Plumbing in the Home/3 bathroom drains plugged and causing back flow
My first floor bathroom has developed very slow drainage, and black soot-like grainy stuff backs up in the shower pan when the toilet is flushed. It has overflowed to the floor a couple times.
I have paid to have the drain snaked-out, but the snake won't go all the way through from the shower pan through the clog (water drainage doesn't speed up), as though it gets stuck at a corner joint and can't get through the bend. Then I had the snake enter through the outside access pipe that is supposed to go all the way across the house in the crawl space, and the snake seems to get stopped before it goes all the way across the house crawl space (approx. 38 feet, but stops around 30 feet in).
There is no other external entrance plug to the main cross-house drain pipe to provide a snake access from the other side.
So far, only the 1st floor bathroom on the South end of the house is experiencing the dirty back flow. The upper floor bathroom above (and slightly East of) the first floor bathroom seems to be able to drain without causing back flow in the 1st floor bathroom, so it seems that the 2nd floor vertical drain pipe does not feed into the 1st floor drain pipes before feeding into the main North-South drain pipe, which has a slight decline from North to south to the point where the main line going East to the street leaves the house.
I have had a quote of about $2400 to add a couple possible external drain access pipes to allow the snake to enter the house from a closer location to the 1st floor bathroom, but I'm concerned that this may not even correct the blockage, since the snakes have already been used from two points (shower pan and access plug on opposite side of the house) without much effect.
I'm guessing that the blockage is fecal matter and residual toilet paper, due to the black soot-like back flow grains, so I was wondering if it would be worthwhile, and safe, to pour some septic tank additive down the drain to dissolve the gunk? Or what about Liquid Plumbr or other "drain cleaners"--though I've heard that "drain cleaners" may be corrosive and possibly harmful to the pipes.
Finally, would you recommend monthly doses of drain cleaner or septic additive to attack buildup of gunk in the drain pipe, rather than waiting until there is a blockage?
Lots of stuff going on here. Based on deduction, it appears that there is a clog in the mainline serving that downstairs bathroom somewhere before the upper floor mains connect. There is another option... Pulling the toilet in the downstairs bath and snaking through that drain line. There is no trap in the drainline for the toilet since the trap is built into the toilet itself.
As far as the snake getting stuck and not be able to cross the house completely, it is possible that there is a backwater valve in the system. These are often installed if your house sits below the level of the main sewer line it feeds in the street. This prevents a backup in the main sewer line for the city from flooding back into your house. If this is the case, it is very important that the snake operator knows that one of these exists. If they do manage to force the snake through a backwater valve they will not be able to pull it back out and now the problems really begin! If there is a backwater valve installed it must have an access point so that it can be opened up and serviced or repaired.
You did not say if your house is on a slab foundation or a raised foundation. If it is on a slab, there's not a lot you can do as far as physically inspecting the actual piping. If it is a raised foundation, someone can go under there and look at the piping to see if there is a back slope in the line somewhere. If the slope, also called "fall" is not enough solids will settle out in the low sloped portion and build up.