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My area gets a lot of precipitation so I want to install French drains and rebuild some retaining walls. I know I need some perforated corrugated pipe, but I’m not sure which diameter to get. Should I get 4 inch or 6 inch?

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  • How much is "a lot"? Is it continuous drizzle or does it come in bursts?
    – Niall C.
    Sep 17, 2021 at 3:51
  • @Niall C it comes in bursts. We’ve had severe flooding a few times this year, where a lot of water comes down at once.
    – LRitter
    Sep 17, 2021 at 11:54
  • I used 6" plastic pipe . Feb 15, 2022 at 15:37

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French drains are a great way to control the flow of water on your property. Building them also requires:

  • gravel without the fine dust found in crushed limestone (it can plug holes and hardens)
  • a good quality geotextile fabric. Don't cheap out on the fabric! Thin or cheap landscape fabric can tear and allow soil to plug the drain.

Usually there is not a lot of cost difference between four and six inch corrugated drain pipe. However a larger diameter takes more work to dig a trench.

Fittings, such a Y connectors and caps are more commonly available for four inch diameter.

If you have the space for the trench and are planning for a "once in a hundred year" precipitation event use two four inch diameter pipes.

The last thing that will help your project is to address where the water will go. Draining onto your neighbor or public property is not always a good idea.

This is where a dry stone well comes in very handy. Determine the lowest area where your drain pipe will finish. If you have plastic buckets that are twelve inches (~24 cm) in diameter cut the bottom out and dig a hole to hold the bucket. Fill the hole with large rocks, uneven size is better to make large pore spaces. Position the drain pipe over the dry well and cover with geotextile or landscape fabric.

Top with soil and you are done: an invisible drain system

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