4.3 times the intensity over a 10-minute period used by the NIWA rainfall data. This is important as the velocity of water on a steep smooth metal roof can compound high rainfall intensity at the gutter very quickly.

Check it out

If in the past you have registered on the NZMRM website then the links below to the Calculator and other documents will work. If you have not registered before, click here on the NZMRM Capacity Calculator do the register and agree to terms and it will then open for you.

The NZMRM Capacity Calculator is a very handy tool which allows you to run a number of scenarios very quickly. The scenarios can range from sizing gutters to reduce cost to testing extreme rainfall events to reduce risk.

The NZMRM CoP provides details on roof drainage design and what drives the Calculator in the CoP section 5.6 Roof Drainage.

For more background, NZMRM have an old article on the Calculator called "Gutter and Downpipe Design". However, it pre-dates the new changes to NZBC E1/AS1 and the addition of NZBC E1/AS2, so ignore the references to NZBC E1/AS1 in their article as they may not reflect the current situation.

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GUTTER & DOWNPIPE DESIGN MADE EASY