Slow drains and backup

Hi,

Bought this 50 year old house two and a half years ago and have been having problems with the drains since the start.

The main bathroom on the main floor (it is a large bungalow with a full basement) has two wash basins, a bidet, toilet and bathtub.

The double wash basins' drains are connected together and the traps and visible pipes are now ABS, showing that some work has been done previously since the rest of the drain pipes (except those newer portion and the ones we had replaced while redoing the kitchen) are steel. I thought they were cast iron,but was told they are actually steel.


The issue is that those two washbasins keep backing up with dirty water that looks strangely like it contains soil or black organic matter from soil. The other drains in the house (whether in the kitchen on the main floor or in the basement bathroom and laundry room are more or less normal in that they drain (a bit slow to my eye) and have no backup.

I will get a plumber to look a this situation, but before I would like some idea of what could be wrong.

Could it be the main house drain connecting to the city drain that is cracked and allows soil in?

If so why would it be only the two washbasins and not also the bath and toilet and bidet that have traps and connections that are lower down than the washbasins?


Could it be something in the roof-vents?

There is also a tell-tale sign that bothers me: the drain pipe in the bathtub and in one of the basement washbasins do not have that cross shaped guard to prevent large objects from falling in. Does this mean that some work has been done to either send a camera or Roto-Rooter type tool down there?


Should I call a regular plumber or one of those outfits that specializes in drains?


What should I be leery of in any estimate given?

BTW chemical drain opener helps the situation for a while but it comes back soon enough.


Thanks

If you main drain lines are galvanized steel, (it's more often cast iron) then there may not be much you can do except repipe. While they can last a very long time, depending on the situation, they can fail early on. They tend to rust from the inside out, and that rusty interior gets very rough which snags all sorts of things easily (hair, lint, etc.). CI could last a century, or not. It's hard to say how long plastic pipe will last, but it could be a very long time as well. At least it doesn't rust!

Find a plumber or drain guy that has a sewer cam. He will be able to locate the source of your problems. Vents NEVER cause plugged or slow drains.

AN incorrect or the lack of a vent can cause a trap to siphon dry (which can be serious), but other than that, not normally many other issues.