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Hello @diohoh
Thank you for sharing your question about how to build a French drain.
Building a French drain is a great way to manage excess water around your property, especially in areas with poor drainage. To start, you’ll want to dig a trench that runs from the area where water tends to collect to a lower point where it can safely disperse this might be a stormwater pit, dry well, or the edge of your yard. The depth can vary depending on your specific needs, but most residential French drains are around 450–600mm deep. If you're dealing with significant surface water, you might go shallower, but if you’re targeting sub-surface water or near foundations, you may need to go deeper.
Yes, the trench does need a fall, a general rule of thumb is a gradient of about 1 in 100, which means a 1cm drop per metre of length. This ensures that water flows properly through the pipe instead of sitting stagnant. Once you’ve dug the trench, line it with a quality geotextile fabric, which helps keep soil from clogging the gravel and pipe. Lay a bed of coarse gravel at the bottom, then place a slotted or perforated drainage pipe like Vinidex 100mm x 20m Socked Slotted Draincoil. Cover the pipe with more gravel, at least 100mm above the pipe.
If you prefer, you can leave the top layer as gravel, which is perfectly fine for areas where you want visible drainage or a low-maintenance finish. Just make sure the gravel is compacted enough to stay in place. Alternatively, you can fold the geotextile over the top and add a layer of soil and turf or mulch to blend it into your garden. Either option works—it just depends on whether you want the drain hidden or part of your landscaping.
Just a friendly reminder that you'll need the services of a licensed plumber to link the French drain to your storm water pipe for safety and compliance.
If you need further assistance, please let us know.
Eric
