Scottville, 49454 Northwest Michigan | Dennis,
Here is a drainage calculator from ADS. http://www.ads-pipe.com/en/resources/calc_ag_drainageknownAcre.asp
We use to always use 3/8 inch as our drainage coefficient. (.375" water removal in 24 hrs.) But all new tile installation is at 1/2" drainage coefficient for us now. This naturally requires larger mains and/or greater grade. As you'll notice from the calculator, you need to set the drainage coefficient that you want to use.
One of the previous posters to your question mentioned the problem of silting in the tile. We've gone to "sand-slot" or "knife-cut" tile to help deal with that problem. You might want to look into, or consider that.
As to your question of whether to have more grade in the laterals or the mains.....like the rest, I would think it would be better to have the greater grade in the mains.
good luck with your project, gordon
Edited by briggsfm 11/20/2009 14:36
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tiling question on grade for mains and laterals
