Masterspec
03 Jun 2021
HOW MUCH DOES MASTERSPEC COST?
With seven different libraries to choose from, Masterspec has all you need to create a quality, New Zealand construction specification for any design....
11 May 2021
NEW PRODUCT PARTNER SECTIONS FOR MAY 2021
In case you missed our Masterspec NOTES at the beginning of the month, here is a list with reference links for the new Product Partner work sections...
29 Apr 2021
BUILDING CODE UPDATES 2020-2021 - INTERNAL MOISTURE
On the 4th November 2021, the changes to NZBC E3/AS1 and the addition of E3/AS2 become the Acceptable Solutions for Internal Moisture. But it is...
29 Apr 2021
LIST OF WORK SECTIONS WITH NZBC CHANGES - MAY 2021
With the updating of the Masterspec libraries, we thought it may be useful to provide the complete list of work sections that have been updated so you...
28 Apr 2021
BUILDING CODE UPDATES 2020-2021 - TIME TO ABSORB & APPLY
On the 3rd of November 2021, the 12 month transition period ends for the NZBC changes introduced in November 2020. This means the previous editions of...
28 Apr 2021
PROTECTION FROM FIRE UPDATES
This article delivers a summary of a number of the more significant updates to C/VM2 and C/AS2. Minor amendments to C/VM1 and C/AS1 were also made,...
28 Apr 2021
STORMWATER DRAINAGE DESIGN – NEW OPPORTUNITIES IN NZBC E1 NOVEMBER 2020 UPDATE
November 2020 brought a number of updates to the Building Code, including a new Acceptable Solution in the form of E1/AS2 in its newest version,...
25 Mar 2021
UPDATES TO MASTERSPEC INTUMESCENT PAINT – STEELWORK SECTIONS
Late last year the FPANZ (Fire Protection Association of New Zealand) released the Code of Practice COP-03 Version 1.0, Specification of Application...
09 Mar 2021
NEW PRODUCT PARTNER SECTIONS FOR MARCH 2021
In case you missed our Masterspec NOTES at the beginning of the month, here is a list with reference links for the new Product Partner work sections...
03 Mar 2021
STANDARDS ONLINE WEBSITE UPDATE
You may be experiencing some problems accessing Standards over the coming days as they are making significant updates to their website. We are aware...
02 Mar 2021
FIRE BARRIERS - FOR VENTILATED CAVITY CONSTRUCTION
Maintaining an unobstructed ventilated drained cavity behind cladding on multi-unit/multi-storey residential or commercial buildings can be made...
18 Feb 2021
ELECTRICAL RELATED FIRES - CAN THEY BE REDUCED?
About 20% of New Zealand building fires are related to electrical systems or devices. So is there anything that could help in new builds and...
18 Feb 2021
TIMBER STANDARD NZS 3604 REVISION PROCESS STARTED
New Zealand's main standard for the design and construction of timber-framed buildings, NZS 3604:2011 Timber-framed buildings, is being revised,...
09 Feb 2021
NEW PRODUCT PARTNER SECTIONS FOR FEBRUARY 2021
In case you missed our Masterspec NOTES at the beginning of the month, here is a list with reference links for the new Product Partner work sections...
12 Jan 2021
NEW PRODUCT PARTNER SECTIONS FOR JANUARY 2021
In case you missed our Masterspec NOTES at the beginning of the month, here is a list with reference links for the new Product Partner work sections...
08 Dec 2020
MASTERSPEC BEGINS A NEW INDUSTRY PARTNERSHIP WITH STEEL CONSTRUCTION NEW ZEALAND (SCNZ)
Masterspec and SCNZ are delighted to announce a new industry partnership to support the structural steel design, fabrication and erection in the New...
26 Nov 2020
STORMWATER DRAINAGE DESIGN – NEW OPPORTUNITIES IN NZBC E1 NOVEMBER 2020 UPDATE
November 2020 brought a number of updates to the Building Code, including a new Acceptable Solution in the form of E1/AS2 in its newest version,...
26 Nov 2020
GREEN STAR UPDATE TO VERSION 1.0
The Green Star Technical Manual v3.2 has been superseded by Green Star Design and As Built NZ v1.0. Also Green Star Office Interiors v3 has been...
24 Nov 2020
GUTTER & DOWNPIPE DESIGN MADE EASY
It is not often that changes to the NZBC Acceptable Solutions and Verification Methods provide a windfall that makes life a whole lot easier for a...
29 Oct 2020
LATEST BUILDING CODE 2020 UPDATES
Earlier this year MBIE ran public consultation on proposed changes to a number of Acceptable Solutions and Verification Methods. Despite a number of...

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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