How to fix downpipe water leak?

ewill

Hi all, any help or advice appreciated.  

With the recent rain we had in Melbourne I noticed a downpipe in the pergola area had water gushing out at the base, just above the concrete slab. 

Not sure if it's due to the amount of rain, blockage somewhere or if the downpipe has incorrect parts. 

MitchellMc

Welcome to the Bunnings Workshop community @ewill. It's marvellous to have you join us, and many thanks for the question about your overflowing downpipe.
 

Does this only happen in a torrential downpour? Or is this an issue you've never experienced?

One potential issue if this was a torrential downpour is the offset connection. Your downpipe, which is 100 x 50 and has an internal area of 5000mm², is feeding into the 90mm stormwater pipe with an internal area of 6361mm². This means that the 90mm stormwater pipe can accept more water than the 100 x 50mm downpipe can deliver, which is great. However, if you look at the offset fitting that connects the downpipe to the stormwater, it drastically reduces the amount of water that can flow into the 90mm pipe. The fitting effectively halves the stormwater's actual capacity to 3180mm². You can now see that if your downpipe is flowing at full capacity, then there is 1820mm² of water that has nowhere to go and will start backfilling up the downpipe and eventually into your gutters. This is just an unfortunate downside to an offset fitting that accepts a downpipe mounted flush to the wall, and a stormwater pipe set out and away from it. I suspect you're seeing this water spurt out the join as it is finding the path of least resistance. Have a look at the rendering I've created for you below, which should help illustrate the issue.

The other cause could be blocked pipes, and if the issue persists, or if you're experiencing it with every rain event, then it would be worth having that investigated.

Please let me know if you have any questions.

Mitchell