Browse Forums Building Standards; Getting It Right! 1 Sep 22, 2016 11:53 am Hi everyone, We have received a quote to replace the gutters at our place (as wella s replacement of metal roof to extension at the back) and had chosen someone for the work. As a last minute check I asked the roofer about whether he would be undertaking the calculations to make sure the replacement gutters and downpipes are the correct size. His response was that all you need is 1 downpipe per 90m2. When I asked what size he was proposing the size he proposed for the gutter is 125mm high front quad. I decided to delve a little deeper and found that AS3500 actually goes into alot more detail than this and that in fact we need a much larger gutter and downpipe to comply with this standard. We have a 70m2 catchment on each side of the house (rectangular roof, gable ends) with a 30 degree slope going into 1 downpipe. The roofer thinks he is designing to HB 35 which is correct. Its all become a lot more complicated than I expected. Can anyone recommend, should I be engaging an engineer to make sure we get the right size gutters and downpipes/connection details or are roofers generally ok to do this sort of thing. I also see there is a CSIRO method for shorter runs. There is a clear discrepancy between the roofers recommendation and AS3500. Not sure which would be the correct way to proceed. Re: Correct Roof Drainage Design Issues AS3500 vs HB35 2Sep 22, 2016 3:57 pm For roof drainage regulatory information, you need to include the State and region you are building in if you want an informed answer. You also haven't included the proposed downpipe size or whether the proposed gutters are standard or slotted. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Correct Roof Drainage Design Issues AS3500 vs HB35 3Sep 23, 2016 10:38 am We are in Sydney NSW. I am proposing 200mm half round gutter and 150mm downpipe based on my calculations whereas roofers think 125mm quad and havent nominated downpipe size Re: Correct Roof Drainage Design Issues AS3500 vs HB35 4Sep 24, 2016 4:23 pm Hi jarunit. Thanks for the additional details. Unfortunately, I still don't know whether the gutters are standard or slotted but I can work around that to answer your queries. Eaves gutter roof drainage compliance references an area's rainfall intensity during a 1:20 Average Recurrence Interval (ARI), the gutter's cross sectional area (carrying capacity), the downpipe size, the roof slope (to factor wind driven rain) and any abutting walls (from an upper roof area) to determine the maximum compliant 'roof harvest area' that can be drained to each downpipe. The roof areas drained to each downpipe is determined by the gutter's high point (HP) and the HP also determines the gutter's slope. Because you are having replacement guttering and downpipes, the replacement must meet current compliance. For obvious reasons, you will want to retain the same number of downpipes as present to minimise costs due to the location of the current stormwater connections. The National Construction Code (NCC) comprises the Building Code of Australia (BCA) Parts 1 and 2 and the Plumbing Code of Australia (PCA) Part 3. The NCC can now be downloaded free of charge. The basic fact is that you are employing the "roofer" and if you want half round gutters, then that is what you should get provided they meet current compliance. Half round gutters are a superior flushing gutter. Roof drainage compliance only needs to meet the inferior BCA requirements as found in the BCA 3.5.2.2 tables a, b and c. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ AS/NZS 3500.3 and AS/NZS 3500.5 are Deemed To Satisfy (DTS) provisions that are superior to the BCA regulations. In Victoria, the Victorian Building Authority (VBA) has wisely implemented a variation within the PCA under Part D1 to remove the BCA as a DTS requirement for roof drainage installations whereas all other States recognise the inferior BCA. The following is found under VICTORIA in Part 3, section D of the current NCC. SECTION D STORMWATER DRAINAGE SYSTEMS PART D1 ROOF DRAINAGE SYSTEMS Delete D1.2 and insert Vic D1.2 as follows: Vic D1.2 General requirements The design, construction, installation, replacement, repair, alteration and maintenance of a roof drainage system must be in accordance with AS/NZS 3500.3 or Section 5 of AS/NZS 3500.5 as appropriate. The following is found under NEW SOUTH WALES in Part 3, section D of the current NCC. SECTION D STORMWATER DRAINAGE SYSTEMS PART D1 ROOF DRAINAGE SYSTEMS In New South Wales roof drainage systems are regulated in: (a) the Environmental Planning and Assessment Act 1979 and the Environmental Planning and Assessment Regulation 2000, which references the Building Code of Australia; and (b) the Local Government Act 1993 and the Local Government (General) Regulation 2005. The ARI for different areas also varies between the BCA and the AS (the BCA is invariably lesser) but your 1:20 ARI as per AS 3500.3 is 200mm per hour. The ARI is based on an average rainfall intensity over a 5 minutes duration. 200mm/hr = an average rainfall of 3.333mm/min over the 5 minute duration. The gutter's cross sectional area is calculated from 10mm below the slots on a slotted gutter or 10mm below the lowest wall on a standard (unslotted gutter), consequently the unslotted gutter has a lesser cross sectional area (carrying capacity) than a standard gutter. There are also new regulations now with regards to overflow provisions. If you study the BCA charts that I have pasted, you will see substantial difference between the inferior BCA requirements and the AS requirements. Just a mention at this point that compliance does not guarantee good design, both the BCA and AS requirements are poorly written and inadequate in parts and I can give glaring examples if you wish. jarunit I asked the roofer about whether he would be undertaking the calculations to make sure the replacement gutters and downpipes are the correct size. His response was that all you need is 1 downpipe per 90m2. jarunit When I asked what size he was proposing the size he proposed for the gutter is 125mm high front quad. Did you check to see whether this guy is licensed to do this work? jarunit We have a 70m2 catchment on each side of the house (rectangular roof, gable ends) with a 30 degree slope going into 1 downpipe. I made mention earlier of the roof slope. The roof also includes the area to the gutter's perimeter and the roof area is generally referred to as: 1. The roof space or the plan area. This is the area as seen 'from above'. 2. The roof harvest area. This area includes a roof slope multiplier that factors wind driven rain during a storm event. If your 70 sq m roof is the roof plan area, then your 30 degree slope multiplier is 1.29 which gives a roof harvest (catchment) area of 90.3 sq m. There are formulas used for larger roof areas and you will see that the BCA charts pasted only show up to 70 sq m of ROOF CATCHMENT AREA per downpipe. Let us assume here for the sake of this example that your stated 70 sq m roof draining to each downpipe is the roof catchment area. If you go to the BCA Table a and the 200mm/hr ARI in the left hand column, you will see that 70 sq m of roof catchment area requires a F gutter (also referred to in Table B) but a lesser 175mm ARI would still require an E gutter (150mm quad). If you look at Table B, a 125 quad gutter is shown as D BUT the applicable 6,300 mm2 minimum cross sectional area is only a standard gutter as the BCA does not mention slotted gutters, one of the reasons why undersized gutters are often specified by those un-knowledgeable in the art. Earlier I mentioned that compliance did not guarantee good design. If you have straight gutter runs, you are very fortunate and if the single downpipes are near the middle rather than at the end of the gutters, then you are even more fortunate. If you have round downpipes, then you are fortunate again. A 90mm round downpipe use to be rated at 3.5 litres per second but many manufacturers now quote 4.2 litres per second. If you have a 70 sq m roof plan area, then one of the 90 sq m roof harvest areas during a 1:20 ARI with wind driven rain could be collecting 90 (sq m) X 3.33 (lpm) = approximately 300 litres per minute. Stating a downpipe's generic flow rate is nonsense anyway because the flow rate depends on the height of the water above the crest of the overflow and that of course depends on the gutter's depth and whether or not it is slotted. Downpipe diameters are also nominal, for example, the common 90mm uPVC downpipe is actually 86.2mm internal diameter which also makes them non compliant when a 90mm minimum internal diameter is required by the regulation but the downpipe pops are smaller anyway and of course the water flows down the smaller pop first...but I digress. jarunit The roofer thinks he is designing to HB 35 which is correct. I think you mean HB39 which is produced by Standards Australia, a private company. Hand books (Guides) are a personal hate of mine because they cause confusion and encourage laziness and poor practice due to them soon being outdated in part. The current regulations should always be consulted. A classic example is the often seen referrals on this forum to the Standards and Tolerances Guidelines, it is impossible for such a 'Guide" to include, for example, certified 'Performance Solutions' now commonly used in increasing numbers by builders. SAA HB 39 1997 was replaced by SA HB 39 2015 on July 15 2015 but many industry 'professionals' still use the older outdated and erroneous SAA HB 1997. Several years ago, the use and legal standing of SAA HB 39 1997 was raised in the NSW Parliament. This guide was widely used in the industry and even referenced at tribunals, in reports and on various (including Government) websites. The following transcript are the answers given by the minister in regards to the Guides legal standing and the text should interest you. Note that the answer is not 100% correct (lol) but the reference to the legal standing in NSW is correct. Also note that the NSW Code of Practice no longer exists. 2361—Fair Trading—AUSTRALIAN STANDARDS AND STANDARD HANDBOOKS Ms Hale to the Minister for Primary Industries, Minister for Energy, Minister for Mineral Resources, Minister for State Development representing the Minister for Fair Trading, Minister for Citizenship, and Minister Assisting the Premier on the Arts— 1. Has Standards Association of Australia Handbook (SAA HB) 39 1997 ever been cited in New South Wales legislation as the standard for determining compliance with gutter overflow requirements? If so, has it now been superseded by AS⁄NZS 3500.3.1998 and AS⁄NZS 3500.5.2000? 2. Do the current Building Code of Australia and the NSW Plumbing Code of Practice require compliance with the Australian Standards⁄New Zealand Standard (AS⁄NZS) 3500.3.2003? 3. Does the Office of Fair Trading require that its officials not refer to old standards such as SAA HB 39 1997 when those standards have been superseded by more recent standards? 4. Is the Minister or Commissioner for Fair Trading aware of SAA HB 39 1997 being referred to in any matter before the Consumer Trader and Tenancy Tribunal even after that standard was rendered obsolete? 5. Did Mr Norm Foster, the Department of Fair Trading's Acting Manager of Building Disputes, on a site inspection of a property in Cary Street, Leichhardt, referred to SAA HB 39 1997 and suggested that overflow measures consistent with that standard would be adequate? Answer... 1. SAA HB39 1997 has never been cited in NSW legislation to determine compliance with gutter overflow provisions. 2. The handbook has no legal standing. The Handbook draws together in one convenient document a number of Australian Standards that relate to the construction of a complete metal roof and⁄or wall system. It was introduced as a quick reference guide and compliments AS3500 and other relevant standards. 3. Yes. In NSW all plumbing work must comply with the NSW Code of Practice. The code stipulates that work must be in accordance with AS⁄NZS3500 unless there is a specific NSW variation. In so far as gutter installations are concerned there are no variations and as such the work must comply with AS⁄NZS3500.3 or part 5. Installing a gutter in accordance with the standard will meet the performance requirement of the Building Code of Australia. 4. I am advised that Standards Australia withdrew the publication of SAA HB 39 1997 in 2004 but is still available to be purchased. Fair Trading officers will refer to the applicable standard that was in force at the time the work was carried out. 5. The Office of Fair Trading was not aware of SAA HB39 1997 being referred to in any Consumer, Trader and Tenancy Tribunal matter. However, I understand as a result of this question Fair Trading notes that SAA HB39 1997 was referenced in the matter of O'Brien v C J M Rood Services Pty Ltd [2003] NSWCTTT 307 (24 March 2003). I am advised that in this matter the Handbook was incorrectly referred to as an Australian Standard when this is not the case. 6. Mr Foster referred to figure 5.6.2 from SAA HB39 1997 as a consideration of an overflow measure. He also referred to AS3500 and the Building Code of Australia as a means to address overflow provisions at the particular house in question. Question asked on 25 November 2008 (session 54-1) and printed in Questions & Answers Paper No. 78. Answer received on 17 December 2008 and printed in Questions & Answers Paper No. 84. EDITED 16/03/2019. I found that the BCA charts were missing. Reposted. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Correct Roof Drainage Design Issues AS3500 vs HB35 5Sep 26, 2016 10:05 pm What an absolutely legendary response. Apologies for my late response as I have only seen this tonight on my way home from work. Thank you sincerely for taking the time to respond so thoroughly as it has clarified alot! I thought it was very strange that the roofers seemed to have no idea about AS3500. And yes these guys were all licensed roofers with 10-15 years experience, had no knowledge of it. You are correct, the roof is ~70m2 on plan with 30 degree therefore the roof harvest area is ~90m2. The other issue we have is that the neighbour has some overhanging trees and it appears that leaf litter will be a bit of a nuisance on one side of the house. My preference based on research is to avoid the leaf guard because of stories of how difficult it is to get underneath when small matter builds up under the guard, therefore my plan was to instead optimise the flow rate with half round gutters, so that any debris can be naturally regularly cleaned out of the gutters easily on their own. I was planning to employ the Tornado rainhead, which states it assists with flow between downpipe and gutter to ensure that blockages do not occur. Do you think this sound like a sensible plan to you H20? http://www.tornadorainhead.com/products.html I do have straight gutter runs, being a rectangular gable roof. For what it's worth I have marked up as below. The yellow dots are the current downpipe locations, the blue lines are the gutter runs. http://imgur.com/a/09hG6 I also take your point about the straight runs. I actually ran the calculator (from http://www.roof-gutter-design.com.au/Downp/csiro2.php) using the CSIRO method of determining downpipe and gutter sizing and for these parameters, and I got that gutter area for rounded sole would be 11,299sq.mm or 170mm half round gutter + 150mm diameter downpipe or 125x100mm diameter downpipe. So the outcome from this is that pretty large gutter/downipes are required. Based on this I was asking the roofers to price based on unslotted 200mm gutter + 150mm downpipe but these sound like pretty huge sizes to me and the roofers said this is more commercial sizing, they suggested it will look a bit out of place.. However the main thing for me is to be sure that we have a good system in place rather than solely aesthetics. I am tempted to consider dropping to more residential sizes of 150mm half round + 100mm downpipe, however I am concerned that this will be at the detriment to the system since it is both lower than both the AS3500 method and the CSIRO method of sizing. So the big question is, how conservative are the figures from AS3500/CSIRO method really? Given that I am running with a half rounded gutter, circular downpipes and also installing a tornado rainhead to improve water flow.. is it likely this could offset the drop in capacity in going from a huge 200mm gutter and 150mm downipe, to a 150mm gutter / 100mm downpipe scenario? (All circular) H20 I what would you do if it was your place? 200mm half round gutter + 150mm dia. DP or save ~$500 and get better aesthetics to go to 150mm half round + 100mm dia. downipe? Thanks sincerely Jaime Re: Correct Roof Drainage Design Issues AS3500 vs HB35 6Sep 27, 2016 3:59 am jarunit I thought it was very strange that the roofers seemed to have no idea about AS3500. And yes these guys were all licensed roofers with 10-15 years experience, had no knowledge of it. Not being in NSW, I don't know when the NSW Code of Practice ceased but the text from the NSW Parliament that I pasted in my previous reply was from 2008, i.e. 8 years ago. If Answer 3 was correct, then the AS applied at the time but I cannot verify as to whether Answer 3 was correct. If the roofers have 10-15 years experience, then they should be aware of the AS and know it anyway as a matter of course but as per my previous post which referenced the NSW roof drainage regulations found in the current NCC Part 3, section D, the BCA is the primary document for roof drainage in NSW, not the AS. Note that this is contrary to what was stated in the NSW Parliament. jarunit The other issue we have is that the neighbour has some overhanging trees and it appears that leaf litter will be a bit of a nuisance on one side of the house. .....my plan was to instead optimise the flow rate with half round gutters, so that any debris can be naturally regularly cleaned out of the gutters easily on their own. Your region has regular rain, a good % of which is medium to heavy and ideal for flushing gutters. Half round gutters have superior flushing characteristics but check as to whether you can fit 100mm downpipes. The 100mm DWV pipe has an outside diameter of 110mm but you can get 100mm colorbond downpipes. jarunit I was planning to employ the Tornado rainhead, which states it assists with flow between downpipe and gutter to ensure that blockages do not occur. Do you think this sound like a sensible plan to you H20? You need to check whether you are able to fit one to a 1/2 round gutter. They look imcompatible but I could be wrong. Vertical downpipes flow by what is called weir flow and the length of the orifice's circumference and the height of the water above the overflow crest is what determines the discharge (flow) rate. Doubling a pipe's diameter increases a pipe's cross sectional area four fold, if you have a 200mm opening draining into a 100mm pipe, you effectively have a 200mm pipe. This requires a big flat gutter. jarunit So the big question is, how conservative are the figures from AS3500/CSIRO method really? You have a best case scenario. The AS (supposedly) factor a worst case design scenario with minimum slope (1:500) as is typical on most new houses (but the AS & BCA do not factor corners!!!). The BCA regulations are poor. jarunit H20 I what would you do if it was your place? How long have you lived there and what size are the current gutters and downpipes? Have you ever seen them overflow? Your largest roof catchment area is nearest the downpipe and the smallest roof catchment areas are at the end of the gutters...where most new houses have their downpipes. You have 7.3 metres between the downpipes and the end of the gutters. As a first step, I would check the drop (slope) along the gutters. Can you let me know? The roofers obviously care zilch for compliance but if the gutters have never had an overflow problem, I would take advantage of their willingness to do non compliant work and hang compliance in this instance and upgrade to 1/2 round gutters with a good slope if 100mm downpipes can be fitted. EDIT 16/03/2019. Gutter compliance now includes the requirement for overflow provisions to prevent water from overflowing into the wall cavity. Slotted gutters are a common choice to satisfy this provision but it must be remembered that a slotted gutter will have a smaller carrying capacity than the same unslotted gutter. A manufacturer's gutter's specifications will show the two different cross sectional areas and compliance is now calculated by the cross sectional area and not the gutter's shape and size. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Correct Roof Drainage Design Issues AS3500 vs HB35 7Sep 28, 2016 12:12 am Even the csiro method accounting for the straight runs seemed pretty conservative still requiring a 170mm half round gutter and 125mm downpipe. I still don't understand why the half round gutters wouldn't work with the tornado rain head? In that case do you think any rain head would be necessary or will the half round gutter and round downpipe get me out of trouble? I have another more well known roofer coming to price the job on Thursday so I want to ensure I give him the right information. Should I A) 150mm half round gutter + 100mm colorbond downpipe B) 200mm half round gutter + 150mm downpipes And worth adding any accessories such as any rainhead/leafguards in your opinion? Thanks again h20 I will have a crack at measuring the existing gutters slope. Although could the slope be adjusted when installing the new gutters? I don't think there have been any past problems with overflow as far as I know. We just recently purchased the house it's an older place built in the 60s. The gutters are full of leaf and debris but we haven't cleaned them yet because we were intending to replace them instead, but now we have been held up by this issue of not being able to determine the correct gutter and dp configuration Cheers Re: Correct Roof Drainage Design Issues AS3500 vs HB35 8Sep 28, 2016 3:03 am jarunit Even the csiro method accounting for the straight runs seemed pretty conservative still requiring a 170mm half round gutter and 125mm downpipe. Yes...but it needs to be. You have two large roof catchments served by optimally located single downpipes but you also live in a high (200mm/hr) 1:20 ARI region plus you have a 30 degree roof slope! The house is also about 50 years old but I assume that had there been an overflow problem up to the present day, then it would already have been addressed. Roof drainage calculations, being generic, cannot take all influences into consideration for practical reasons but your situation is that if you want compliance even with the lax BCA, you either have to have an industrial size gutter and downpipe or else have a regular size gutter but two downpipes draining it at two new locations. In your situation, I would just do what will work by being clever and save some money as a bonus. jarunit I still don't understand why the half round gutters wouldn't work with the tornado rain head? I could be wrong but I can't see how they could fit. I don't think that you would need them anyway. Of interest, AS/NZS 3500.1 Section 8 Figure 8.3 shows the weir flow rate down a vertical pipe that is inverted in a tank of water. A 125mm pipe with a 20mm, 30mm and 40mm depth of water above the crest will flow at 2.21, 4.07 and 6.26 litres per second respectively. It must also be remembered that a downpipe receives flowing water. jarunit I have another more well known roofer coming to price the job on Thursday so I want to ensure I give him the right information. Should I A) 150mm half round gutter + 100mm colorbond downpipe B) 200mm half round gutter + 150mm downpipes I would go for A) but insist on a good slope. Just tell him that you want to upgrade to the half round and ask whether they can be fitted with a 100mm downpipe. The issue will be his adherence to compliance and whether he will insist on only installing a solution that is compliant as is his requirement. Also make sure that he knows the new regulations for overflow provision. What are your current gutter and downpipe sizes? jarunit And worth adding any accessories such as any rainhead/leafguards in your opinion? Flushing a mass of organic matter into the stormwater should be avoided if at all possible but I cannot recommend any of the current leaf diverters. I was working on one until a year ago and I will get back to it once my current work backlog is under control but for now it is on the back burner. I particularly don't recommend the bucket catchment type. jarunit I will have a crack at measuring the existing gutters slope. Although could the slope be adjusted when installing the new gutters? The roofer will most likely check your current gutter height on the fascia for compliance, it is called a h/f formula. This will give him an indication. A new type of gutter will have different brackets. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Correct Roof Drainage Design Issues AS3500 vs HB35 9Sep 28, 2016 7:41 am Thanks H20. The existing has 125mm quad gutters with 100x50 downpipes. I think I am set on the 150mm half round with 100mm colorbond downpipes. Just to be clear, in case it does fit, have you come across this "Tornado" rain head, it is not just a standard rain head but it claims to improve flow by 5 times due to vortex action it creates. Given that we are going with the 150mm gutter/100mm downpipe, it seems any additional flow we can get would be worth it. Do you think these claims have any merit, and would it be worth paying the extra to install these? Link to the product is below: http://www.tornadorainhead.com/products.html Alternatively is there anything which could improve flow at the connection between the 150mm gutter and the 100mm downpipe? Or would the standard 100mm DP fixed straight into the 150mm gutter generally be sufficient? Re: Correct Roof Drainage Design Issues AS3500 vs HB35 10Sep 29, 2016 12:38 am jarunit The existing has 125mm quad gutters with 100x50 downpipes. Replacing like with like is the easy option but I would be 'surprised' if the gutters never overflowed during heavy rain. 100mm x 50mm downpipes are inadequate for your roof drainage requirement. jarunit I think I am set on the 150mm half round with 100mm colorbond downpipes. They will be a substantial step up with low/no maintenance benefits, a good choice. jarunit .....have you come across this "Tornado" rain head, it is not just a standard rain head but it claims to improve flow by 5 times due to vortex action it creates. Water flowing into a pipe by what is known as weir flow naturally induces an annular vortex. This is why water spirals down a round pipe and why standard downpipes never have less than 3/4 air unless blocked. Weir flow is a very inefficient drainage method because of the vortex it generates. If a large orifice drains into a smaller pipe, the smaller pipe will be flooded during heavy rain and consequently drains faster by what is referred to as bubble flow, a process with a higher % of water volume and the third of the four priming stages that purge syphonic drainage pipes of air. It is the initial larger surface area's capacity to drain more water to the smaller pipe under turbulent conditions that generates the increased flow, not the vortex. jarunit Alternatively is there anything which could improve flow at the connection between the 150mm gutter and the 100mm downpipe? Or would the standard 100mm DP fixed straight into the 150mm gutter generally be sufficient? A 100mm downpipe is more than capable of draining your gutters as per the flow rates at different heights above the crest of the overflow that I previously posted. The half round gutter is a reasonably tall gutter and the 150mm version will have plenty of reserve carrying capacity. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Correct Roof Drainage Design Issues AS3500 vs HB35 11Sep 29, 2016 9:21 am Thank you H20, you have cleared up alot of confusion and saved me and my family alot of heart ache. If I understand correctly regarding your comments on weir flow, you do not think it is worth spending the extra for the Tornado rain head? Should we just run with standard connection detail between the half round gutter and the downpipe ? Re: Correct Roof Drainage Design Issues AS3500 vs HB35 12Sep 29, 2016 5:17 pm jarunit If I understand correctly regarding your comments on weir flow, you do not think it is worth spending the extra for the Tornado rain head? Should we just run with standard connection detail between the half round gutter and the downpipe ? The Tornado rain head is manufactured in Australia by an Australian company, I also manufacture in Australia and I support other Australian companies who also manufacturer quality products in Australia. When I offer advice on forums, I don't recommend against a product that I might think is not needed for a particular situation in case a recommendation is interpreted as a criticism of the product itself, what I try to do is give information that will allow the reader to understand the dynamics so they can make an informed decision. I don't think that the Tornado rain head can be fitted to a half round gutter but you are best to contact the manufacturer and ask them for that information. I have checked and confirmed that half round gutters can be fitted with 100mm downpipes. The flow characteristics of a half round gutter combined with a 100mm downpipe will give superb flushing and a more than adequate drainage capacity for your straight gutters. It must be remembered that the 100mm downpipe's pop/outlet services the half round gutter's entire sole and extends up the walls. The half round gutter flows with a greater velocity in comparison to other gutters. The greater velocity and the outlet/pop's characteristics also increases the downpipe's drainage capacity due to the generated turbulence entraining air at the top of the downpipe. The Tornado rain head's larger surface interface increases the initial drainage capacity but it also significantly increases the smaller downpipe's flow capacity due to the turbulence also entraining air which induces bubble flow as previously explained. The vortex does not cause the gutter to drain faster, the gutter drains by a combination of design, slope and the size of the gutter's outlet/pop/rainhead. Reducing the vortex significantly increases the downpipe's flow capacity by allowing the downpipe to fill with more water which also flows with greater velocity due to minor head pressure being generated during bubble flow. You will see this in the Tornado rain head video. The 100mm downpipe for your gutter will also be subjected to some turbulence during heavy rain but it won't need this 'assistance' to cope. The downpipe outlet/pop should be fitted to the gutter's underside so as to eliminate the ridge that is made when the pop is fitted inside the gutter. Decaying organic matter is negatively charged and this will rust metal gutters, it is best to eliminate any possibility of sediments accumulating. Rain Harvesting Pty Lty is an Australian company that makes a 100mm uPVC outlet/pop for half round gutters and it is widely available. The part number is GSG006. A little off topic but for some other applications, the vortex can be eliminated by fitting an anti vortex baffle above the pipe to block the vortex once the water level rises above the baffle. Once the vortex is eliminated and the pipe purged of air, the water will flow vertically (not spiral) under head pressure at high velocities that generate sub atmospheric pressures. The negative pressures are greatest at the top of the pipe and the negative pressure literally sucks water into the pipe. Negative pressures can however cause pipes to buckle asymmetrically and so fit for purpose pipes must be used. The method is called syphonic (also siphonic) drainage and it is used on virtually all large roofs world wide. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Need more photos from around the house including your gutters. 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