How to add drainage under deck?

Hi everyone,

I’d like some recommendation regarding under deck drainage. I’m building a low profile deck but in order to be flash with the door I have dag around 30mm below the rest of the backyard.

Initially I thought to use an agi pipe and connect it to stormwater. But I read an article that might cause issues in an flooding event.

My thought is to install the pipe nearly 2m away from the house.

Any thoughts?

Community manager's note: Check out How to build a low-level deck for expert advice.

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Comments

EricL

Hello @GeoKoro13

You raise an excellent point when planning drainage for a low-level deck. In clay soil, water tends to drain very slowly and often alters the consistency of the soil making it soft and pliable.

When planning drainage for the bottom of your deck, I suggest putting a slope in the soil so that it points towards your backyard drain pit. You can also use a smaller diameter agi pipe to facilitate drainage. The last recommendation of course is to put in spoon drains. If you decide to put in spoon drains, I suggest building them yourself so that you can customize them to your decking layout.

Generally, a spoon drain is a shallow channel that has been cemented over and allows water to enter from the sides and direct it towards the main drain. I've placed a sketch below to give you an idea of how it is made. In order to get a smooth rounded surface, I suggest using a 100mm PVC pipe and running it over the concrete surface to get a perfect round channel.

If you need further assistance, please let us know.

Eric

GeoKoro13

https://www.workshop.bunnings.com.au/discussion/comment/23608#Comment_23608

Thanks for the response @EricL .

I attached a pic of my design with the drain location and drain pic.

So, around the deck I want to lay fake grass. But as I mentioned the soil under the deck will be 20-30mm deeper than the surrounding ground.

If I run an open drain under the deck will be still lower (approx 30mm) than the stormwater drain plus will be in the middle of grass.
Am I missing something here?

George

EricL

https://www.workshop.bunnings.com.au/discussion/comment/74453#Comment_74453

Hi @GeoKoro13

My apologies, I totally skipped right over that detail. That does present a bit of an issue, it will still be necessary for you to put a drain under the deck. I'm afraid you'll need to dig a channel so that you can link into the drain near your hot water heater. It might be necessary to dig a secondary emergency drain in case there is a very large downpour.

If the deck is going to be covered, I suggest putting a gutter at the end of the roof so that the rainfall will not travel down to the bottom of the deck. I still recommend the laying of weedmat and drainage gravel to facilitate water drainage. However, before you start digging, I suggest checking the rules and regulations of the local council in your area about connecting to your local drain. Some states require that only a registered plumber make the connection.

Let me call on our experienced members @Adam_W and @ProjectPete for their recommendations.

If you need further assistance, please let us know.

Eric

GeoKoro13

https://www.workshop.bunnings.com.au/discussion/comment/100805#Comment_100805

@EricL,

Just to make sure I understand that correctly.

- Install an open drain (not ag pipe) from the decking area towards the drains next to the tank

- Connect the open drain to the drain somehow (given approvals and everything from council if applied)

- The area I’ve dag is enough for the height of my deck with some room under the joists. Do I need to dig more to lay drainage gravel?

George

EricL

https://www.workshop.bunnings.com.au/discussion/comment/117298#Comment_117298

Hello @GeoKoro13

I'm sorry, please ignore the open drain suggestion as this will not work for your situation. It will be necessary to use agi pipe underneath the deck. I suggest conducting a soil drain test in your proposed decking area. I propose digging a hole approximately 200mm wide and 300mm deep. Pour half a bucket of water into the hole and measure how long it takes for the water to drain. If it takes more than 10 minutes, you have soil that is high in clay content. However, if it drains quickly that means that you have good draining soil and water will not stand under your deck.

If you have high clay content in your soil, it's important that the agi drain be properly laid out so that water inside falls away from your decking area. I recommend engaging the services of a registered plumber so that you can consult them about your drainage plans. I propose putting in the weed mat and fixing it place with plastic weed mat pins to keep it in place.

If you need further assistance, please let us know.

Eric

GeoKoro13

https://www.workshop.bunnings.com.au/discussion/comment/126818#Comment_126818

Thanks so much @EricL! I really appreciate your responses

GeoKoro13

https://www.workshop.bunnings.com.au/discussion/comment/126818#Comment_126818

Hi @EricL,

I was just thinking, could it be easier to install another catch basin (see photo) in the middle under the deck to collect the water and connect with pipe to the other drains next to my hot water?

George

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/133169#Comment_133169

Hi George (@GeoKoro13),

@EricL will be back after the weekend, and I trust he'll add anything I've missed. What you have there is more so an inspection outlet. It's designed to allow access to your pipework for cleaning. If you place a singular water catchment device under your deck in the middle, I'd recommend a stormwater pit. It's a similar idea but has a much larger capacity and is less likely to become blocked by debris. Having a drainage device in the area will help in torrential downpours. As per the rudimentary image below, adding Agiline over the site is the best drainage method. Your not only building a low-level deck, but you've sunken the area below the surrounding soil. With your plan, you'll need to remove all the water from the site as soon as possible after rain, or you'll see earlier decay of the deck's structure.

Please let me know if you have any questions.

Mitchell

GeoKoro13

https://www.workshop.bunnings.com.au/discussion/comment/137077#Comment_137077

Thanks for the response @MitchellMc.

So, you reckon it would be more effective to remove the water under the deck but installing agi pipe in the formation on the pic to cover more area.

The pipe has to have a fall towards the drain and then connecting the agi pipe with an adapter to the stormwater.

If you look the pic of the drains above (the one you see the chair), will I connect the pipe to the drain on the left (not the one with the overflow for the tank)?

What’s the minimum depth do I need to dig for the agi pipe?

Do I get the concept correctly?

George

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/139777#Comment_139777

That's pretty much it, Geoff (@GeoKoro13).

You'll connect the Agiline to the buried stormwater pipe, not the drain itself. Those two drain outlets will likely connect together directly under the soil. I've put a rendering together for you below to illustrate better. Your stormwater might not run off in the direction shown. You'll need to dig down to locate it.

The depth of the stormwater pipe will govern the fall you can establish. If the stormwater pipe is relatively close to the surface, then your Agiline will need to sit higher, so the water is transferred. If the stormwater pipe is 30cm underground, you'd need to have the Agiline buried10-20cm underground to establish an adequate fall between the two.

As @EricL mentioned, although you can do all the digging work and lay out the pipe, you might need a drainage professional to make the connections.

Mitchell

GeoKoro13

https://www.workshop.bunnings.com.au/discussion/comment/141626#Comment_141626

Thanks so much @MitchellMc!

Did you just draw that?!?!? Amazing illustration!
Ok, got it. I’ll do all the hard job and then I’ll pay someone to do the easy job 🤓…

You guys are super helpful!

Thanks for the support!

George.

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