Eaves Gutter capacity changed

The new Code AS/NZS 3500.3:2015 is now out and about. When a code is updated, confusing things are sometimes clarified, a few grey areas are made less grey, and a few more requirements  are added etc.

Sometimes a code is (hopefully) expanded into areas where no man has been before, and Angles fear to tread.

However this time the capacity of eaves gutters flatter than 1:500 has been changed every so slightly. Probably just due to a more accurate drawing of fig 3.2.5(C). However it has a very interesting effect.

Previously for eaves gutters flatter than 1:500, a flow of 6 L/s required an eaves gutter area of 22,900 sq.mm. The new code requires 22,700 sq.mm. Admittedly this is only a minor change of 200 sq.mm. and for this size gutter equates to a reduction of only 1mm of depth. (Which is neither here nor there.) This change is proportional for all flows. So lets look at the effect of this.

The design of the downpipes depends on the size of the eaves gutter. For instance in Table 3.5.2 for an eaves gutter flatter than 1:500, a 90 dia DP is required for an eaves gutter of between 7,901 and 8,900 sq.mm. anything over this requires the next size on the table which is 100 dia.

However considering the table is rounded to the nearest 100 sq.mm we can assume that a gutter size of 8,951 (rounded up to 9,000) requires a 100mm downpipe. Whereas a gutter of 8,949 (rounded down to 8,900) requires a 90 mm downpipe. 

So a difference of only 2 sq.mm can change our Down pipe size.

So do we start pulling our hair out now, or later? (Too late for me. I pulled mine out long ago.)

Do our previous jobs still comply?

Thankfully - Yes they do. It means that we can now use a slightly smaller gutter than before, and as downpipes are related to gutter sizes, there may be cases where a small decrease in gutter area could result in a smaller downpipe as well.

In reality, the end result is, on certain jobs, my tests indicate that we could sometimes get away with one less downpipe.

So another panic narrowly averted. We can all go back to putting out the daily bush fires, and putting band-aids on everything else.

Check out the new formulas here

An eaves gutter steeper than 1:500 remains unchanged.

Ken Sutherland is a  Hydraulic/civil Engineer who spent too much time analyzing the Plumbing Code, associated formulas, and computer programming. He has now moved up the ladder to other things, and offers the benefit of this research and some of the resultant design programs,  for free on the internet, for the benefit of the Construction industry, Architects, Engineers, Hydraulic consultants, students, and every other man and his dog.