What you'll need first:
- Longest horizontal run of the house
- Roof Pitch
- What products do you want to use?
- If using a custom product, what mm2/m or mm2 per unit it is
- What type of roof is it?
- Eaves and box gutters are also things to note for the choice of products
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Please note that Cathedral rafter roofs with no air gap may require above batten ventilation with vapour permeable insulation.
Vents must be evenly distributed as per ABCB Housing provisions standard 2025 10.8.3
Climate zones 6-8 - Ref NCC Climate Zone Map: https://ncc.abcb.gov.au/resources/climate-zone-map
Ventilation Information
How-to Guide for the Roof Metrics Ventilation Calculator
Step-by-step instructions for compliant roof ventilation design
Overview:
Good ventilation is key to roof health – helping prevent moisture buildup, heat stress, and costly long-term damage. This guide shows you how to use the Roof Metrics Ventilation Calculator to quickly work out your legal ventilation requirements based on the latest NCC 2022 Section 10.8.3 and ASI/NASH guidelines.
It’s designed to keep your design compliant and your project moving forward – without getting buried in code references or guesswork.
Important Information
This calculator was designed to align with the ABCB housing provision standard 202210.8.3. It serves as a guide only, and users should exercise special care to account for the unique requirements and constraints of each job and design.
The information and design were developed from the NCC, ABCB housing provision standard 2022 section 10.8.3, as well as ASI NASH guidelines for ventilation of steel roofing systems.
For additional details, please consult:
Consult the disclaimer below for details on proper usage.
Step-by-step instructions
1. Builder or Client Name
Enter the name of the builder or client. This will automatically become part of the report generated by the calculator.
2. Site Address
Enter the street address of the job. This becomes the report name in your final PDF.
3. Roof Description
Provide a description of which part of the roof you’re working on. (This will help you distinguish between different roofs on the same job.)
For example:
Main roof
Top story roof
Side skillion
Main roof
Top story roof
Side skillion
4. Longest Horizontal Run
Measure the longest span horizontally across the roof. Accurate measurements here are essential – double-check your plans or use site data. If you're unsure, check out the link above for information on ventilation in steel roofing.
5. Roof Pitch
Select the roof pitch from the dropdown menu. This choice influences the options and calculations below.
6. Roof Type
Select the roof type from the dropdown menu. This choice influences the options and calculations below.
7. Roof Cavity
If there are multiple roof cavities, do each one as a separate calculation – every space needs its own vents.
8. Eaves
Indicate whether the house has eaves by selecting “Yes” or No – this changes the ventilation options available.
9. Box Gutter
If your roof has a box gutter, select “Yes” – this will tailor the ventilation options accordingly.
Calculations & Vent Options
Once your roof data is entered, the calculator will:
Show the required high-level and low-level ventilation (in mm²)
Suggest vent types and configurations based on your setup
Calculate the number of vents required for each level
Split venting between one or both long sides (based on roof type)
If your job includes a hip/valley or gable roof, vents will need to be spaced evenly across both sides. The calculator helps automate this.
Show the required high-level and low-level ventilation (in mm²)
Suggest vent types and configurations based on your setup
Calculate the number of vents required for each level
Split venting between one or both long sides (based on roof type)
High-level ventilation options
Choose the preferred high-level ventilation option from the dropdown menu. This selection determines the type of venting to be used.
Low-level ventilation options
Select the desired low-level ventilation option. This affects the type of venting employed for the lower sections of the roof.
Ventilation requirements
The calculator will populate both the legal high-level and low-level ventilation requirements in square millimetres. This ensures compliance with regulatory standards.
Number of vents
The calculator will automatically determine the required amount of venting based on legal standards.
Long sides ventilation
For houses with hip and valley or gable roofs, both sides of the roof must be vented evenly. The calculator will populate an option for one or two sides, depending on the roof style.
Total low-level vents
The total required low-level vents are calculated based on the long sides.
Additional high-level venting as combination
The option to use a combination means in some jobs (e.g. long ridgelines), a passive-only solution may work. For gable roofs, you might be able to skip whirlybirds altogether.
Additional low-level venting as combination
Not as common, but you can mix eaves vents and above-fascia vents if needed – the calculator will flag options.
Generating the Ventilation Report – Print PDF
Once complete, you can generate a comprehensive PDF report. This report includes all required calculations and is ready to be included in working drawings or submitted to building surveyors.
No guesswork. No extra paperwork. Just clean, compliant outputs ready to go.
Conclusion
Roof ventilation isn’t something to wing. The Roof Metrics Ventilation Calculator makes it easier to get the numbers right and comply with regulations and requirements – without slowing down your workflow. Just enter your site details, check the vent options, and download a compliant report in minutes.
Whether you're dealing with box gutters, skillions, or gable roofs, this tool helps you build smarter and stay compliant – every time.
Legal Disclaimer
The Roof Metrics Ventilation Calculator is provided as a guide only and is intended to assist users in calculating ventilation requirements based on inputted roof metrics. While every effort has been made to ensure the information and results generated by the calculator are accurate and reliable, users are advised that:
The calculator is not a substitute for professional advice, including engineering, architectural, or regulatory compliance consultation.
All results and recommendations provided by the calculator should be independently verified by qualified professionals before implementation.
Users assume full responsibility for the use of the calculator and any decisions made based on its outputs.
We do not guarantee compliance with local, state, or national building codes or regulations. It is the user’s responsibility to ensure compliance with all applicable laws and standards.
We shall not be held liable for any errors, omissions, or inaccuracies in the calculator or for any damages arising from its use.
By using the Roof Metrics Ventilation Calculator, users acknowledge and accept this disclaimer and agree to use the tool at their own risk.