What size and slope for main drain?

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DIY Webber

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Orangeburg SC
A few questions concerning main line.

What size (3 or 4) should I use for main drain line. Per code, a 3 inch is large enough, but was thinking about using 4". Considering nothing other than performance will going with larger pipe be better or worse?

I have plenty of headroom in crawlspace, SO....is it best to get more fall, say 3% slope verses 2%?

OR what combination of size and slope is best?

Thanks
 
To much pitch and water will travel faster then the solids leaving solids in your pipes
 
In my area, code allows 3" for a standard 2 1/2 bath house. Minimum for 3" is 1/8" per foot.

Personally, for a small 2 bath house, I would install 3" with a little bit more fall than 1/8" per foot, say 3/16" per foot.

The past few years doing drain calls, along with camera inspections on a lot of the clogged drains, the single biggest issue I see is "flushable" wipes in cast iron. Next in line is bellies (backfall) in PVC.

I do occasionally go clean a re occurring clog in a 4" PVC line. I have been getting calls for this line for 3 years now. The original plumbing contractor has already cut open the floor and replaced the double wye that caused the original problem. There is absolutely no backfall on this line, it drains completely once I clear the clog. But it has a 4" line serving 3 toilets and 2 lavatory sinks. And every time, it clogs in the 4" past where all of the toilets connect. So this makes me think that the added velocity of a smaller line very well might make a difference in keeping problem lines open.
 
I agree. With today’s ultra low flow toilets you get less water to carry [emoji90] down the tube. Use 3” and will carry waste a bit farther
 

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