Roof Plumbing Plan

Hi everyone,
I have just been researching my roof plumbing plans for a double storey house, and would like some feedback to see whether I am on the right track. I've been following AS3500.3, but because this is new to me, I'm not sure if my decisions are correct. Could someone please help me out by doublechecking my plans, please?
OK, here it goes:

Location is Melbourne, Australia
Eave gutters (not box gutters)
Annual Recurrence Interval (ARI) = 20 years
5min duration for Melbourne = 127mm/hr
Roof pitch = 25degrees
Slope factor = 1.23
Upper storey roof plan area = 181.5m2
Lower storey roof plan area = 79.5m2
Gutter Area = 90m x 0.1 = 9m2
Upper Catchment Area = 181.5 x 1.23 = 223m2
Lower Catchment Area = 79.5 x 1.23 = 98m2

All gutters to be installed with minimum fall of 1:200 (2mm for every metre).
All upper storey downpipes flowing onto lower storey roof to drain directly into gutter, not onto roof (asphalt shingles).
Eaves gutters must be supported by brackets securely fixed at stop ends and at no more than 1.2m centres.

Gutter Specification:
QUAD Hi-front, unslotted
Cross Section Area - 5809mm2
Colour = Colourbond Monument

Maximum size of catchment is 40m2 (conservative)
Minimum number of downpipes = 223/40 = 5.6 downpipes = 6 downpipes (conservative)

Please see drawings for location of downpipes.

http://i65.photobucket.com/albums/h228/akapacker/UpperRoofPlumbing_zpscc1b997c.jpg

http://i65.photobucket.com/albums/h228/akapacker/LowerRoofPlumbing_zps52c62c5c.jpg

Downpipe Specification:
Rectangular, 100x50mm or Round 90mm (To be determined)
Colour = Matched to background

Valley Gutter Specification (ARI < 200mm/hr):
Sheet width = 355mm
Effective depth = 32mm
Effective width = 215mm

Overflow measures installed in accordance with Clause 3.5 of AS3500.3
= To Be Determined

Gutters installed to flow down from High Point (HP) towards downpipes.

Are there any standards around the design of the underground pipes to take the downpipe discharge to the Legal Point of Discharge?

Appreciate any comments, and any suggestions on things that I have missed.

Thanks

David

Hi David,

Just a couple of corrections. You have done very well. For what it's worth, non compliant roof and storm water drainage is endemic on new houses. Have a read of the page (and the links) linked below from one of my websites for an informed insight.

http://gutterpumper.com.au/gutters-overflow.html

The Melbourne 1:20 ARI was upgraded from 127 to 130 mm/hr a few years ago.

The roof overhang is generally reckoned as 50 mm. Deduct that from the gutter width to arrive at the necessary perimeter addition.

Your roof calculations are correct as is the multiplier for slope.

1 in 200 is not 2mm/m.

I don't know what you mean by "All upper story downpipes flowing onto lower story roof to drain directly into gutter, not onto roof".

A concentrated discharge directly into a gutter during heavy rain could cause problems. This is why you use spreaders. It is also best design practice to have a downpipe located at the lower roof area where the upper roof concentrated flow drains to. You have probably noticed that a lot of 2 story houses have a spreader discharging to one end of a gutter but the downpipe is often at the other end of the gutter and often around the corner. This is volume builder nonsense, avoid doing the same. Were you planning to discharge directly above and into a larger downpipe?

The maximum factored roof area with the gutter and downpipe described will be about 45 sq m.

You should have a HP at a valley but if the HP is close by, it doesn't really matter much.

223 sq m = 5 DPs but each roof area would be borderline and extremely difficult to manage. 6 as you have calculated is the go.

Re above, can you edit the diagrams and show the shaded (factored) roof areas in sq m thanks? I have something important to point out when you do. Also, is N to the top?

You need to consider where the greater roof harvest area is but of course 2nd story downpipe position is also influenced by windows and the lower roof if applicable etc. The worst place for a downpipe is at the end of a wall because it will be furthest away from the greater roof area on a gable roof but that is where most of the project builders have them. Aesthetics subjugating function!



Yes. Read the thread below. This is another area of common non compliance.

viewtopic.php?p=1111630#p1111630

Any walls abutting lower roof areas also have to be calculated for wind driven rain as part of the lower roof catchment area.

Half round gutters are the most efficient but they don't aesthetically suit all houses.

Don't forget the hf calculation to determine the maximum height of the gutter's back wall to the top of the fascia...another area of common non compliance.

90 mm round downpipes are more efficient than 100 mm x 50 mm rectangular. You will get storms of greater intensity than a 1:20 ARI. Look at what Geelong had last week and what Melbourne had 3 years ago. A well designed system will easily handle greater than a 1:20. Melbourne metro gets more localised rain now and some localised activity can be intense without warning.

Thanks SaveH2O, you are a champion!
Didn't know about that rule of thumb for gutter overhang of 50mm, so thanks!
1 in 500, it should say - typo on my behalf. Thanks for picking it up.
I found very little information on handling rainwater from a primary roof to a secondary roof. There's not much in the BCA or the standards. I was reading this US site:
http://www.nachi.org/forum/f16/downspou ... roof-4685/
and they recommended that rainwater from a primary to a secondary roof should be better controlled. Didn't know about the rainspreaders, but still not super-impressed by them. I wanted the downpipe to track across the roof, and directly into a gutter, but didn't know that that would be a problem. Thanks for the advice! Can you use a rainwater head instead, to alleviate the problems?
"The maximum factored roof area with the gutter described will be about 45 sq m."
Did you read this from the chart in AS3500.3 - Fig 3.5A? I went super conservative with my calculations, so that I don't run into problems, and came up with catchment area per downpipe of 40 sqm.

Just to test my understanding, you want the Downpipe (not a high point) close to a valley gutter, so that this high volume of water is cleared as quickly as possible, and so that it does not unnecessarily load the gutters (BCA 3.5.2.5.c). Actually, I'm really confused here - Re-reading your advice, you recommend having a High Point near the valley gutter. And so does AS3500.3:2003, I think. In Example calculation Appendix H, CASE 1 Step (i), it states "a high point is located at an outlet to a valley gutter". Is there a conflict here, or are my interpretations way off the mark?

If there is a good reason for positioning due to function, I'm happy to forgo the form. I placed the downpipes to try to get an even harvest area from the gutters each side of the downpipe. But after reading your excellent website, maybe they are not optimised for a hipped roof.

I have added the areas, and realised that I have made a mistake - the areas I used for the calculations were not the plan areas, but the sloped areas (used to calculate how much asphalt shingle material I will need to cover the roof). So, the upper roof plan area is about 165m2, not 181.5m2. The upper roof factored area is then about 210sqm, so, I still require 6 (210/40) downpipes.

Here are the roof plans with harvesting areas:

http://i65.photobucket.com/albums/h228/akapacker/UpperRoofPlumbingAreas_zpsb68e63c6.jpg

http://i65.photobucket.com/albums/h228/akapacker/LowerRoofPlumbingAreas_zpsd416042f.jpg

What I did not thoroughly work through, are the lower storey rooves, which the upper storey gutters drain into. You're right, the lower storey gutters also must consider the wall area above them, but I was too lazy to calculate this, since the upper storey eaves overlap the lower storey, and I thought the areas are so small, that I would not worry about them. But looking at the harvesting areas that they will be servicing, I am actually over the limit of 40sqm on many of the lower storey gutters. This is particularly concerning on the North and South boundaries, which have shortened eaves (on property boundary) and overflow will be closer to the brick veneer wall. Any advice on this, SaveH2O, other than to add in more downpipes on the lower storeys?

North is the top of the page.

Was reading about the half round gutters, but haven't seen them physically, so not sure if they will work aesthetically. However, I was considering using these along the lower storey North and South boundaries, where aesthetics do not matter. This might be another option to having more downpipes on the lower storeys.

Yep, thanks for reminding me about the hf calculations. I was leaving that till later with the blanket statement of Overflow measures to be determined. Any recommendations on continuous overflow measures? I was thinking about the 10mm gap between the gutter and the fascia, since I heard all the concerns about slotted gutters not being a compliant overflow measure and leaving ugly stains, not to mention the reduced Effective Cross-Section bearing capacity.

I'll go with the 90mm round downpipes. I chose the rectangular because I thought that it would be easier to fix to the wall, but if round is more efficient, then round it is. In hindsight, maybe round is easier to fix to the wall? Also, I intend on having a 10kL concrete underground rainwater tank under the drive, so I will have to convert rectangular to round pipes, anyway.

I'll read up about the design of the underground pipes to LPOD! Thanks for the link!

And thanks for all the advice!

Hmmm. I read this post:
viewtopic.php?p=1111630#p1111630
but, it might not have answered my question (which might have been too vague, as well!).
I want to specify the pipes that will convey the water from the down pipes to the legal point of discharge. This specification will include the pipe diameter, the fall, how to connect the downpipes together, whether I need any inspection outlets, how deep the pipes need to be buried, that sort of stuff. Maybe I overlooked something in the thread. Anyway, I'm reading further through AS3500.3 and I suspect the regulations are in Section 7. I'll have to read further.

This will be a quick fire reply as I am way behind with my work and about to go out. I have also had tezzab waiting on another thread for a reply for about a week and so if I don't get back on the forum for a few days after this, I won't have abandoned the thread (or tezzab...who gets the next reply).



The 50 mm overhang isn't regulatory, just advisory but standard practice to prevent blow back. Some houses have more overhang but that can restrict access to clean the gutter. For a new house, you just calculate the roof drainage area from the gutter's outside perimeter.

I have always considered the roof plan area (less guttering) to be the overhang but I am going to check this to make sure.



1:500 is a minimum requirement. You really don't want less than 1:350 which is ok.



You won't find anything about best design. Just because something is compliant doesn't mean that it will work well. Roof drainage provides plenty of examples.

A common mistake is to use a spreader that is too small as the dispersal needs to dilute the concentrated flow. Everything needs to be factored for a 1:20 ARI but it rarely is.

The roof area drained to a lower roof has to be added to the lower roof's sq m area. This is another reason why the lower roof downpipe locations are so important. A lower roof harvest area could be 12-15 sq m yet the upper roof could be 35-40 sq m.

The downpipe placements and roof harvest area on your diagrams show that you have a good understanding of the basic design and compliance principles. Some of the upper level downpipes will go to ground but you need to account for the ones that drain to a lower roof. I was going to point that out when you had added the roof harvest areas but I can see that you now realise that after filling in the roof areas.

You mentioned shingles in the opening post. Will you actually have shingles?



The roof areas, when doing roof drainage compliance calculations, have to show the factored areas because it is those figures that you are using to determine compliance.




I'm in Melbourne and so I knew it but I still checked.



A HP should be at the valley so that:
1. The concentrated flow will evenly disperse to two downpipes.
2. The concentrated flow is going into a gutter that has less depth than it would have if it was a low point that had all of the water flowing to it.

Many regulations are in conflict, you have picked up on one of them.

The BCA prefers a downpipe within 1.2 m of a valley but AS/NZS3500.3:2003 does not state this. The BCA also states that a provision for overflow must be installed if a downpipe is not installed within 1.2 m of a valley but AS/NZS3500.3:2003 again does not state this. The BCA is the primary document and has hierarchy but AS/NZS3500.3:2003 is a DTS document and builders and plumbers often overlook hierarchy. For what it's worth, both sets of regulations need improving.



You have to remember that the abutting walls are included in the calculations because of wind driven rain during an intense storm event.



You have to match the upper roof harvest areas to the lower roof harvest areas. If an upper roof harvest area is large and you don't want a lot of downpipes on the lower level, you could simply have bigger gutters and 100 mm downpipes on the lower level to increase the compliance area. You could also reposition the upper roof's HPs and add an extra downpipe so that a smaller roof area drains to the lower roof. I haven't actually transposed the two roof areas at this stage due to time limitations but all this is very easy.

For what it's worth, large numbers of new houses have non compliant roof drainage AND badly positioned downpipes but they get passed. Downpipe positioning is the important factor. A well designed system will perform much better than one that is poorly designed (most are) yet compliant.



Yeah, slotted gutter do overflow a lot more than they should but large numbers drain non compliant roof areas. There is also a ratio re effective gutter flow in relation to the gutter's height and width. Slotted gutters have greater flow resistance due to their lower depth (height of the slots).

The best overflow device is a low fronted gutter but high fronted gutters are chosen for aesthetics.

Overflow provisions are classified as continuous and non continuous.

The 10 mm gap between the gutter and the fascia is effective but not the best aesthetically.

Having a flashing is effective and neat.

Inverted nozzles are also effective. The Gutter Pumper is super effective.

The gutter's HP should always be at a height that allows built up water to flow to the next downpipe. If the slope is too great or the distance between downpipes too long, this can be prevented from happening and the roof harvest area between each HP will effectively be a dam. Having a good slope without compromising the HPs function of allowing water to pass over to the next downpipe is critical but not always understood.

Re the hf, most plumbers just use a minimum 20 mm gap as the calculations are difficult for many.



I get a lot of calls about problems with underground tanks. Things to consider are...
They are expensive.
The water needs to be filtered prior to entering the tank.
Submersible pumps supply water from the anaerobic zone.
Don't use copper pipe with rainwater.
Beware of being told BS about what you have to have for a mains water switching device.
Make sure the plumbers don't use small diameter high friction loss pipes to any fixture...including outside taps.
viewtopic.php?f=35&t=61588

I would use a settling tank to supply an underground tank if I was to have an underground tank...which I wouldn't.

SaveH20, again, you are a STAR!
Yep, I'm going to have asphalt shingles, and I intend to install myself, so I can ensure the flashing is laid to handle overflow of the gutters.
I'll have to take a look at this gutter pumper!
I was intending on building the underground concrete water tank myself, since my uncle has built about 3x 10kL and 5x20kL tanks. He has the formwork already made up, and a lot of experience as well. I will definitely be trawling through your posts about water tank design to get some excellent tips.
Once again, thanks for taking the time to reply to this thread. It's pretty amazing, to be honest. Well done for contributing to everyone's knowledge. THANKS!

Oh yeah, also, guess I am being conservative with the 40sqm, rather than the 45sqm. I'm ok with this, if it means my roof doesn't leak! And you're correct, 1:20 ARI is the bare minimum.
1:350 fall on the gutters - I can do that. That's like 3mm for every metre. Surely that is not hard to install, right?

The minimum 1:500 is only 2 mm per metre and so having 1:350 (3 mm per metre) isn't much of a step up. It's the minimum that I would have. Just have the spreaders as close as practicable to the downpipes.

You have obviously worked out your best downpipe locations. You don't have to go up much in gutter cross sectional size on the lower roof to be ok for the larger roof area compliance as you are already at 45 sq m. There are many gutters to choose from. Going to the 100 mm round downpipes with the bigger gutters on the lower roof also has other advantages.

The fall on stormwater pipes is a minimum of 1:100. Plumbers mostly use (cheap and flimsy) 90 mm stormwater pipe draining to 150 mm DWV, depending on the flow rates. The regs are in section 7 & 8.

Read this post re gluing PVC pipe. It is easy to weaken the thin stuff.

viewtopic.php?p=941557#p941557

If you get a chance, you should find bashworth's blog full of handy hints and interesting stuff. Well worth a read.

http://www.anewhouse.com.au/

Do you know that plumbing in Victoria is strictly regulated? Some other States (and the ACT) allow you to do the work that you are intending but not here. You don't want to get caught out with compliance issues re certificates etc.


The best laid plans of.....

Hi David,

Also check section 5, surface drainage systems - design.

Talking about plumbing regulations...Did you notice anything 'unusual' about the information in table 3.3? HUGE hint - 90 mm downpipes and the heading indicating required minimum downpipe sizes.

Thanks SaveH2O. Yes, I've been reading your posts about pipes leaking with interest. I could think of nothing worse than pipes leaking within your slab. It would be a nightmare. I will certainly specify DWV pipes for my stormwater system. This is opposed to "90mm stormwater pipe" that you talk about here:
viewtopic.php?f=31&t=68030&p=1098749&hilit=onsite+stormwater+detention#p1098749
I guess I have to be just as vigilant when the plumber installs the pipes for the black and grey water plus vents - make sure they use the DWV and that the joints are done correctly. I wonder how the plumbers will feel with me telling them how to do their job? Will have to discuss it at the quote stage, when everyone is nice!
To be honest, my eyes glazed over on Table 3.3. It took me a long time to figure out what was going on, but then only looked at the line that was relevant to me, and moved on. Looking back at it, it has a lot of very similar options, and I tend to wonder why so much detail is needed?
AS3500.3 is a very good standard, if you ask me. The examples in there are very easy to follow. But, at the moment, I am wading through Onsite Water Detention. This on the otherhand, is Crazy, with a capital K. I'm surprised how much work gets put into the hydraulics. I shouldn't really be, but I am. Reading up on the cost of OSD is staggering.
Thanks for your advice, SaveH2O!

Oh, and I was wondering about the ARI for Melbourne, and how I could get more up-to-date information, if I were using obsolete data. I think the data in AS3500 is from the Annual Rainfall and Runoff Study from 1987 (AR&R87). This data is available in a much easier format on the Bureau of Meteorology's website, plus more data from 2013!
http://www.bom.gov.au/water/designRainfalls/ifd/
I couldn't figure out why the 2013 data I was getting, was so far off the 1987 data. I just realised that I have to multiply the X min duration by 60/X to get mm/hr. For instance, a 20yr ARI, 5min duration in the 2013 data for my latitude/longitude is: 5%, 5min = 11mm x 60/5 = 132mm/hr.
Have I figured this out correctly, SaveH2O?

Are you are required to have Onsite Stormwater Detention (OSD). Yes/No.

If Yes, then the first thing to do is find out from the council the maximum discharge rate allowed. This is usually stated as litres per second (lps). If the maximum is given as let's say 5.5 lps, then the maximum overflow rate will be 330 litres per minute (lpm).

If your 1:20 ARI roof run off is calculated as let's say 640 lpm (I haven't read back to get an accurate figure), then a hydraulic engineer will calculate the detention capacity (detention compartment) needed to absorb the 310 lpm excess flow with a safety margin. It must be understood that the roof run off will most likely already be in excess of the overflow capacity prior to the rain intensity reaching a 1:20 ARI status. In the above scenario, the detention capacity required may be 2,500 litres.

An above ground detention tank has a small overflow pipe at the bottom of the detention compartment and a larger overflow pipe at the top. The smaller pipe's flow rate increases as the detention compartment fills because it operates with a variable hydraulic head.

If the smaller overflow outlet is meshed and the mesh gums up, the upper pipe will be a safety valve of sorts. I say "of sorts" because the overflow capacity will not match the inflow. The upper pipe is fitted to give a safety margin in case there is a prolonged storm, not because of any consideration given to the smaller pipe's mesh clogging but this scenario must be considered in a well designed system. It is best to use a flap valve that allows the smaller overflow pipe to discharge into a junction fitted to the larger overflow pipe connected at the top.


You can see in the above photo that a 90 mm stormwater overflow pipe drains into a 100 mm DWV fitting. The smaller detention overflow pipe drains into an invert taper that connects to the DWV fitting.

EDIT:
Can you clear up the reference to OSD and your intended use of the buried concrete tank thanks. I had assumed that this was going to be a retention tank. If the concrete tank is in fact intended to be a detention tank, then a whole lot of new dynamics apply such as slope, pipe locations etc. The concrete tank's overflow cannot be additional to a detention tank's hydraulic plan if it is intended to be a retention tank.

Have you read the link below that mentions nominal Vs actual pipe sizes and AS2200 flow rate compliance? For your roof area, '90' mm uPVC stormwater pipe would be borderline for compliance anyway.

viewtopic.php?p=1145148#p1145148


Answer: The second column's header states: "Internal size of vertical downpipe mm". These figures refer to the MINIMUM size!

The internal diameter of most 90 mm uPVC stormwater pipe is 86.2 mm!

3.3 is easy to read, the gutter's cross sectional area is on the left side. Once you know that, you just look at the RHS for the downpipe size. Just make sure you look in the 1:500 slope or greater column.

Re the ARI, the BOM announced a while ago that they were moving to a new system to determine a Intensity Frequency Duration (IFD). I like the old ARI system as it is easier to explain and quicker to look up but the IFD system is more zone specific. AS/NZS 3500.3 stormwater drainage compliance still refers to ARI, the Standards haven't changed. The links below lets you work out your area's IFD. Note that AS/NZS 3500.3:2003 Appendix D states the guidelines for determining IFDs.

http://www.bom.gov.au/hydro/has/cdirswe ... webx.shtml

Click "Create an IFD".

The link above is off bashworth's blog...a gold mine of information. http://www.anewhouse.com.au/category/su ... ity/water/


Yes. Well done.

Hi SaveH2O,
Once again, thanks for the advice. I might start a new thread for Onsite Stormwater Detention, to keep this thread pure. I'm beginning to understand why you recommended against underground water tanks. But, I'll elaborate more in the new thread.

Hi akapacker,

Just to let you know the spreader bars are a great idea as I often see the asphalt shingles deteriorate prematurely where the water off the main roof is let to run off the roof below. However, the MOST IMPORTANT thing is to use the best underlay. IKO Roof guard premium synthetic underlay will last a lifetime. Even if the shingles start to deteriorate the underlay will stop any water from entering.

We have stopped using the asphalt saturated felt paper because it breaks down easily and can leak. It is just PAPER with an asphalt saturant coating. It is worse in the areas where water builds up. If your whole roof is covered in the synthetic underlay and the valleys are lined with IKO Armourguard rubberised membrane you will never have a problem.

I am replacing a block of ten units where no underlay was used on a 10 degree pitch - huge mistake.

Low slope roofs need "Rubberised Membrane" between 8.5 and 18.5 degree pitch & valleys.
All other areas need premium Synthetic underlay - like IKO Roof Guard synthetic underlay.

Kind regards,

Richard.

I've been reading this thread with interest as I'm in the process of doing the same as akapacker and have worked out my downpipes pretty much the same.
One thing I am confused with though is the slope factor. Any of the slope or pitch factor charts I have seen say that a roof with a pitch of 25º has a slope factor of 1.103 not 1.23 ?
I'm about to work out the slope factor for my roof which is 15º and accordingly the SF should be 1.036 or is this the wrong figure ?
I thought it was as simple to work out your actual roof area by multiplying the plan area x the SF.
What am I missing here ?

Stewie

Hi Stewie,

AS/NZS 3500.3:2003 Section 3, table 3.2 is my reference for eaves gutter roof area factoring.

The former Victorian Plumbing Industry Commission (PIC), now incorporated into the new Victorian Building Authority (VBA), had a technical solution sheet that explained the factoring by example. It now simply refers to AS/NZS 3500.3 and HB114...a handbook! Go figure!!!

I checked the NSW Office of Fair Trading website and they now also use the 'upgraded' technical solution sheet.

http://www.pic.vic.gov.au/__data/assets ... water3.pdf

Nevertheless, if you calculate the roof plan area, you will find that it is 185 sq m (two areas of 10 x 15 & 5 x 7). If you then add up the catchment areas shown, they total 222 sq m.

The roof pitch is still stated in the linked document as 23 degrees. This is factored by 1.21 as per AS/NZS 3500.3 (calculation example no longer shown on technical solution sheet) but the doc still states that the maximum roof area drained to the indicated gutter's cross sectional area and downpipe size is 47 sq m. Given that the average of the 5 roof catchment areas shown is 44.4 sq m, you can see that the 23 degree roof pitch shown in the example has had a factoring of 1.21.

Aha !
15º Roof slope factor = 1.13
Thanks for that.
I just downloaded AS/NZS 3500.3:2003 and had a look at the charts.

Stewie

Correct.