Looking at your drawing ( excellent illustration by the way Sketchup woop woop!!! ) the one degree slope will not be enough to empty out the line at the bottom. It will however act like a giant trap. Water will stay at the lowest point of your pipe run and will top off at the half meter point where it needs to over flow into the existing drain. Every time it rains water will flow through the trap but you will possibly get the following effects.
- Pipe water overflow - When there is too much water coming from the roof and gutter and the water in the pipe is not flowing out fast enough to accommodate the volume. This is caused by that trapped portion of your run which acts like a bottle neck to the speed and flow of water.
- Vacuum trap syndrome - When air is trapped in the down pipe due to its length and run. Usually happens with two story buildings a vacuum break is sometimes installed when the run is too long to avoid this occurrence.
Recommendation : Why not run a larger diameter down pipe on the right hand side near the drain. I suggest a 100mm down pipe to accommodate excess rain water to avoid having to run it under the deck. I suggest a 45 degree slope off the top of the gutter or start of the run to increase the waters velocity as it comes down the pipe. To increase the waters flow to the pipe you can increase the slope of the gutter by an extra 1 to 2 degrees.
French drain : If constructed wide enough and deep enough it should provide you with adequate drainage, however I would not place it under the deck. Should it ever get water logged you will need access to it to fix the issue. In some situations I would use the down pipe and french drain combo but in this instance I would rather you re-direct the water in to the drain. I hope these suggestions help. Good luck and stay safe.
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Its great to see you've already received a helpful reply from @redracer01 and he has covered my thoughts. The main issue I see is that in a heavy downpour the downpipe on the left is going to backfill up faster than the charged pipe is going to be able to release the water. Where is the stormwater line in this scenario? If it is on the right, can the connection be lowered 50cm to allow a suitable fall of all pipes into it?
Thanks for the reply Red. Yep I'm a planner, gotta make sure I get things sorted before I start construction.
Ok, so the French drain underneath the decking is out
With the recommendation to use a 100mm downpipe I still have a concern/question. I did these quick mock ups of the existing guttering, downpipe and ground level so you can see. About half of the house goes to the one downpipe. I have an old house and currently when it rains heavy the gutters overflow above the back door opening. This is why I wanted to add the new downpipe to the left of the pergola. I don't have enough space to be able to increase the fall on the existing gutter.
I could increase the diameter of the existing downpipe from what is probably 70-75mm to 100mm but I still feel that this is not enough to prevent the overflow with heavy rain? Do you think this would be sufficient? If I still add a new downpipe to the left of the pergola I could run the new stormwater pipes around the outside of the new decking but I'm not sure how much that will help me? I would still need to put a fall on the stormwater and this would mean that I am still relying on a "charged line" to get the water up to ground level to be able flow into the drain. I'm still unsure what to do here?
Thanks for the reply. Unfortunately I cannot lower the connection by the 50cm. I've put more detailed info with additional pics in my previous reply.
Thanks for the detailed diagrams, makes things much easier.
Just to sort of unpack this whole plan.
- A charged line can be very effective however the junction you have created will not work. Where the right-hand downpipe meets the uprising charged line coming from the left is the main problem. You will have two water flows meeting head-on and they will both back-up. See the attached quick diagram but you may be able to work around this by creating a 'V' or wishbone junction however I would recommend that the pipe they meet is a larger diameter. Also... you need more than 1˚ of fall. I believe 1 in 100 is standard.
- A thought... Could you instead put a 5000L water tank on the left and run the overflow to a rain garden?
- I have concerns about the awing-house roof interface and how water is being managed there. What will happen if the awning gutters overflow to the house side, which they will?
- A French drain is a rubble or gravel drain, these days often with an ag-pipe down the centre. Two issues there... They can require maintenance as they can clog up with silt over time - you won't be able to do that. And they still need fall and they still need to actually drain somewhere. In a landscape situation they can be used as dispersion & absorption drains but I'm guessing that you don't want the foundation of your house waterlogged...
Thanks for all the feedback.
Ok, so you've talk me out of the French drain AND the stormwater pipes being run under the decking
So here are the plans now using a 5000L watertank. Now I'll start investigating what type of pump I will need to go with the watertank.
We have an expansive range of rainwater tanks to choose from; you'll find they come in a multitude of different configurations and styles that suit your needs. We've even got under deck options which would be perfect for this scenario. We also have you covered with many different pump options. If you see one of our helpful team members in-store, I trust they would be delighted to pair up a great tank and pump combo for you.
I look forward to hearing what options you choose and seeing your results after installation. Please let us know if you have further questions.