Information you need first :

• Site location

• Length of box gutter

• Catchment area of roof PLAN view (no pitch)

• Roof Pitch

• Catchment area of wall area, and back of paraphets

• Catchment area of top of paraphet capping, any other spreaders, or catchment running from other roofs

• Goal Width of box gutter (can be adjusted later)

Information you need first :

• Site location

• Length of box gutter

• Catchment area of roof PLAN view (no pitch)

• Roof Pitch

• Catchment area of wall area, and back of paraphets

• Catchment area of top of paraphet capping, any other spreaders, or catchment running from other roofs

• Goal Width of box gutter (can be adjusted later)

Information you need first :

• Site location

• Length of box gutters

• Catchment area of roof PLAN view (no pitch), both roof areas

• Roof Pitch

• Catchment area of wall area, and back of paraphets, both roof areas

• Catchment area of top of paraphet capping, any other spreaders, or catchment running from other roofs, both roof areas

• Goal Width of box gutter (can be adjusted later)

ROOF AREA 1

Roof area 2

Calculations

Information you need first :

• Site location

• Length of box gutters

• Catchment area of roof PLAN view (no pitch), both roof areas

• Roof Pitch

• Catchment area of wall area, and back of paraphets, both roof areas

• Catchment area of top of paraphet capping, any other spreaders, or catchment running from other roofs, both roof areas

• Goal Width of box gutter (can be adjusted later)

ROOF AREA 1

ROOF AREA 2

Calculations

Box Gutter Calculator

Sizing calculators for all box gutter situations per AS3500.3 

How-to Guide for the Roof Metrics Box Gutter Calculator

Step-by-step instructions for accurate box gutter calculations

Overview

Getting your box gutter sizing right is key to ensuring you meet the legal requirements of AS3500.3 and avoid damage down the track. This guide explains how to use the Roof Metrics Box Gutter Calculator - what info to enter, what to consider, and how to work out catchment areas and gutter dimensions for your job. 

What you’ll need before you start

Before diving into the calculator, ensure you have the following info:

• Site location: Specify the state and city to determine rainfall intensity.

• Length of the box gutter: Measure the full length of the box gutter.

• Catchment area of roof (plan view): This should exclude any roof pitch.

• Roof pitch: This affects the catchment area and must be included in calculations.

• Catchment area of wall and back of parapets: Include vertical areas contributing to drainage.

• Catchment area of top parapet capping: Calculate square metres for any parapet caps.

• Additional spreaders or catchment from other roofs: Factor in catchment from higher-level roofs or spreaders.

• Width of the box gutter: This parameter can be adjusted based on design needs.

Rain head or sump type

Start by selecting the rain head or sump you're using. This helps work out the calculations and design constraints required. 

Options include:

• Box gutter to rain head 

• End sump with side overflow

• End sump with high-capacity overflow

• Central sump with side overflow

• Central sump with high-capacity overflow

Rainfall intensity

Select the state and city where the job is located.

The calculator pulls rainfall data directly from the AS 3500 (2025) standards. This number helps determine how much water your box gutter needs to handle during heavy rain and is used to calculate flow rates, gutter depths, and outlet sizing.

Catchment areas

Our calculators work off 100% rainfall catchment and 50% for wall catchment. Unlike the standards that have adopted a European method for partially protected roofs, this gives a slightly higher allowance than the AS 3500 method. We prefer this approach because extreme weather can come from any direction – it’s a more practical way to size gutters in real Aussie conditions.

Roof catchment plan view

Start with the roof's catchment area in plan view – just the flat outline, without adding pitch. This gives you the base area for your calculations.

Catchment area with roof pitch

Add the roof pitch factor to the plan view catchment area. This is calculated using the length and width of the box gutter, plus the "Slope factor" for roof pitch. The calculator will adjust these values automatically once you enter your numbers. 

Wall and vertical parapet catchment

If there’s a second story or raised walls next to the catchment, include those vertical surfaces in the calculations. The calculator automatically adds 50% of these areas to the total combined catchment area, as per AS 3500.

Additional catchment from cappings or spreaders

Include catchment areas from wide parapet cappings or spreaders from roofs above, such as a second story. These add extra square metres to your total catchment area.

Total combined catchment area

Once all the catchment areas are entered, the calculator adds them up to give you the total combined catchment area. This is the basis for determining your box gutter sizing and design flow.

Width of the box gutter

Choose the box gutter width from the dropdown in the calculator. This selection will automatically adjust the depth of the box gutter according to AS 3500 standards. If you are in between widths, please round up.

Box gutter grade

A gradient of 1 in 100 is recommended for optimal drainage, while 1 in 200 is the minimum allowed.

Ventilation above the box gutter

Check if ventilation is needed above the upstand. This depends on your building’s design, climate zone, and specific building requirements. The calculator factors this in by allowing for a ventilation gap above the gutter material before the roof sheets, which can be filled with ventilation materials such as corvent.

Box Gutter Depth

The calculator gives you two depths – high end and low end.

The high end shows the minimum depth required at the start of the gutter, based on AS 3500. The low end is worked out using the gutter’s length and your selected fall (grade). This helps you plan the drop and confirm compliance.

Rain Heads, Sumps, and Overflow Heights

The size and height of your rain heads, sumps and overflow points are also based on AS 3500.

The calculator uses your catchment area, flow rate and gutter size to figure out what’s needed. These outputs match the rain head or sump type you chose at the start, so everything works together properly.

Final Output

Finally, once all calculations are completed, the resulting details are compiled into a professional-grade PDF. This document serves as a comprehensive representation of your drainage design, ready to be integrated into working drawings or submitted to building surveyors to meet Deemed-to-Satisfy (DTS) provisions. This streamlined process transforms complex calculations into actionable data, paving the way for effective implementation of your drainage solutions.

Once you’ve entered your info and the calculations are complete, all the details are compiled into a professional-grade PDF. Now you have a comprehensive representation of your drainage design, ready to be integrated into working drawings or submitted to building surveyors to meet Deemed-to-Satisfy (DTS) provisions.

It’s all done for you – no missed details, extra maths, or back and forth.

Conclusion

Box gutter sizing isn’t something you want to guess or half-arse. Using roof metrics sizing calculators for box gutters requires careful input of the data for each job, consideration of catchment areas, and adherence to design standards in the AS 3500. By following the steps outlined above, you can ensure that your calculations are precise and your drainage system is effective. Taking into account rain head, sump, and box gutter dimensions will help safeguard the structure against water-related issues while ensuring everything works as it should. 

Legal Disclaimer

The roof metrics box gutter calculators provided are intended solely for informational purposes and should not be interpreted as professional advice. These tools are based on data sourced directly from the AS 3500 standards of 2025 and are designed to assist users in their calculations. However, it is important to note that we are not hydraulic engineers, and the calculators do not replace professional hydraulic engineering evaluations. The user assumes full responsibility for the application and results of these calculators, and they are used entirely at their own risk. We expressly disclaim any liability for damages or inaccuracies arising from the use of these tools. Users are strongly encouraged to consult qualified professionals to validate their design calculations and ensure compliance with relevant regulations and standards.