I may be alone in my opinion here but I think this is such a non issue that I wouldn't even disclose. When you live on a property with trees or if your neighbors have trees even if you don't, this is just one of the things that can happen and is part of owning a home. If you have trees then you have to clean your gutters regularly, too or they can overflow - again, just part of owning a home, so would you feel the need to disclose that? There are many home maintenance items.that will be part of a learning curve for a new homeowner - that does not mean there is something wrong with your house or that you need to disclose before selling. If you choose to leave a sheet of your maintenance schedule or companies you have used for the new owner, that's a different matter and is a nice gesture.

The roots aren't the only problem. Roots get into the sewer lines through loose joints or cracks in the pipes. The Rootx doesn't fix the real problem.

This problem isn't fixed. Disclose.

Normal unbroken sewer pipes cannot have root penetrations. It's physically impossible. What you have is a problem that you've been addressing the symptom of all of these years. If I were a buyer, I'd insist that the line be replaced. It should have already been replaced as a health concern. It is leaching out contaminants into the groundwater.

hollysprings that just is not true. PVC pipe is impenetrable but there are joints in cast iron and clay pipes that can be infiltrated by tiny almost thread-like roots that grow larger once inside the pipe and then cause problems with back up. I've lived in homes with clay or cast iron sewer pipes my entire life in a heavily treed suburban area and maintaining sewer pipes is just part of ownership and no one thinks twice about it. Here, unless someone tears down an old house and builds new with PVC, the housing stock is old enough that there are joints in sewer lines not only for residences but also for the city lines. As for contaminating the groundwater, the 10 million or so people of metro Chicago seem to be managing and we are hardly the only ones with old housing stock. Not every buyer is going to flip out and demand replacement for a system that isn't broken. We don't know the age of the house but the OP said the line under the house is cast iron.

I am planning to disclose, my question was whether the average buyer would insist on fixing the root issue, which is pretty costly and difficult since the problem is under the foundation, or whether it was normal to have to put root killer down the pipes regularly. I know a lot of people get roots in their pipes, it doesn't always mean the pipe is broken, as dim2000 said, with cast iron, you get roots coming in through the joints. I've attached an article about it.

'Root clogs also can be cleared without replacing the sewer pipe, if identified early, Specht says. Both contractors recommend annual or semiannual drain cleaning, followed by a video inspection.'

Here is a link that might be useful: Clogged Sewer Lines

Cast iron or clay pipes are obsolete and should be planned to be replaced as a normal part of upgrading things around the house. If roots can get in, do you really think that water and bacteria can't get out?

No, they usually line them from the inside out. But, digging may be cheaper in some areas.

I have never heard of lining clay or cast iron pipes. In my experience, they cap off the old cast iron or clay and run new PVC.
IMO, the OP's sewer line is not "performing the purpose for which it is intended". Which is the bar that has to be crossed when determining if something needs to be disclosed, or for what a buyer should expect during inspections.

I am in the camp where I wouldn't think twice about roots in the sewage pipe. Seems like a very common issue around here. My old house had them (it didn't even cross my mind to disclose), and I wouldn't be surprised if this new house did, too. I think we had to call the plumber out two times in the 8 years we lived at the old house. He said some people have a standing twice yearly appointment to deal with it. It almost seems similar to spraying roundup on the weeds that grow between the driveway and the garage.

Spraying roundup on weeds is a voluntary chore that costs a couple dollars. Weeds in a lawn is something expected. Calling a plumber out twice a year to clean out the lines is not voluntary and is costly, and most homeowners do not consider it normal.

We live in a 100+ year old house. We know from cast iron pipes. We have yet to call out a plumber to clear the lines in 21 years of living here. New town sewer lines have been replaced in the road a few years ago, but not the ones that run from the house. Twice a year cleaning? Wow ! That would be something I would definitely expect to be disclosed by the seller. NancyLouise

If you disclose it, plan on them insisting it is fixed before the sale is completed, or they offer much lower (or not at all) due to it. Esp if your realtor is telling you not to disclose, and you choose to, you are putting yourself at a disadvantage of all the others in your neighborhood who are choosing not to disclose at the advice of their realtor. (ethical or not, you will be "the loser" in comparison to your neighbors).

I'm not cleaning out the pipes twice a year, I put root killer in the pipes twice a year. You just pour it into the toilet and flush.

I had to have the roots cleaned out right after I moved in, then started using the root killer, then had to have them cleaned out again when I forgot to use the root killer. In 11 years, I believe I've had to have Roto Rooter come out 3 times. Obviously if I had to have that done twice a year it would be a much different issue.

Some cities will inspect the sewer lines for free. The downside is if they find anything wrong they will charge a surcharge on the sewer bill if the problem is not corrected within a certain time frame. It might be worth looking into.