Need drainage and ground levelling advice for backyard

[we2jin]

I'm seeking some advice regarding a drainage and leveling issue in my backyard.

About a year ago, I had six rectangular concrete slabs (approximately 1m x 2m each) installed in the backyard. Unfortunately, the slope wasn't done properly at the time. While there haven't been any drainage issues during regular rainfall so far, I am concerned about the potential for flooding during heavy rain events, like those in 2011 or 2023.

The red circle on the attached image indicates the location of the drain. It's slightly raised, and I’m worried that in extreme weather it may not function effectively. Additionally, my overall ground level is a bit lower than my neighbors', so I’m considering raising the ground level by around 8cm.

The area marked in yellow is approximately 4.4m x 3.5m. I'm thinking about laying gravel in this space. One option is to create a slope using crushed dust, then install a weed mat, stabiliser grid, and top it with gravel. Another option is to simply place the stabiliser grid and gravel directly on top of the existing concrete slabs to raise the surface, as creating a proper slope by myself may be quite difficult.

One of my concerns is whether water wil actually flow toward the drain when using crushed dust, or if it will wash away during heavy rain. Some people have also said that using crushed dust won’t really solve the underlying drainage problem.

I'd really appreciate your professional opinion or suggestions on the best approach. Thank you in advance for your help.

 

[JacobZ]

Hi @we2jin,

Thank you for your question about improving drainage in the back patio area of your home.

Is the issue that the concrete slabs fall towards the house? Or just that the drain is slightly above their face?

Unfortunately, adding drainage gravel or crushed rock over the concrete would not solve the issue, as the slabs will still be sloping underneath the gravel. Gravel is useful in drainage because it is stable, while still allowing water to flow through it. In your case, it would allow water to drain between it until it hits the concrete slabs, at which point it will flow in whichever direction the concrete slabs are sloping. 

If the main issue is the height of the drain, your best option would be to have a plumber come out and reduce the height of the drain to below the height of the concrete slabs. 

If the slope of the concrete is the main issue, then you will either need to have them ground down to add a slope towards the drain or away from the house or have them lifted and packed out so that they slope away from the house.

Something you could consider if you haven't already done so is having agi pipe installed against the house side of the concrete and having a plumber connect it into the existing drain. This would help to route any water that flows towards the house away from it and into the existing stormwater system.

Allow me to tag @Noyade, @TedBear and @Nailbag to see if they have any thoughts.

Let me know what you think.

Jacob

[we2jin]

Thank you for your reply.
The drain is slightly higher than the surface of the concrete slabs. There are six rectangular concrete slabs with approximately 10 cm gaps between them, and 1 cm of gravel has been laid in the gaps.

Although water that doesn’t go into the drain seems to escape through the gaps between the slabs, the ground beneath the concrete slabs is quite hard. It has been about a year since the slabs were installed, and there haven't been any major issues so far. However, we haven’t experienced any heavy rain like in 2022, so I'm not completely sure how it will perform during extreme weather.
Do you think this will cause any serious drainage problems?

Also, you mentioned using crushed rock, but would it still work if I used crushed dust to create a slope? Wouldn’t the water still flow over the crushed dust?
Are crushed rock and crushed dust the same material?

[JacobZ]

Hi @we2jin,

It would be hard to say how your drainage would cope with a major weather event without seeing and experiencing it firsthand.

What is referred to as crusher dust is not the same as crushed rock or drainage gravel. Crusher dust is a mixture of fine stones with extremely fine aggregate that can be used as a compactable base material. It is less free-draining than something like drainage gravel as the fine aggregate plugs up the gaps between the stones. It would, however, still allow water to drain through it, just less so than stone.

The problem I have with adding either crusher dust or crushed rock over the concrete to create a slope is that water will still drain through them. Crusher dust would be better for creating a slope that water would flow over, but it would still allow water to drain through it in the same way regular soil does. Concrete, on the other hand, will cause water to pool on it. 

If you were to add crusher dust over the concrete to create a slope, the water would still drain through it and sit on top of the concrete, draining as it is now. All you would be doing is creating another layer for the water to drain through.

My honest opinion is that you likely don't need to worry about it. If you have seen it handle rain this year without any concerns, you may experience issues with a major weather event, but adding a slope is not going to fix this issue in the event of a flood.

I hope this helps.

Jacob

[we2jin]

Thank you Jacob 

Your advice really helped me and clarified my confusion.