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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 rearrangement5.3.2 Rainfall Intensity Inserted mention that payment is now required to use NIWA's HRIDS tool.
2 - Editing and rearrangement5.3.2.1 Designing for Climate Change We have changed the recommendation for calculating rainfall intensity to using the new feature on our online calculators.
2 - Editing and rearrangement5.4.7 Gutter Capacity Calculator Enable 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 rearrangement5.5.7 Valley Capacity Calculator Enable 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 rearrangement5.7.3 Downpipe Capacity Calculator Enable 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 rearrangement5.8.1 Maximum Area Above Spreader Calculator Enable 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 rearrangement7.1.4 Maximum Run Calculator Enable 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 rearrangement9.4.4 Maximum Area Above Penetration Calculator Enable 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  v26.4 - April 2026 Urgent update to introduce rainfall intensity calculator.
 

 

Clause: 
019_001_000_000_000_000_000_000_000
Clause Number: 
19.1
/cop/revision-history/april-2026
Revision Category: 
4 - Clause Inserted
Revision Detail: 

Urgent update to introduce rainfall intensity calculator.