Downpipe Compliance - general discussion

goody59 · Aug 12, 2015 8:39 pm

SaveH2O

goody59

So what I take from that is that my 7 downpipes to be compliant will only drain by design to be compliant 37 sq m each. A shortfall for me of 2.3 sq m per downpipe so multiply that by 7 is 16.1 sq metres short fall for my roof drainage.
Have I got that right SaveH20?

Yes...but the figures aren't exact because I didn't know the exact roof area...but the drainage will come up short because the figures will be close and the discrepancy is large.

The calculations factor the maximum roof area that can drain to each downpipe, it is not the average drained to each downpipe because downpipes invariably drain different size roof areas.

Your builder will (if willing) be able to tell you the roof space area and the roof catchment (factored) area.

If you return to fzxbeetle's opening post on this thread, you will see that he has posted his roof areas as given to him by his builder. Note that the builder (Henley) were wrong with their drainage calculations as per those figures because they basically didn't have a clue.

Cheers SaveH20, what this discussion does show besides the numerous builds with non compliant roof drainage is that it is a rather difficult task on behalf of the average punter to have any idea if their roof drainage is compliant. Even our so called experts are not getting it right by looks of things.

The VBA technical solution sheet 0.04 that I have linked below also discusses roof drainage compliance with an example. Note that the technical solution sheet is a re-vamp of a former Victorian Building Commission technical solution sheet but it is a poor attempt...some things like the roof slope factoring isn't properly addressed and they have even left a downpipe off the last house diagram on page 6! Good on 'em. I give it a 4/10.

http://www.vba.vic.gov.au/__data/assets ... mwater.pdf

SaveH2O · Aug 12, 2015 9:28 pm

tom_air

How do you work out how much of the roof area each downpipe is draining? Is it possible to work it out based on the plans?

The builder should supply the area of roof space when asked...note that this must also include the gutter area. Read the opening post to see the OP's reply from his builder as an example. To calculate the roof area drained to each downpipe, you also need to know where the gutter's high points are.

You also need to know the roof slope, the number of downpipes and their size plus the gutter specified for use and it's cross sectional area as defined by the regulations. Also ask whether the gutter will be slotted or unslotted.

I can only see 7 downpipes, the 7th is in the family room corner but hard to see because of the stamp.

Downpipes are usually shown as nominal positions on house plans and it is left up to a tradesman further down the food chain to design the system for best practice during installation...which rarely happens. This allows builders to build their display homes for aesthetics rather than function and this is why there are often no downpipes showing on a display home's street view. The plans invariably reflect the display home.

tom_air · Aug 12, 2015 10:14 pm

SaveH2O

tom_air

How do you work out how much of the roof area each downpipe is draining? Is it possible to work it out based on the plans?

The builder should supply the area of roof space when asked...note that this must also include the gutter area. Read the opening post to see the OP's reply from his builder as an example. To calculate the roof area drained to each downpipe, you also need to know where the gutter's high points are.

You also need to know the roof slope, the number of downpipes and their size plus the gutter specified for use and it's cross sectional area as defined by the regulations. Also ask whether the gutter will be slotted or unslotted.

I can only see 7 downpipes, the 7th is in the family room corner but hard to see because of the stamp.

Downpipes are usually shown as nominal positions on house plans and it is left up to a tradesman further down the food chain to design the system for best practice during installation...which rarely happens. This allows builders to build their display homes for aesthetics rather than function and this is why there are often no downpipes showing on a display home's street view. The plans invariably reflect the display home.

There are only 7 downpipes that is right.

I've asked the builder for information regarding the gutters and also the catchment area.

The roof pitch is 22.5 degress and it is a colourbond roof.

The downpipes have been installed and are 100 x 50.

I'm going to the house over the weekend so will take a look at where the downpipes ended up being installed.

SaveH2O · Aug 12, 2015 10:49 pm

An early ball park figure is that you will need a gutter with a 6,000 sq mm cross sectional area. The 100mm x 50mm downpipes will be compliant for this size.

You are better off having larger gutters than more downpipes although in the southern states, having a wide gutter sole does compromise the gutter's flushing ability. Half round gutters have the best flushing/self cleaning ability but aesthetically, they don't suit all houses.

EDIT
This thread deals with VICTORIAN ROOF/STORMWATER drainage compliance. If you are in another State, then please start another thread because other State's regulations are different as Victoria does not allow the inferior BCA regulations to be used as discussed in this thread many times. If you ask a question in this thread and you do not state that that you are not in Victoria, it can only be assumed that you are in Victoria and the advice given will reflect that.

SaveH2O · May 15, 2016 4:51 pm

I am bumping this thread due to the large number of Victorian builders who are still not using the Australian Standards for roof drainage solutions as mandated by the Victorian variation within the PCA under Part D1 that removes the BCA as a Deemed To Satisfy requirement for roof drainage solutions.

Thousands of non compliant homes are still being wrongly certified as compliant and handed over annually.

Crazyk · May 15, 2016 7:30 pm

It's a very sad state of affairs.

I know my builder B Homes contractors called up Section 5.4 of the AS which allows them to use their experience and local knowledge of the area to design the stormwater system instead of doing the calculations which would take them all of a couple of hours to do.

In my instance I called the builder on an undersize stormwater discharge system based on calculation and they just called up section 5.4.

Poor poor workmanship and care factor if you ask me.

SaveH2O · May 15, 2016 9:56 pm

Crazyk

It's a very sad state of affairs.

I know my builder B Homes contractors called up Section 5.4 of the AS which allows them to use their experience and local knowledge of the area to design the stormwater system instead of doing the calculations which would take them all of a couple of hours to do.

That would be section 5.5 NOMINAL METHOD which states:

"The 'nominal method' may be used for single dwellings in non-urban areas, and in single dwellings on urban allotments with less than 1000 m2 in area. Pipe design shall be determined according to local practice and experience (without specific design calculations), and according to the minimum diameter (Clause 7.3.4), cover (Clause 7.2.6), gradient (Clause 7.4.6) and other relevant criteria of this Standard."

Hydraulic calculations are required as per 5.4 and these would take no longer than 10 minutes by anyone competent in the art. Eaves gutter roof drainage is simply the roof area multiplied by the five minute 1:20 ARI average intensity then compared to the subsurface drainage pipes maximum flow rates when referencing the size and slope as per AS 2200.

5.4.11.2(c) states "Select the pipe material and the Colebrook-White roughness coefficient from AS 2200, or table 5.6 for normal conditions, and determine from Figures 5.1 the hydraulic capacity of the pipe drain for the selected DN."

5.4.11.2 (d) states "If the hydraulic capacity is less than the design flow, assume a new DN for the pipe drain and repeat Step (c) until the hydraulic capacity exceeds the design flow. To reduce the possibility of overflow from stormwater pits due to increased energy losses, the full-pipe velocity in the outlet pipe is recommended not to exceed 1.5 m/s and shall not exceed 2.0 m/s."

I can only conclude that the 5.5 NOMINAL METHOD was introduced to give a compensation to trade professionals who cannot do simple arithmetic and it hasn't worked.

AS 2200 itself also has fault due to the flow calculations applying to the pipe's DN (nominal diameter) and not the true internal diameter (ID). A glaring example is to compare the DN 90mm stormwater pipe's flow rate of 360 lpm with a 1:100 slope to a DN 100mm DWV pipe's flow rate of 480 lpm with a 1:100 slope.

If you calculate the internal volume of a 90mm and a 100mm pipe and then calculate the velocity per second for each pipe, you will see that they are near identical yet a flimsy SN2 90mm stormwater pipe has an ID of 86.2mm and if you use the stronger DN 90mm SN4 stormwater pipe, the ID reduces further to 85mm because the walls are thicker. The DN 100mm SN6 DWV pipe meanwhile has an ID of 104mm. If you then recalculate the pipe's internal volumes per metre and use the same velocity per metre, the lpm is much different to what AS 2200 states for the DN 90mm and 100mm pipe sizes.

Crazyk · May 15, 2016 9:59 pm

Yes it is 5.5 then. I couldn't find the old email however what you state is correct.

Carlopogi · May 08, 2024 4:27 pm

Does anybody know what is the minimum Slope for the Downpipe? Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

SaveH2O · May 08, 2024 7:49 pm

I have never seen one stated but the National Construction Code (NCC) defines "drainage" to mean "any sanitary drainage, liquid trade waste drainage or stormwater drainage system."

Now, a "stormwater drainage system" would reasonably (but wrongly) be expected to include downpipes in with the subsurface drainage pipes but the NCC and the overall hierarchy system have contradictions and errors from the Building Act down as to what constitutes stormwater and rainwater. Leaving this aside and if we, for convenience, group downpipes in with stormwater pipes as both being part of a stormwater system, then the minimum 1:100 slope for drainage pipes (except for the very large ones) should/would also (arguably) apply to any sloped downpipe as can be found in opposing sections of applicable regulatory hierarchy.

As an aside, stormwater elbows are 90 degrees but the equivalent Drain Waste Vent (DWV) elbows (called bends) are 88 degrees!