plumbing topology

Hi,

I'm considering replacing all of the existing PCV plumbing under my house. I suspect that the layout must be incorrect because I always get a clog at one end about every 3-4 years.
It's a ranch one story home and a pretty simple linear existing layout. The far end is the kitchen sink and down about 35 feet is the bathroom tub (this is where the clog usually is), another 5 feet and then a 90 degree turn or "L" and there is the bathroom faucet connected to the vent (2" pipe I believe" then the sewer pipe "T" section, (toilet is right above here) then on to the septic tank.
Does the main pipe which feed into the septic tank have to be a certain size? It's 2" I believe, but so are the feeds from the sinks and tub. They are either 1 1/2 or 2" but definately the same size. I'd like to know what a good physical topology should be so there is good flow. Seems like I should have 3" main drain and eliminate the "L" turn in the main drain?! I can post a link to a diagram if I confused anyone. Thanks for your comments

Helwoe!

Are you saying your main drain is only 2"? I find it hard to believe that a home built in the modern era would be plumbed that way, but strange things happen. The main drain to the sewer should be at least 3", although 4" would be better. If you only have a 2" drain that's what is causing the clog. I wonder if the person who plumbed this bothered to give the system enough slope, too. A drain is required to have 1/4" slope per foot in length. 90 degree turns should be made with long sweeps rather that sharp elbows. I suggest you contact a local plumbing contractor to analyze your entire drain system to determine just what you have and what should be done to it.

Thanks for the reply. It's not a modern built home, 1950's. The steel drain pipe was replaced probably a couple years before I bought (1993) I can tell because they left the old pipe under the house. I double checked and yes the main is 2 1/4 diameter PVC. I was planning on doing the job myself since I'm short a couple hundred dollars... Sounds like if I can make the turn using 30 deg angles or a couple 45's. I believe the slope is close to what you said but could be sloped even more. I'm guessing the connections from the sinks/tub should be angled in as well.
We just had the whole house insulated top, sides and bottom and now it seems to have sealed in mice. Guess I'll lay down some poison when I go down below.

Helwoe!

The toilet drain needs to be at least a 3" line.

You would be better off trapping mice than using poison.
I think the idea of leaving dead rotting mice in the crawlspace will wind up stinking.

Thanks for the reply. The toilet sits on top of a cast iron pipe (I think it's 4" at least) with two side inlets, one to the septic and other connected/reduced to the existing 2" main drain. Still can't believe someone used such a small 2" size main. I can tie into that with 4" PVC with a boot joiner.

I suspect it was the "handy-man" the realtor hired before we took possession of our house. There was dry rot in the bath room that had to be independently repaired. Cost me $500. The idiot did a really poor job so I had to re-do the whole thing myself. He probably did the plumbing as well based on how well thought out it was, NOT. I agree with the mice, I decided traps make more sense especially with a couple of small dogs we own.
Had it been still summer we probably wouldn't have this problem. Oh well..
Thanks for all the input.

Cheers!

Helwoe!

Sorry I confused myself as well. 4" cast iron with two inlets and one outlet to the septic tank. One inlet is from the toilet and the other 3" (outside diameter), the inside is actually a bit less than 3" I used a rubber coupler to my 4" PVC main drain.
The crawl space is three feet so it's been a real pain doing the work.

Helwoe!

drain pipe maintenance

Well job is finished and the only thing that stood out besides the small 2" main was that there was a 7 foot sag between the far end connection to the kitchen drain. This whole section was packed solid with the white clay gunk. And about 3/4 the way down from the tub and all the way to the 90 degree bend was a slight sag and partially full of same. I replaced the main with 4" and spent a good 2 hours making sure there was a 1/2 inch drop every couple feet and put in pipe hanger supports about every 4 feet.
I wish I'd done this 10 years ago when I was in better shape.

Does anyone suggest a good maintenance to keep the pipes clear? I've heart baking soda and vinegar every couple months but that doesn't seem to get past the P trap to do any good. I really Appreciate the replies folks. Thanks!

Helwoe!