How to add a French drain to stop shed flooding?

[fitzy87]

Hi all,

Recently moved to a new property and the shed slab has either sunk over time and the grass has raised.

The shed floods in heavy rain and where the colour bond meets the grass has rusted.

I have started to dig a trench to make a French drain as the water is pooling very bad and entering the shed.

The shed has down pipes that run to the house downpipes and into the water tank. I don't think it will be a good idea to run the flooded water into this system.

Could I get away with a French drain with no outlet ? Plan to use 10mm bluestone, corrugated socked 100mm piping and geotextiles matting and about 200mm deep.

Appreciate any guidance with my first new project. I should mention I do not have the ability to connect the French drain into the normal storm water piping due to retaining walls and other landscape.

     

[MitchellMc]

Welcome to the Bunnings Workshop community @fitzy87. It's sensational to have you join us, and many thanks for your question about creating a french drain.

You're definitely on the right track thinking about how to manage the water pooling around your shed. From what you’ve described, it sounds like you’re actually creating more of a soakwell rather than a traditional French drain. French drains are typically designed to capture water and redirect it to another location—usually downhill, into a stormwater system, or to a lower-lying area in the yard. Since you mentioned you don’t have access to stormwater due to retaining walls and other landscaping, and the area tends to hold a lot of water, a soakwell system is likely a more suitable solution for your situation.

To make this work, you’ll want to dig a deeper and wider pit than a standard French drain—ideally around 600mm deep and 400–600mm wide or larger, depending on the amount of water you’re dealing with. Line the entire pit with geotextile fabric to prevent soil from clogging the system over time. Fill the pit with aggregate until you're about 100mm from the top. Fold the fabric over, then top it off with soil or grass, depending on what you’re planning for the surface.

If you can, it’s still worth investigating where your nearest stormwater line is—even if it's not close—because that would offer the most effective long-term fix. 

Please let me know if you have any questions.

Mitchell
 

[fitzy87]

Thanks so much Mitchell that's a great idea. So to confirm I woudnt need the corrugated socked piping would I. Just dig down deeper and then backfill with my 10mm bluestone.

[JacobZ]

Hi @fitzy87,

That is correct. 

The idea is that the gravel acts as a path of least resistance down through the soil.

It will then sit at the base of the trench and slowly drain into the earth beneath.

Let me know if you have any further questions.

Jacob

[fitzy87]

Thanks Jacob. I have started to dig the trench 600x600 , I have the geotextile fabric to line and pegs to hold in place.

I have done some measurements at 8m long around the shed and 600 x 600 I will require approx 3 cubic metres / 3 tonne off 20mm arragate gravel. Does this seem correct?

Some more photos for anyone with the same problem to track my progress.

 

    

[JacobZ]

Hi @fitzy87,

Great work with the digging so far, I'm sure you've had quite the workout.

My calculations also come out to roughly 3 cubic metres.

Thank you for the update.

Jacob