Cleaning out main sewer line

My main sewer line is backed up. Anytime a faucet runs or the shower runs, water starts filling up in the boiler room from a small pit in the back of the building. Sewage smell then wafts through the basement into the building. The water accumulates in the boiler room and then very slowly heads down the floor drain in the basement.

I've looked for a clean-out but haven't been able to find one. This is a 100 year old building in Brooklyn, NY.

I called a local drain cleaner "$49.95 drains". The tech told me the house trap is buried in the dirt pit in the front ( The main water supply comes up through that pit.) He wanted $250 to expose the house trap. I told him I'll dig the trap up myself.

I've dug about 2' deep so far and no trap. It's tedious work.

What are my options on cleaning this main sewer line blockage up? Here are my guesses from googling....

1. Pull the basement toilet and snake it.
2. Cut the fresh air vent and snake through the pipe back toward the building. I'm assuming the fresh air vent is connected with wye to the main sewer line.
3. Dig the front pit to expose the trap, if it's even there. Then I'd have to break the rusted plugs with a cold chisel. Then snake the street side followed by the house side.

What size snake should I get? Cable size and length? The building is 60' long and the run to the street is another 20'

I do suspect trees roots are the issue. My neighbor has a large tree close to my sewer line. And there's a tree on the sidewalk that could be growing into my sewer line.

Thanks for the advice.

I would recommend using a reputable drain cleaning co....if you have never used a snake before and you have roots this is not the time to learn..the cable can bind up quickly
next thing you know is your fingers are wrapped up in the cable

I wouldn’t try it if it’s your first time. Also before you start digging I would recommend finding out how far down your housetrap is. I’ve seen some deep ones and you can get a sense by looking at the drain pipes or asking your neighbors if the houses are similar. GL

For 250 you may be better off having him find it and then you will always have its location. I have a feeling when its all said and done you will be happy you paid that 250.

I hired a drain cleaner guy who tried to snake it from my basement toilet. The snake came out through a floor drain in the back of the house.

So I dug the pit where the trap is located. I found the a running trap with vent down there.

I'll call the drain tech back and have him snake to the street and then to the back of the house.

Thanks!