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What is the better way to improve roof gutter drainage?

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What is the better way to improve roof gutter drainage?

Hi all,

My roof gutter down pipes are roughly 10m spacing but  i was considering adding /cutting in down pipes every  5M and angling the 90mm pipes to the other existing down pipes. As most pipes  have concrete paths stopping easy access to existing ground  storm water pipe.  I am not to worried about the looks a am more worried by torrential rain events over flooding  gutters as  some water has back flowed into the eves occasionally.


Or  is there a deeper gutter out there or alternatives ?


Thanks for you thoughts once I finish land scaping  I will switch to roof maintenace as house is 22 years old some rusty  gutter corners etc.



Community Manager
Community Manager

Re: What is the better way to improve roof gutter drainage?

Would you be able to share some photos @Jewelleryrescue? Then helpful members like @EricL@MikeTNZ and @TedBear might be in a better position to suggest next steps.






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Bunnings Team Member
Bunnings Team Member

Re: What is the better way to improve roof gutter drainage?

Hello @Jewelleryrescue


Can you please tell us the current size of your downpipe and what material is it made of? Sometimes older houses have the traditional galvanized 50mm x 80mm box drains. In order to increase the drain capacity, I propose upgrading your downpipe to 90mm round PVC drainpipes. You can even go as big as 100mm but it will be very noticeable. 


We look forward to seeing the photos of your roof gutter and drains.




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Amassing an Audience

Re: What is the better way to improve roof gutter drainage?

Hi @Jewelleryrescue,

I think we need to see what you have already got there, before we can make any sort of an assessment of what you might need.

We don't need much, just a few basic photos of the spouting and the current down-pipe arrangement, down to the ground.

That will give us a decent idea on what you might need.

Does the spouting only flood on one side of the house or is it all the way around?

Knowing this also would be really handy.


Thanks for your time.



Mike T.

Re: What is the better way to improve roof gutter drainage?

Hi @EricL  @MikeTNZ  @Jason


@MikeTNZ   in a down pour several gutters over load around the house 3 places this is normal every blue moon or less.  Severe rain is normal at present but while i  am about to repair/replace sections I might as well super size it and  stop all together and head off future issues.


FYI  when the house was being built i had council inspectors dig up the storm water pipes as  i thought they might be flat in angle to  storm water  system but they where legal  1:10 fall so they said.


 Existing 100 x50 box down pipeExisting 100 x50 box down pipe


existing 100 x 50 box down pipeexisting 100 x 50 box down pipe


Yes the house does have the basic 50 x100  metal  box drains.   I  replaced one box  50 x 100 down pipe with a plastic 90mm down pipe and discovered while fitting it the problem might be at the gutter box outlet as a normal but narrow  outlet. So  your  response  has  given me food  for thought. thanks @EricL 


So do appreciate where you are heading with wider down pipes 90mm  even 100mm is fine once they painted up they look like they  have always being there. But if  i dont incease hole size at the top and bottom of the pipe i think thats where the issue is mostly.


So no matter the pipe diameter I need to put the larger fitting at the top to allow faster drainage. is  part of the problem.  Then there's the base of the rectangular  box down pipe the cement has taken on the form of the square down pipe.   Last time I had to used a hot air  gun and heat a  plastic 90mm to square  fitting to get it to fit rectangular  ground shape  to get it to insert properly with out leaks.  (90mm  pipe fittings  with rectangular pop slide over are  larger than 100x50 mm box rectangular pops which is easy to deal  with at gutter.  But trying to get the 90mm to rectangular pops back into the ground rectancalar hole needs a shoe horn or a hot air gun to reshape smaller greating bottle neck see the narrow exit with huge 90mm pipe over it in photo. I might have to hack  concrete but  Aircon external unit in front of it to. sigh might need to tempary move it aside.


Temporay down pipe until better soloutionTemporay down pipe until better soloutionNarrow pop outlet into large pipeNarrow pop outlet into large pipe


Solution 1: maybe

So maybe If I install  I/n4790370 90mm round down pipe pop (if it fits in standard metal guttering width (i havnt checked yet) )

So I have the capacity to cement chisel down to the 90mm storm water pipe at need to insure 90mm all the way down.


But still open to  better solutions


Re: What is the better way to improve roof gutter drainage?

Hi @Jewelleryrescue


I agree with changing your outlets to the Fielders 90mm Zinc Round Downpipe Pop. Unfortunately, the Holman 100 x 80 x 90mm PVC Stormwater Downpipe Adaptor although convenient for transitioning from box type to round PVC, it is very restrictive and cuts the effectivity of the downpipe by at least half. So instead of getting the benefits of a round-down pipe, the drain is still very slow.


Your assessment of the bottleneck at the bottom is spot on. If the downpipes have been adjusted to a bigger size but the ground pipes are still fixed to the original 65mm or 75mm outlets it will create a choking point as the amount of water can't get out fast enough. This might be something that you'll need to investigate as well.


Please keep us updated with your progress, we look forward to seeing your downpipes fixed.




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Re: What is the better way to improve roof gutter drainage?

Hi @Jewelleryrescue, one simple question, how old is your house that plastic junk like that got installed in such a poor way? 😁

(Don't take that personally)

That actually looks like it was removed from an old dairy shed and used to provide storm-water relief to a house.

What I would do is get a quote from a place that specialises in spouting and storm-water disposal.

Yes, you could do most of this yourself as in the spouting.

In the second and third pictures suggests a lot of excavation, breaking of concrete and bringing the whole installation to where you have

round down-pipes and collectors at the base of the concrete.

Is the spouting the same on the other side of the house?

Personally, I don't like the idea of anything being concreted into the ground, where it cannot be removed easily down the track without either an excavator or a concrete saw.

This looks like a reasonably modern house, the spouting lets it down.

Rain collectors are a MUST at the base of down-pipes, that fitting in your third picture, where that down-pipe reduces down from 100 mm to what ever the size is heading underground, is where a collector should have been fitted.

Installing one of these on each side of your house, is what I thought the Council inspector would have liked to have seen.


By all means @Jewelleryrescue,

If you need further advice, let me know, I'll help you out where I can.



Mike T.

Re: What is the better way to improve roof gutter drainage?

Hi @MikeTNZ  @EricL 


Like  I was saying the round 90mm was a temp fix  (choked at the bottom) with out getting to carried away on that whole house project  starting the concrete busting.  I thick  EricL is right here putting in 90mm round pipe  AKA dairy shed pipe will greatly improve flow once its allowed to connect full to 90mm storm water  ground  connection no choke points. In fact that can be increased to 100mm the extra diameter will store more  volumn of water like a water collector Mike  meantions a good idea (I have only seen  water collectors  used on factory roofs never domestic they use them to regulate the  flow (slow) water into the storm water system. not an Oz requirment as far as I can tell on domestic) 


Reason  for temp repair. I had a steel gal down pipe rust out on me and  leaking all over air con  unit so I wasnt to keen on the electrical  and water mix possibly shorting electrics out or  additional corrosion of the unit as they are water  resistant but not designed to sit under a near water falI.    I am also  surmising  re  roof eves getting wet some times the rushing  water off the roof hits the  guttering insides and splashes up  backwards  or this  is more possible if gutter allready  full  off waterI might look into some aluminium flashing or plastic as a back up .



WEll I have an open front yard to redo  Old clapped out timber fencing and gate and new veg patch and a bunch of other smaller projects.  And a major project a koi pond filter and pond redeisgn my most complex project todate at least to my mind. But I am always is  planning ahead so  Like Mike says I might find a   guttering reseller a good idea.   Or  guttering sale?  At least I will know what hardware to look out for. I probably wont start until for 6 months . I also shut down  hot out door projects at full summer its just to uncomfortable  as I approach older years

MikeTNZ I read your help full notes on guttering from the garage lean to post that was very helpfull to thanks.


Thanks for your help  people.

Re: What is the better way to improve roof gutter drainage?

Hi @Jewelleryrescue,

The reason you have to install a rain collector at the base of a down-pipe (and I've oddly enough mentioned this in other threads here),

is so that the down-pipe does not "flood" during heavy rainfall.

If you fill the whole pipe up with water, it will airlock and not drain at all, causing flooding issues in the spouting above it

The collector, with the grate that it has in the top cover, allows the down-pipe to vent to atmosphere, it also stops leaves and such things clogging the storm-water drains.

Nothing is ever perfect, but if you install a decent storm-water system, you can save yourself a LOT of money in building maintenance down the track.

And that is what we are all about here.



Mike T.

Re: What is the better way to improve roof gutter drainage?

Hi @MikeTNZ 

Thanks for  explaining the concept of the rain collector at the base of a down pipe. I can see a full pipe being in an air lock situation and  normally I try dealing with leaves before they get in.  I Think and internal 19mm garden hose back to the roof level could work as a  air lock snorkel at first novice response to  be up dated with more learning.

But  you definatly have  a good  practical understanding of storm water managment so will honour  your advice and do some  more product research but I am serious  when I say I never hear of such a system in OZ.  This is just  showing my level of ignorance here so I will  start learning more.  If I find commercial hard ware  I will see  what you think.  Just so you know I have a 5000L water tank on one down pipe feed by surrounding roof. I might try to pick up more  water from other some  other corners not down piped as you have said they are bad  flow  points.  Hang in there I will put together a down point house plan  we can modify.

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