Practitioner Education Series - Plumbing: Common Plumbing Enquiries

For more information, visit the Victorian Building Authority website at www.vba.vic.gov.au, email [e-mail] or phone [phone].

[@rohansprenger6902]

Some points:
The work by Dam Buster proves beyond any shadow of a doubt most of this waffle is totally unnecessary.
There is simply no good reason box gutter design cannot be standardised. That is - to be sized for the maximum ARI of any given state and a maximum area draining to it.
Quite simply a minimum depth of 200mm and width of 300mm at a fall of 1:100 nationally, and a maximum area (by state) draining to it. (a minimum fall of 1:200 should never be used - ponding occurs too frequently where dust/dirt is not removed - all reference to that fall should be removed from the standard). ONLY any gutter outside of that should require calculation/performance solution.
The whole 'no change of direction' thing stems from a misinterpretation of the much earlier "Box gutters must be laid straight" (as in not having ~dips and bumps, NOT that it may not change direction). It needs to be rectified. Yes, water does indeed 'bank up' at a change in direction, but not appreciably at velocities seen in box gutters having a fall of 1:100....
A slotted overflow being permissible for a sump but not for a rainhead (which since it is required to be sealed to the box gutter is identical in function to a sump) is ludicrous.
If - as in the earlier editions of the roofing code of practice - the rainhead was not sealed to the gutter (to provide for box gutter expansion), having an overflow open to atmosphere would make sense, but that is not the case we deal with today.
Apart from that, a roofer is not typically consulted during the design phase - they normally only see a job after it is framed - usually according to drawings done by someone who puts a little qualifier on the plan "*roof to be laid to AS3500" but does not allow for it themselves.... And then if the roofer objects, the builder simply finds another roofer who will stick it on and run away....
That's to say - no amount of 'practitioner education' will solve the underlying problem of poor design. To fix it you need to make the standard easier for the average draftsman to understand - consider adopting my suggestions above. Feel free to reply here if you need anything clarified.

[@rohansprenger6902]

*re. the box gutter bit. 😁

[@piety23]

Valid points right there