BOX GUTTER DRAINING TO RAIN WATER HEAD (Simplified)

at the end of the box gutter.

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Note:This Program is free to use, and is based on the Australian Plumbing Code AS/NZS 3500.3:2021.
However it can be used for any Plumbing Code. Find out how
This is a simplified version of the other box gutter calculators.
There is no allowance for vertical faces or rain shadow effects.
However the user can allow for vertical faces by adding half the
vertical area to the catchment area.
By not allowing for rain shadow effects, rain is assumed to be coming
from all directions at once. This may result in a slightly larger box gutter.
However this effect is insignificant for flattish roofs, or small vertical faces.
Legend: Boxes shown like this are results. No input needed.

Enter Details

Catchment Area (sqm)

Box Gutter Slope (1 in ?)

Roof Slope (degrees)

Trial box gutter width (mm)

Rainfall: Either choose a Location (Important)

or enter intensity(mm/hr)

How to find the Intensity for other places.

Unit Conversions.

Note! Box gutters are notorious for causing mega damage.
The problem is usually with the design, construction, or lack of maintenance. You do not want to be first cab off the rank if something goes wrong.
Read about overflow provisions for some extra design protection.
Also it doesn't hurt to get a copy of the calculations with all the necessary plumbing Code references for your records.
View A Typical Design
Read about the calculation PDF file.

Activate now

By the way, this note does not appear after activation.

Box Gutter Sizes

Min Gutter Depth with slope (mm)

Runoff (Litres/sec)

Min Gutter Depth no slope (mm)

Suggested Down Pipe dia (mm) (Whats this?)

Rain Water Head Sizes

Enter a Trial DP size nearest to the suggested dia above. (Available sizes are 75,90,100,125,150,225,300)

Depth of RWH (mm)

Length of RWH (mm)

Width of RWH (mm)

Extra Features click here to activate

Try a Rectangular Down Pipe

Depth RWH

Length RWH

Width RWH

DP Side1(mm)

DP Side2 (mm)

Get a copy of the calculations to AS3500 for your records.
Shows calculation steps and Code references. It allows insertion of Job numbers, Descriptions, etc. and is designed for printing out on letterhead. Includes all 'Extra features' plus calculates for flows greater than 16L/s. CLICK HERE.

View a typical template. Flow < 16 L/s
View a typical template. Flow > 16 L/s

Don't like the idea of a circular down pipe? Then experiment with any size of rectangular down pipe.

CLICK HERE to activate this extra feature. This will also activate the printable version, and enables flows greater than 16L/s to be calculated.

Important Note
regarding flows
greater than 16 L/s

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Disclaimer: It is the users responsibility to unsure that the program is suitable for their roof configuration,
and that data is entered correctly, according to accepted practice and the attached instructions.