In another question I was advised to use pea gravel for french drains.
Now, as it happens, a previous owner put down a lot of weed matting and covered it with perhaps 5cm of gravel. My best guess, from looking at this page, is that the gravel is equivalent to 3/8" crushed stone. Can I use that instead of pea gravel?
Crushed gravel is the media of choice for providing drainage. What you want is to have the maximum amount of airspace in the drainage layer so water has somewhere to go. Normally you would use 5/8" or even 7/8" crushed gravel but 3/8" will do. Pea stone gravel would not be my first choice as it will pack down over time and provide less airspace than the irregular shapes of crushed gravel. If you want to be diligent you can give the gravel you dig up a quick wash to separate out any dirt or other loose material. And, when in doubt, add more drainpipe with sleeve! There is no reason you cannot have several layers of drainpipe on top of each other in particularly difficult to drain areas.
