Browse Forums Building A New House 1 Feb 17, 2023 1:03 pm Bit of context - KDRB currently underway and had a engineered stormwater plan which showed no OSD requirement, large number of downpipes as shown and the following contained within the tender: Provide stormwater drainage lines and pits as per details provided by XXXXX Engineering Hydraulic detail plan (see attached) including the following:- * 100mm diameter PVC downpipes in lieu of 90mm * Provide Quad Hi Front gutter to home (Non slotted) in lieu of standard. Now that construction is underway I've been informed that they couldn't fit the 100mm pops in the base of the specified gutter so they have installed 90mm pops and subsequent 90mm downpipes, suggesting they checked with the engineer that it was fine to go to 90mm given there is no OSD requirement. I questioned why 100mm was specified in the first place and they suggested it was just a default for OSD on larger roofs but they never updated it. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Note: 30deg slope of roof, so there are faster velocities to consider They've also installed the classic style bends ash shown here (does this impact friction losses?) Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ My concern here is the consequences of overflowing gutters due to velocities and volumes, given that they may be telling furphies about discussing it with the engineer (I'm prepared to ask their verification of the 90mm design) as they didn't consider the installation when they specified the gutter to meet the engineers specifications re: gutter pop sizing. saveH2O - Some of your knowledgeable advice if I should pursue this or not (if possible) would be fantastic. Re: Downpipe size 2Feb 17, 2023 9:21 pm I probably have more questions than answers! Do you know the chosen gutter's cross sectional area? This is a critical compliance check. What emergency gutter overflow provision are you having? Downpipe and gutter sizes for different regions and their maximum roof areas drained vary with each region's 1:20 Average Recurrence Interval (ARI) rain intensity. Your 1:20 ARI is 212 mm/hr which is based on an average rain intensity over a 5 minute duration and so your 212 mm/hr equates to an average intensity of 3.53 mm/minute over a 5 minute duration. At this point I will explain that this is the minimum qualifying intensity to be considered a 1:20 ARI. Let's say your 1:50 year ARI is 4.5 mm/m, you could be experiencing a rain intensity of 4.4 mm/m and have gutters overflowing yet you will still be in a 1:20 ARI. The greater the rain intensity, the larger the downpipes and gutters need to be but overflows usually do not happen at the downpipe despite the downpipe being fitted at the gutter's low point. If you look at the plan's bottom RH corner, there is a 3 line table that shows the gutter getting smaller as the roof area drained and the downpipe size gets larger! The smaller gutter area of 2467 sq mm, well......that is a tiny, tiny gutter. By comparison, here in Melbourne where our 1:20 ARI is 130 mm/hr (2.166 mm/m), the most commom gutter fitted is the slotted 115mm quad with a cross sectional area that is usually about 5,250 sq mm. The plan also uses the term "roof catchment area" which we consider to be the roof plan area x the roof slope multiplier to allow for wind driven rain. The multiplier used for a 30 degree roof slope as per AS/NZS 3500.3 is 1.29. AS/NZS 3500.3 is referenced by the NCC for roof drainage. The plan's roof catchment areas total 323 sq mm which factors back to a roof plan area just a tad more than 250 sq m. Do you know approximately what your roof plan area is? Is it approximately 250 sq m? A confusing thing is that if I use the pool's wetted area as being 3.0 m x 6.5 m as shown on the plan, I arrive at a roof plan area of about 400 sq m! Another area of confusion in the plan is with the lower RH table stating that a 90 mm downpipe can only service a maximum roof area of 27 sq m. This is clearly wrong as NCC Part 2 3.5.3 states that a 90mm downpipe can service an 'E' gutter (9,000 sq mm cross sectional area) servicing a roof area of 60 sq m with a 1:20 ARI of up to 255 mm/hr. By comparison, AS/NZS would allow a roof catchment area of about 47-48 sq m. They also don't define the 27 sq m as being either the roof plan or the roof harvest area but it should refer to the roof harvest area. If so, then 10 downpipes x 27 sq m = 270 sq m yet their roof harvest areas total 323 sq m but as already pointed out, the 27 sq m figure is wrong. Roof drainage regulations are found in the 3 Part National Construction Code (NCC) and AS/NZS 3500.3 is referenced as a Deemed To Satisfy Solution (DTS). AS/NZS 3500.3 is stricter than the NCC and Victoria has never legislated to use the laxer NCC but other States use the NCC as the primary document. As such, your applicable regulations only need to satisfy the NCC. According to the NCC Part 2 3.5.3, a 50 sq m roof catchment area can have either a medium rectangular gutter with a 6,500 sq mm cross sectional area or a 125mm D gutter with a cross sectional area of 6,300 sq mm. Both of these gutters can have a 90mm round downpipe. I used a 50 sq m roof catchment area in the above because one of your roof areas (middle bottom) shows 43 sq m. As well as showing the downpipe positions, the red lines also show the gutter's high points (their slope apex). Downpipe size has nothing to do with an OSD requirement but pvc pipes are used for downpipes when the system is charged. The "classic style bends" on the selected gutter will not negatively impact on friction loss. Gutters that narrow towards the bottom have less resistance and better flushing characteristics. I think that you are confusing flow rate with velocity. The flow rate shown is the amount of water falling directly downwards (no wind) on the roof during a minimum qualifying 1:20 ARI rain intensity. Gutters flow at different rates along their length because rain hasn't yet learnt how to fall in a concentrated burst just short of each of the gutter's high points! The downpipes are also well positioned but don't forget my first question. 3in1 Supadiverta. Rainwater Harvesting Best Practice using siphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost, siphonic, eaves gutter overflow solution. Re: Downpipe size 4Feb 18, 2023 7:23 pm Your floor plan is 4 sq m larger than the total of the plan's stated "roof catchment areas", a most difficult task as the latter must also include the gutters but seriously, it shows that the plan's stated roof areas are the roof plan areas, not the 30 degree slope factored roof catchment areas needed to calculate the gutter and downpipe sizes. With the 1.29 factoring, the 43 sq m roof section becomes 43 x 1.29 = 55.5 sq m which means you have to look at compliance for a 60 sq m roof catchment. You can download the NCC Part 2 free of charge or you can see the tables in the link below. They are 2016 NCC tables but still current. You take your figures from the 200 mm/h line and the gutters needed are shown as B or E which have cross sectional areas of 7,900 sq mm and 9,000 sq mm respectively. http://www.pantexroofing.com.au/docs/NC ... npipes.pdf Your current gutter with a 5,809 sq mm cross sectional area is not compliant if the gutter must be calculated to service 60 sq m (slope factored) roof catchment but the 90 mm downpipes are compliant and will easily handle the flows. LATE EDIT: Note that the roof drainage plan states that AS 3500.3 must be used for stormwater drainage. The Australian Standard Deemed To Satisfy (DTS) regulations are stricter than the NCC roof drainage regulations but a quick reference to the NCC regulations will confirm the inadequacies of the roof drainage plan without needing to do compliance calculations by referencing the AS 3500,3 manual which few homeowners will have access to. Also note that 80 sq m of roof is shown on the plan as not draining to the tank, hence my reference below to only 243 sq m of roof draining to the tank. I also have concerns about your water tank's overflow capacity given that it will be collecting water from 243 sq mm of roof plan area (8/10 downpipes and no wind). During a minimum qualifying 1:20 ARI, the tank will collect 243 x 3.53 = 858 lpm which has to discharge without overtopping when the tank is full. I recommend that you check what discharge capacity the tank's overflow is designed for as a standard overflow outlet will fall a long way short of your required capacity. The recent thread below may also interest you. viewtopic.php?p=1911554#p1911554 I recommend designing the overflow to manage at least a minimum qualifying 1:50 ARI. 3in1 Supadiverta. Rainwater Harvesting Best Practice using siphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost, siphonic, eaves gutter overflow solution. Re: Downpipe size 5Feb 20, 2023 8:00 am ![]() Your floor plan is 4 sq m larger than the total of the plan's stated "roof catchment areas", a most difficult task as the latter must also include the gutters but seriously, it shows that the plan's stated roof areas are the roof plan areas, not the 30 degree slope factored roof catchment areas needed to calculate the gutter and downpipe sizes. With the 1.29 factoring, the 43 sq m roof section becomes 43 x 1.29 = 55.5 sq m which means you have to look at compliance for a 60 sq m roof catchment. You can download the NCC Part 2 free of charge or you can see the tables in the link below. They are 2016 NCC tables but still current. You take your figures from the 200 mm/h line and the gutters needed are shown as B or E which have cross sectional areas of 7,900 sq mm and 9,000 sq mm respectively. http://www.pantexroofing.com.au/docs/NC ... npipes.pdf Your current gutter with a 5,809 sq mm cross sectional area is not compliant if the gutter must be calculated to service 60 sq m (slope factored) roof catchment but the 90 mm downpipes are compliant and will easily handle the flows. I also have concerns about your water tank's overflow capacity given that it will be collecting water from about 243 sq mm of roof plan area (8/10 downpipes and no wind). During a minimum qualifying 1:20 ARI, the tank will collect 243 x 3.53 = 858 lpm which has to discharge without overtopping when the tank is full. I recommend that you check what discharge capacity the tank's overflow is designed for as a standard overflow outlet will fall a long way short of your required capacity. The recent thread below may also interest you. viewtopic.php?p=1911554#p1911554 I recommend designing the overflow to manage at least a minimum qualifying 1:50 ARI. I agree and was one of my concerns re: gutter size. It may be a suck it and see item now as they have not fitted the appropriate size. with the tank overflow, I am considering two things post OC, one of which is removal of the tank, and the other is to fit a low-high overflow at say halfway volume (with reduced orifice and perhaps a NO valve) with a secondary high-high overflow This will reduce my capacity to store water although I can use the valve to change that and will also provide extra capacity at storm events. Building Standards; Getting It Right! It's all good. You can get both wye piece and the coupling with or without thread. The cost difference is negligible and means the plumber only has to carry the single… 1 6401 Thanks so much! So i really need to run this pipe into a soakwell? 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