I plan on using 4" corrugated pipe with perforations (weep holes) to drain my back yard. I understand I must use crushed limestone in the trench and place the weep holes facing down in the trench, but is there a required slope (1/4" or 1/8") per foot? Thank you.
I'm no expert on this, but if I were doing it, I would put 1/4" of slope on the drain tile, just as is required for plumbing drain pipe.
Putting the drain tile in the middle of a sloped trench of gravel is the way to go, but in my opinion, if you put the drain holes on the top and bottom, the tile won't be carrying the water anywhere and it might as well not be there.
I would put the drain holes on the sides so that the excess water can be gathered and carried away in the bottom half of the pipe to wherever you're discharging it (drainage ditch, storm drain system, dry sump, etc.).
And I would most definitely put a fabric sleeve (made for the purpose) on the drain pipe to keep out clogging dirt.