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April 2026

On 21 April 2026 the Roof Drainage Calculators were updated outside of the standard update sequence.

This update was to address the issue of the HIRDS tool from NIWA changing to a pay-per-use tool:

  • There were no changes to recommendations or calculation methodologies.
  • Existing project calculations and figures are unchanged for the same parameter.
  • All calculators now enable selection of Location to utilise rainfall intensity figures from NZBC E1/AS1 Table A.
  • Intensities determined from HIRDS or other methods can be entered as 'Custom'

Changes for Roof Drainage Calculators April 2026

CategoryClause Number (prior to update)Clause (As of current publication)Details
2 - Editing and rearrangement5.3.2Rainfall IntensityInserted mention that payment is now required to use NIWA's HRIDS tool.
2 - Editing and rearrangement5.3.2.1Designing for Climate ChangeWe have changed the recommendation for calculating rainfall intensity to using the new feature on our online calculators.
2 - Editing and rearrangement5.4.7Gutter Capacity CalculatorEnable selection of Region/location per NZBC E1 Table A.  Calculations are unchanged for a given Rainfall Intensity, and Rainfall Intensity can still be manually entered if determined from HIRDS or other methods.
2 - Editing and rearrangement5.5.7Valley Capacity CalculatorEnable selection of Region/location per NZBC E1 Table A. Calculations are unchanged for a given Rainfall Intensity, and Rainfall Intensity can still be manually entered if determined from HIRDS or other methods.
2 - Editing and rearrangement5.7.3Downpipe Capacity CalculatorEnable selection of Region/location per NZBC E1 Table A. Calculations are unchanged for a given Rainfall Intensity, and Rainfall Intensity can still be manually entered if determined from HIRDS or other methods.
2 - Editing and rearrangement5.8.1Maximum Area Above Spreader CalculatorEnable selection of Region/location per NZBC E1 Table A. Calculations are unchanged for a given Rainfall Intensity, and Rainfall Intensity can still be manually entered if determined from HIRDS or other methods.
2 - Editing and rearrangement7.1.4Maximum Run CalculatorEnable selection of Region/location per NZBC E1 Table A. Calculations are unchanged for a given Rainfall Intensity, and Rainfall Intensity can still be manually entered if determined from HIRDS or other methods.
2 - Editing and rearrangement9.4.4Maximum Area Above Penetration CalculatorEnable selection of Region/location per NZBC E1 Table A. Calculations are unchanged for a given Rainfall Intensity, and Rainfall Intensity can still be manually entered if determined from HIRDS or other methods.
4 - Clause Inserted April 2026Urgent update to introduce rainfall intensity calculator.
 

 

Clause: 
019_002_000_000_000_000_000_000_000
Clause Number: 
19.2
/cop/revision-history/april-2026
Revision Category: 
1 - Minor Errata
Revision Detail: 

Correction to clause links