How to build a French drain along a fence line?

SmeesDIY

Hi all,

I’m hoping to get some DIY tips on building a French drain along a fence line on my property. Our yard slopes down toward the fence and whenever it rains we get pretty bad pooling. It’s starting to rot the fence, so I’m keen to fix it properly.

My rough plan is:

• Run socked ag pipe the full length of the fence (about 30m give or take)
• Continue it under the front gate
• Direct the water to the front yard so it can run off into the street gutter
• Finish the top with decorative stones

A couple of things I’m unsure about:

• Any tips to make sure I install the French drain correctly (depth, gravel, fabric etc.)?
• What’s the best thing to do with the excess soil from the trench?
• Can I plant star jasmine along the fence line if the drain is slightly offset from the fence?

Would really appreciate any advice, tips, lessons learned or a step-by-step guide if you’ve done something similar. Thanks!

JacobZ

Hi @SmeesDIY,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

You are on the right track with your plan, and a properly installed French drain should significantly reduce the pooling issue along your fence line.

You might like to check out @Adam_W's guide on How to install garden drainage as a starting point.

https://www.workshop.bunnings.com.au/discussion/97183/how-to-install-garden-drainage

One of the most important aspects is the fall towards the outlet. A general guideline is a minimum fall of 1:100, which equates to a 10 mm drop per metre of pipe. For your 30 m run, that would mean a total fall of around 300 mm from the highest point to the outlet. If you can achieve slightly more, such as 1:80, that will improve flow, but consistency is more important than steepness. Avoid any low spots where water could sit.

You'll want your french drain to be at least 300mm wide and 300mm deep, to accommodate a base of around 50mm of drainage gravel, then your agi pipe with gravel on each side, then 100-150mm of gravel on top. As the fall needs to be consistent across the full length, this needs to be considered when digging the french drain, but at the highest point, it should be at least 300mmx300mm.

You should also consider the fact that if the agi pipe simply discharges on to your front lawn to then run out to the gutter, that this can cause erosion and contravene council rules. While natural runoff is generally acceptable, concentrating water and discharging it generally needs to be done to a lawful discharge point, so it is worth checking what is considered a lawful discharge point in your local council area. Generally, this means having a plumber attach the agi pipe into a stormwater pipe, or by having them run the agi pipe directly to an outlet in your gutter.

In terms of materials, make sure you use a quality non-woven geotextile fabric to line the drain, ensuring it covers the base and both sides of the channel with spare at the top to fold over. This https://www.bunnings.com.au/grunt-1-x-10m-non-woven-geotextile-membrane-drain-mat-fabric-black_p0320352 is a good option. You can then place your drainage gravel in the base of the channel, then lay your socked agi pipe over the gravel, before pouring additional gravel over the top and around the sides of the agi pipe. Once the channel is full, wrap the fabric over the top to finalise the french drain. You can then put your choice of soil or decorative stones over the top.

For the excavated soil, reuse is ideal if possible. It can be used to regrade other parts of the yard or build up low areas. If the soil is clay heavy, it is best not to place it back over the drain zone, as it will restrict infiltration. If that is the case and you have no use for it, then I would see if there were any business in your area that do free soil collection or advertise it to see if anyone would like to collect it.

There shouldn't be any issues planting star jasmine against your fence as long as the drain is slightly offset.

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

Jacob