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How to merge shed and house downpipes

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How to merge shed and house downpipes

Hi everyone,


I would like some advice on how to join the downpipe from a new shed to my existing house downpipes. The house roof is 200m2 and the shed roof is 86m2.


The house is currently connected to the watertank via 90mm PVC piping. Both gutter outlets join to a 45° bend and a 90° elbow, as shown in the photo below. The two 90mm PVC pipes then meet at a joining tee before flowing into the tank.



My plan is to replace all 90mm pipes and fittings with 100mm to improve capacity (I've seen it overflow with very heavy rain). I'd also like to replace the 90° elbow with an 88° junction (drawn in red below) allowing me to connect the shed downpipe (blue):



Would the 88° junction be likely to overflow in a heavy downpour?


Also I'm thinking the tee could be replaced with a 45° junction + 45° bend facing the gutter pictured (i.e. shed downpipe direction), and a couple of 45° bends facing towards the far gutter, to improve flow.


Any advice would be really helpful thanks!

Bunnings Team Member
Bunnings Team Member

Re: How to merge shed and house downpipes

Hello @MatthiasMcRobbi


Welcome to the Bunnings Workshop community. It's a pleasure to have you join us, and thank you for sharing your question about your downpipes.


If the current system of 90mm overflows in strong rain, you can just imagine what would happen if you combined it with the flow from your shed. You'd be surprised at how much rainwater an area of 86m2 can catch. Gutter overflow often happens when there are not enough exits on the gutter. Imagine ten people on your roof slowly tipping a bucket of water on your roof every 30 seconds. At the end of the gutter is one person emptying the gutter with two buckets every 2 seconds. As more water is poured down your roof eventually the person emptying the gutter will be overwhelmed. 


The more exits you provide for the rainwater means it will not gather in your gutter. With more exits, the flow will be smaller and even compared to a single exit where it is large and causes a deluge. The water has time to flow out of the system and is not forced into a single outlet. Increasing the diameter of the pipe will definitely help. My only tip for this particular setup is to make sure that the pipes are well mounted and secured with steel straps to prevent movement. 


So can you still put in that "T" junction? Yes, you can, however, I suggest putting a bypass in case of too much rain. I suggest using Blue Mountain Co Plumbing 90mm Water Diverter. To save that diverted water, I suggest putting in Clark Tanks 3000L Poly Slim Water Tank - Rivergum behind your shed. You can also create your own storage system if you can source adequate containers.


let me tag our experienced members @TedBear and @Adam_W for their recommendations.


If you need further assistance, please let us know.




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Kind of a Big Deal

Re: How to merge shed and house downpipes

Hi @MatthiasMcRobbi , here is how I would consider tackling your downpipe issue:

I agree with @EricL that adding the extra pipe, even to a new 100m pipe could cause you problems during downpours.

I would replace just the 90mm pipe that runs off to the tank with 100mm (given that you said it currently only overflows occasionally),

but I would not connect it to the two 90mm pipes via a T piece, as at present.  Instead I would fit a 100mm Rain Harvesting Leaf Eater just below where the T piece is now (assuming the run-off slope allows it) and connect it via a Rt Angle connection to the 100mm pipe.  Then feed the two 90mm pipes over its intake. 

I would then run the proposed new downpipe connection (90mm should be fine) all the way to the Leaf Eater and let it drain into there too.

(May need a 45^ connector at the end to turn the water inward toward the catcher?)

That way you will get less leaves into your tank and if there is a large downpour the excess will flow out at the Leaf catcher (i.e. outside) and not

back-up in your gutters where it could possibly flow back into the roof space.


To give a little more height for this, consider replacing the current 45^ gutter outlets with right angle outlets  (only $1.90 each), so that the pipe is a little higher where it leaves the gutter end. (joining two, as at present to give the direction required, but going out before dropping down, rather than going both down and out that the current 45^ outlet is doing.) 

Workshop Legend

Re: How to merge shed and house downpipes

Hi @MatthiasMcRobbi I'd say your overflow issues are caused by two things;
- inadequate fall on the actual pipes.

- the sharp angle of the outlet.
Large or surge volumes of water don't flow well when they hit a right-angled junction. Basically it causes the water to deflect straight back the direction it came and causes heavy turbulence as a result. This reduces flow & causes water to back-up. It can even trap a pocket of air which can effectively dam the pipe as it builds up, compresses & clears. You would see this as the gutter or downpipe inlet overflowing in surges - overflow, runs, overflows, runs...

Switching the immediate outlet sections to more gentle angles may help.
Until you remedy the existing issues I wouldn't look to add a second pipe & even then be careful as you may overload the existing pipes.

If you do add that pipe make sure the existing outlet is switched to a 45˚ like this one (pictured is threaded but you get the idea) as that allows water to 'merge' rather than 'collide'.

Re: How to merge shed and house downpipes

Thanks @EricL@TedBear & @Adam_W for your advice. You've each given me some things to consider, and I'll be sure to come back here when I'm ready for the next steps!

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