Connected water tanks to gutter => Gutter overflowing in heavy rain?

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boland

547 posts

Ultimate Geek

#311690 7-Feb-2024 15:24

Our house is ~20 meter long with on one side over the whole length a gutter. On both ends, I've connected 200L rain tanks, that will collect any rain into these tanks if they aren't full. I've cut out ~10 centimeter and installed the connector that came with the rain tank.
Two on one side (connected to each other), and one on the other side.

When it's raining hard, the gutter is overflowing around the center. This causes water to get under the roof (metal roof) and has caused leakage in one of the rooms.

I've cleared out the gutter, my suspicion is that due to the rain tanks the flow rate has decreased, causing a water build up in the middle. Is that a reasonable assumption?

It only seems to happen in the center, so it may also be that it's not 100% level & it's slightly lower in the middle.

Any thoughts and possible solutions? Hopefully there's an easy solution, e.g. installing a pipe in the middle will require me connecting it to the storm water, which won't be easy.

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1238 posts

Uber Geek

#3191969 7-Feb-2024 15:40

Could you try testing to see how your drain flows.
Why not block off the water tank drains and full the gutter with water. You will soon see where the water is banking up and what part will overflow first.
That will confirm if the middle is lower.

I assume that if the down pipes to the tanks are not blocked then if the are smaller diametre you could be right about this being the cause.
Maybe there is a sharp bend in teh water tank down pipe that is causing the water to bank up with heavy flows?

Maybe you could install a second downpipe at one end and create a small half height dam in your gutter. This would mean that uder normal rain your tanks would fill,
but if the rain causes the water to build up and top over the dam it would flow into the second downpipe and drains?

boland

547 posts

Ultimate Geek

#3191971 7-Feb-2024 15:52

KrazyKid:

Could you try testing to see how your drain flows.
Why not block off the water tank drains and full the gutter with water. You will soon see where the water is banking up and what part will overflow first.
That will confirm if the middle is lower.

We're in Wellington and under level 2 restrictions. Technically I'd be allowed to do that, but not feeling good about it.
Good suggestion to check it out first though, next time it's raining heavy I'll go check it out.

I assume that if the down pipes to the tanks are not blocked then if the are smaller diametre you could be right about this being the cause.
Maybe there is a sharp bend in teh water tank down pipe that is causing the water to bank up with heavy flows?

Maybe you could install a second downpipe at one end and create a small half height dam in your gutter. This would mean that uder normal rain your tanks would fill,
but if the rain causes the water to build up and top over the dam it would flow into the second downpipe and drains?

There is no bend in the pipe going down as far as I can tell.

That second down pipe with a dam sounds like a good idea!

wellygary

8342 posts

Uber Geek

#3191984 7-Feb-2024 16:21

Couple of questions.

How "wide" is the roof gable..... @ 20meters long, if your roof is 5m "wide" you've got 100sqm of roof,

For every 1mm of rain you are generating 100 litres of water,

I'm guessing your tanks are back filling the downpipe causing flow off the roof to slow and allow the overflow,

You could stop the backflow by putting an over flow hole in the top of the tank that is lower than the downpipe inflow to allow the excess out

You could also have an overflow pipe ( a Y or angled joint would do it) on the downpipe that relieves any backfill

boland

547 posts

Ultimate Geek

#3191999 7-Feb-2024 16:27

wellygary:

Couple of questions.

How "wide" is the roof gable..... @ 20meters long, if your roof is 5m "wide" you've got 100sqm of roof,

For every 1mm of rain you are generating 100 litres of water,

I'm guessing your tanks are back filling the downpipe causing flow off the roof to slow and allow the overflow,

You could stop the backflow by putting an over flow hole in the top of the tank that is lower than the downpipe inflow to allow the excess out

You could also have an overflow pipe ( a Y or angled joint would do it) on the downpipe that prevents any backfill

Think about 100 m2 goes into that gutter. It's indeed a lot of water, never calculated it myself. Theoretically, a couple of mm of rain and the tanks should be full again (from empty).

That overflow for the water tanks is a brilliant, easy idea.

Bung

6501 posts

Uber Geek

Subscriber

#3192010 7-Feb-2024 16:43

Did the downpipes originally go to stormwater? If your 200l tanks don't have overflows and the downpipes are sealed into them you will overflow the gutter. Your overflow could go into existing stormwater.

I have a 10m x 7m roof going into 1 x 200l tank that drains into hoses going to gardens. The tank will quickly overflow in steady rain. The excess just goes onto lawn.

Senecio

2718 posts

Uber Geek

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Lifetime subscriber

#3192011 7-Feb-2024 16:51

What type of downpipe diverter did they supply with the tank? If it is like this then it won't be impacting the flow significantly.

Marley Twist Rainwater Diverter - 80mm White - Bunnings New Zealand

tweake

2397 posts

Uber Geek

#3192030 7-Feb-2024 17:30

boland:

When it's raining hard, the gutter is overflowing around the center. This causes water to get under the roof (metal roof) and has caused leakage in one of the rooms.

the gutter was never installed properly to begin with. if water is backing up say from a blocked downpipe it should overflow at the downpipe end. overflowing in the middle means the middle is the low spot because someone didn't install it correctly.

2ndly water should overflow the gutter typically over the back and go down between the gutter and eave. if its overflowing into the house, the gutter is probably mounted way to high.

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mattwnz

20177 posts

Uber Geek

#3192040 7-Feb-2024 17:39

Why not install leaf divertors in the downpipes below the guttering, where if the downpipe is obstructed, it should overflow out the top of the divertor?

tweake

2397 posts

Uber Geek

#3192041 7-Feb-2024 17:40

boland:

Think about 100 m2 goes into that gutter. It's indeed a lot of water, never calculated it myself. Theoretically, a couple of mm of rain and the tanks should be full again (from empty).

That overflow for the water tanks is a brilliant, easy idea.

got a pic of the setup?

what size pipes and how is it done.

100m2 would need at least a 100mm dia downpipe, especially if its connected at the ends. that fed into a 200l tank will fill it in minutes. i think its the marely website that has calcs that you can use to work out pipe size and gutter sizes. tanks should always have overflows.

i installed a detention tank system over winter (due to last years floods) so went through all the sizing. we actually found someone installed the wrong size downpipes on the garage so thats going to need its own detention tank at some point. my 1000l tank in big storm conditions will take all of 10-15 minutes to fill from a roof half the size as yours.

boland

547 posts

Ultimate Geek

#3192131 7-Feb-2024 20:55

First, thanks all for your replies! It's amazing to see everyone chipping in. Very much appreciated.

Senecio:

What type of downpipe diverter did they supply with the tank? If it is like this then it won't be impacting the flow significantly.

Marley Twist Rainwater Diverter - 80mm White - Bunnings New Zealand

Not a fancy one, it's one of these. It's very likely to impact the flow.

tweake:

the gutter was never installed properly to begin with. if water is backing up say from a blocked downpipe it should overflow at the downpipe end. overflowing in the middle means the middle is the low spot because someone didn't install it correctly.

2ndly water should overflow the gutter typically over the back and go down between the gutter and eave. if its overflowing into the house, the gutter is probably mounted way to high.

Two very valid points, think you're right. While we have 10 year builders warranty, not sure I want to go down that path.

boland

547 posts

Ultimate Geek

#3192132 7-Feb-2024 20:56

Senecio:

What type of downpipe diverter did they supply with the tank? If it is like this then it won't be impacting the flow significantly.

Marley Twist Rainwater Diverter - 80mm White - Bunnings New Zealand

Not a fancy one, it's one of these. It's very likely to impact the flow.

boland

547 posts

Ultimate Geek

#3192134 7-Feb-2024 20:57

tweake:

the gutter was never installed properly to begin with. if water is backing up say from a blocked downpipe it should overflow at the downpipe end. overflowing in the middle means the middle is the low spot because someone didn't install it correctly.

2ndly water should overflow the gutter typically over the back and go down between the gutter and eave. if its overflowing into the house, the gutter is probably mounted way to high.

Two very valid points, think you're right. While we have 10 year builders warranty, not sure I want to go down that path.

boland

547 posts

Ultimate Geek

#3192135 7-Feb-2024 20:57

tweake:

got a pic of the setup?

what size pipes and how is it done.

100m2 would need at least a 100mm dia downpipe, especially if its connected at the ends. that fed into a 200l tank will fill it in minutes. i think its the marely website that has calcs that you can use to work out pipe size and gutter sizes. tanks should always have overflows.

i installed a detention tank system over winter (due to last years floods) so went through all the sizing. we actually found someone installed the wrong size downpipes on the garage so thats going to need its own detention tank at some point. my 1000l tank in big storm conditions will take all of 10-15 minutes to fill from a roof half the size as yours.

Pipe size is good I believe, but I think the problem is that the lowest point is in the middle (instead of at the end), like you said.

Photo of one end here:

mattwnz:

Why not install leaf divertors in the downpipes below the guttering, where if the downpipe is obstructed, it should overflow out the top of the divertor?

Another great tip, thanks. I'm still learning about all of this.

Jase2985

13465 posts

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Lifetime subscriber

#3192141 7-Feb-2024 21:13

thats 100% going to cause a backup when it rains heavily, you can see the restriction in the pipe diameter.

tweake

2397 posts

Uber Geek

#3192170 7-Feb-2024 21:40

boland:

Pipe size is good I believe, but I think the problem is that the lowest point is in the middle (instead of at the end), like you said.

don't believe, measure.

sorry i don't have time tonight to look into it but can you get pic/measurements of the whole system. you mention both ends? got two downspouts? really need layout and size of roof feeding it etc. i notice the gutter is the higher flowing version. off the top of my head 80mm is probably ok if there is two, but the center should be the high point. it sounds like they have the slope the wrong way, maybe intended to have a single big downspout in the middle.

the adapter thing looks highly restrictive.

i would fix it asap.

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