As I understand it, the drain lines are supposed to slope by 1/4" per foot of run.  Several of the lines in my house have no slope at all.  I believe this is because the drain pipes enter the fittings (T's and Y's) at right angles.  

If the fittings don't have a built-in slope, how do you get the slope?  Do you force (bend) the pipe upwards to give it a slope?  

Do you exit the fitting and go into an elbow or Y which can be angled upwards?  Doesn't that impede the drain flow and make cleaning the drain line more difficult?  

there is usually enough wiggle room in the fittings to get the slope. I assume you are talking PVC?

J-

Having 6" of drain pipe with no slope is not really a problem.  20ft of drain pipe with no slope is a problem.

The drain lines I'll be installing are ABS (plastic).  The original lines are cast iron fittings and galvanized pipe.

I'm asking because the tub and shower drain lines that run to the vertical drain stack are level (no noticeable slope).  These level drain lines are about 6 to 8 feet long.  I was thinking the installers did a crappy job.  Then, I started thinking about how they would have done it with such rigid fittings and rigid pipes.  When I replace the lines I am going to give them a slope.  

Is the following a reasonable plan?  

sloped 2" drain line from shower --> long sweep Y --> sanitary T <--long sweep Y <-- sloped 2" drain line from tub


The long sweep Y should make snaking the line a bit easier.  The use of a Y, instead of an elbow, allows for a clean out to be installed at the turn.

Are there any web sites that show the proper way to install drain lines - what fittings to use, what not to use, how to configure them, etc?

For instance, can I use a sanitary cross (tee?) in a horizontal line?  That way, I could have two branch drains enter the waste line at the same location.  Do I have to use two separate long sweep tee's, instead?

Just put the pipe in with the slope the easiest way.  If it's 2" pipe the slope can be had by not seating the pipe in the fitting square, but instead tilted so one side is unseated 0.04" or something just over (0.01" over) 1/32".  Use plenty of primer and the joint will be fine.

ABS or PVC?

ps
either way, be sure to use the correct solvent cement and primer for the pipe type.

ar-jedi

As was said before, there is enough play in the pipe alone to allow the proper pitch.

1/4 inch per ft for 2" and smaller
1/8" per ft for 3 & 4 in pipe
Mainly just enough to achieve 2 ft/ second flow

ABS, which is black in color.  I used an ABS medium cement.  

I spent some time doing a layout of the drain pipes.  I now have a design that drops about two inches per foot and dumps almost directly into the my main drain line leading to the City sewer line.  It will be easy to install, too.

I wish I had a way to present my design for comments.  The City should probably see them, too.

I was hoping to find some waste line design info online.  Are there any web sites that show the proper way to install drain lines - what fittings to use, what not to use, how to configure them, etc?  

For instance, can I use a sanitary cross (tee?) in a horizontal line? That way, I could have two branch drains enter the waste line at the same location. Do I have to use two separate long sweep tee's, instead?

No on san t's on horizontal, and definitely no on a cross.

If you ever had to clean out a line, a snake would go straight across the cross, and not downstream,

All fittings with horizontal flow, or coming from vertical to horizontal should be long radius type fittings.

Too much pitch can also cause problems, so don't get too carried away!

I've never seen one.  There may be a book at Home Depot.  Folks generally just go through the parts bin like it's legos.   Use the cross, T, sanT or Y, whichever gives the smoothest fit and the smoothest flow.  Have designed piping for processing, homes, labs and did the install.  It's like legos.  It's a drain, so there's no complicated design considerations, just slope them (0.25"/ft) so the water keeps moving, but make sure it's not too fast in long buried runs or the liquids will outrun the solids and cause you grief.  As above short straight runs are fine.

Use plenty of primer before using the cement.  Somewhere there will be a vent in the piping you're replacing.  Keep it.  If there is none that's a problem...

That's short, to the point, and is a code requirement.  You're a terrific asset, Jake.

Save the sanitary tees for anywhere in the venting system and the horizontal to vertical waste transitions.  

Always use a sanitary tee for the first vertical drop after a p-trap.  If you use a long sweep tee there is the potential to siphon the p-trap.  

This link is a decent example.  Along with the proper fitting use if you look on the right side you can see how the sink (#18)  "wet vents" the tub and shower drains-  Link