I hope you watched how they cleared it, and the equipment they used. Are you talking about the "main drain"?

If it is the main drain, maybe you have tree roots growing into a crack in the line. I know someone who, for the time being, has their main drain power-augered each year to a length of 140' to get to the obstruction, which is just before it meets the street line and is beneath a massive oak tree.

BTW, maybe a different toilet paper could help a little. Some of them disintegrate better than others. Eg. while the kirkland brand from Costco is premium and nice to use, it seems that it wouldn't fall apart easily (based on using it like a paper towel an seeing how it holds together.)

The blockage is in the basement so no tree roots.

It's an old house, the main trap is cast iron and 6" not 4 as I mentioned.

Thinking about it more I finally realized that heavy materials in water from toilets and sinks must sink in the bottom of the main trap and build up, needing periodic clean out.

I looked at what the plumber did and it is a vertical pipe attached to the top of the U on the trap with rubber joints fixed with clamps so easy to get to.

They tried a roto rooter thing without success. The clean out is about 12' from the trap with a right angle turn in between. It was only after they did the upright and were able to reach the trap directly did they clear it.

I guess I've answered my own question! It was later after posting I got to thinking about the settling thing.

grease and other soluables stick to the sides of the pipe. in the trap they build up a little more and this catches everythign else. then the clog forms. changing what goes down teh drains is a better approach to longterm fix of the problem. drain your meats in a bowl and toss teh grease in teh trash instead of lettign it go down the sink. don't wash out paint brushes/etc in teh sinks/tub. and put only human waste and toilet paper in the toilets, everything else goes in teh garbage can.

If you have an old enough installation to have a trap in the main line that is part of the problem.

They have been prohibited for many years since they catch stuff and clog.

The pressure in a drain line is very low, essentially zero unless a clog develops.
Once water starts to pile up, it takes an awful lot of backup to get any real pressure. Water about 28 inches deep only creates 1 pound per square inch of pressure.

buckeye

That's interesting about the need of a trap! And good advice on what to put down the drain.

Anyway problem solved as the man that winterized our house prior to leaving for CA for two months came and found the blockage about 6' down after the cleanout. Removed it easily.

He said that once the water is drained and the house shut down whatever is in the pipes eventually dries up, becomes stiff and maybe even freezes. Gets hard to move.

Wow I'm so glad he diagnosed the problem as I was all ready to dig in trying to unplug the trap.