How to add drainage for low level deck?

I am looking to utilise the space under our current deck for another decked area extending to the fence to increase the pool area (moving fencing back which will cut across new deck area)

The issue I have is clearances, the current under deck height is 2100. Therefore the preferable height would be for the deck to finish level with pool slab which is 100mm from ground.

In order to do this I am aware I would need to dig down. I was planning to use joints on tuff block to keep it as low as possible. Although the under-current deck area will be sheltered by the top deck and roof from some rain I am still concerned about pooling under the deck if it effectively sits in a pit. What would you recommend for drainage solutions? or is this overall an unworkable idea?

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Comments

EricL

Hello @dave_022

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your low-level deck project.

If you were to dig a pit into the ground for the deck to fit in, you would have to make sure that the drainage for that area is flawless and is capable of taking on a typhoon deluge. Should the drainage fail water will pool in that area and could possibly cause damage to your property. However, before you begin, I suggest checking your local council's rules and regulations in regards to building a low-level deck under an existing deck. What are the space and height restrictions in this type of build.

On the condition that the rules for building a low-level deck does not have restrictions on height between decks, I propose building above ground using standard timber post footings.

Let me call on our experienced members @Nailbag, @AlanM52 and @Dave-1 for their recommendations.

If you need further assistance, please let us know.

Eric

dave_022

https://www.workshop.bunnings.com.au/discussion/comment/59467#Comment_59467

Thank you for your reply. As far as I am aware there are no restrictions for space an height on decks under other decks. I am in Townsville and the local council regulations stipulate planning permits for all decks as it is a cyclone area, this focuses on adequate anchor support.

As mentioned, the current ground to joist for the existing deck is 2100mm, which is minimum height for non-living rooms; technically this is not a room.... but if I add a deck at say 200mm above ground then this space will reduce to 1900mm which would make it very tight for taller people.

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/92151#Comment_92151

Good Morning @dave_022

I am hedging away from installing a raised deck. Majorlly because of the height you will end up with. Going to all that effort to clock your head or clip your arms as they wave about tends me to think of some other type of surface.

I dont like the idea of digging the surface down by much, more so because of the concrete foundations of the piers you have then drainage.

Living in Townsville you will have periods of intense rain that will dump a lot of water. Mmmmm

As much as you may not want to, maybe paving becomes an option. Your yard looks like it slopes towards the back fence so you can have some slope to it to help with pooling. Instead of the stock standard paving you could go with the larger sandstone type pavers/tiles which would match well with your garden and pool. Tuscan Path 600 x 600 x 20mm Sand Porcelain Paver Tho the picture shown dosnt do the tile justice.

If you paved it you could keep your 2100mm distance, I would really try not to decrease that.

Dave

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/92151#Comment_92151

I'd tend to agree with @Dave-1 that paving might be a better option, considering the challenges @dave_022. However, if you can install adequate drainage within the lowered area and have the ability to send that water elsewhere, I don't think digging down would be out of the question.

You would lower the area, then dig a trench around the area down another 200mm. Place your agline within it and backfill the trench with gravel. The agline can then be run out of the area to a stormwater pipe or an area to runoff.

Mitchell

Nailbag

Hi @dave_022

Looking at the photos there appears to be the concrete foots already starting to be exposed around the top deck posts. Though probably not likely, but digging down could potentially compromise their integrity.

So, with that in mind I too also agree with @dave_022 and @MitchellMc and consider paving instead. This means only having to level the ground of the high-points, laying a min 50mm sand base down then 40mm pavers.

Nailbag

dave_022

https://www.workshop.bunnings.com.au/discussion/comment/59526#Comment_59526

I have passed the idea of pavers up to the boss and she didn't seem opposed to the idea.

Given the great advice on here, I think I will go with that option. The other advantage is that under the matting is a broken concrete path as well so I may be able to remove some of that to tidy the who area up with pavers as well.

I will post some photos as I go along

EricL

https://www.workshop.bunnings.com.au/discussion/comment/92180#Comment_92180

Hi @dave_022

Please keep us updated with your progress, we look forward to the start of your paving project.

Eric

dave_022

https://www.workshop.bunnings.com.au/discussion/comment/112147#Comment_112147

So, I managed to put some work in last weekend. It turns out the ground was so hard I had to hire a 1.8T digger; but got down 150mm in the end and backfilled with 100mm of road base. Then it was Christmas, and now it is raining.... hard.... water is pooling at one end. Is there anything I should consider doing at this point or will it work out when the sand and pavers are in (bearing in mind they will slope towards back of garden)?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/123842#Comment_123842

Hi @dave_022,

Water is naturally going to pool there if it is lower than the rest of the area. Regardless of whether you are adding sand and pavers over the top, water will still pool like it is currently doing; it will just do it beneath the sand and pavers.

I'd suggest you need to install agi pipe, in that area and run it to a suitable drainage point, preferably a stormwater pipe. If you run it to a stormwater pipe, you will need a plumber to assist with the connection.

Check out How to install garden drainage and How to fix a muddy lawn for some further guidance.

Let me know what you think, and if you have further questions, please don't hesitate to ask.

Jacob

dave_022

https://www.workshop.bunnings.com.au/discussion/comment/131281#Comment_131281

Unfortuinately the storm water pipe runs down the side of the pool closest to the house and as the pool slab is 100mm higher than the rest of the area then that is not a direct connection. The other side of the slab runs very close to the fence to another block wall, so no way out there either. Would some kind of sump and pump work?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/135819#Comment_135819

At the moment, it looks like you’ve essentially created a low point where water collects @dave_022, which is why it’s pooling so badly. A sump and pump could work, but it’s worth considering whether there are simpler ways to manage the drainage first.

For example, what’s to the right-hand side near the shed? If possible, you might be able to dig a short ag line there to divert some water away into another part of the yard. Also, the lowest point appears to be down at the fence. If the aggregate base is levelled correctly, a lot of that pooling should reduce naturally. Once your paving is installed, water won’t penetrate as easily and will mostly run off, especially if you grade the paving slightly towards the backyard.

Given the current water level, I’d recommend building up the aggregate so the top of the base sits at or just above the current water level. Then when you lay your paving, it will sit higher than the water, so even heavy rain shouldn’t cause pooling on top of the pavers.

Essentially, ensure the aggregate is raised enough to give the water somewhere to soak in without sitting on the finished surface.

Mitchell

dave_022

https://www.workshop.bunnings.com.au/discussion/comment/138923#Comment_138923

Thank you for the advice. Since my last post it has rained some more which has been more telling. Given enough rain the whole area fills up.

As seen in the photo we intended to level the pavers with the path next to the wall and adding road base to the water level would create a step up to this.

I think a large part of the problem may be the downpipe wchich runs on to a paver and basically into the hole. I am wondering if this was re directed into some ag-pipe running to the back o the garden this may help? I just wish there was a way of getting it to the front of the house as there is a 400mm drop to the. road that side....

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/141013#Comment_141013

Thanks for the extra detail, @dave_022. That extra rain has definitely helped reveal what is really going on. From what you have described and from the photos, the downpipe discharging into the low point is a major contributor to the problem. In heavy rain you are not just dealing with groundwater or surface runoff, you are actively feeding stormwater into an area that already has nowhere to drain to, so it is no surprise it fills up quickly.

Redirecting that downpipe into a properly laid stormwater line would make a significant difference. Running it into ag pipe that daylights somewhere lower in the garden could help a little, but ag pipe alone is not ideal for roof water as it can silt up and it still needs a clear discharge point. If there is no way to get it to the front where you have that 400 mm fall to the road, then your options become more limited and more “engineered”. This is where either a new stormwater connection, subject to council approval, or a sump pit with an automatic pump starts to make sense, particularly if the area is already acting like a bowl during prolonged rain. You could potentially take the stormwater pipe at deck level and send it down the side of the house to the road. It would be attached to the side of the house. With that height and amount of fall you might be able to atleast send the the water from the roof to the road.

You are right to be cautious about building up road base and creating a step or changing levels around the path, as that can create new problems and still will not solve the core issue, which is water evacuation. Before spending too much more time or money, I would strongly recommend speaking with the council’s drainage or engineering department and explaining exactly what is happening. They can advise whether there are any legal downhill connection points available, whether at the rear or side, or whether a pumped solution is acceptable in your area. It is frustrating, but once you have a compliant way to get that roof water out of the yard, the rest of the drainage issues around the deck become far more manageable.

Mitchell

dave_022

https://www.workshop.bunnings.com.au/discussion/comment/142556#Comment_142556

I did have a thought as I walked round the pool. Could I connect the two down pipes as shown and direct all roof water from that side into stormwater?

The white pipe is not overwhelmed and then I would only need to sort out the water that falls directly on the garden,,,

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/143661#Comment_143661

Yes, you could @dave_022. However, you'll need to check where that drainage channel connects to, as you don't want to just shift the problem elsewhere.

Be advised that alterations to roof drainage points could be considered plumbing work, requiring a licensed plumber.

Mitchell

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