Under Oregon landlord tenant laws can tenant be billed for toilet overflow cleanup

Billed for toilet overflow cleanup.: We moved into our current residence in Portland, Or in April. The bathroom in the house is old and the toilet has been slow flowing since the day we moved in. In early October it overflowed and waste material leaked into the heating duct. We contacted the property management team and next thing we know a company calls advising us that they will be out to clean up the heating duct. The week after the cleanup the property managers husband who is the acting maintenance person, came to the house the "check" the toilet. We advised him that the toilet has been slow flowing since we moved in, but this was the first time it overflowed. He flushed some toilet paper down the toilet and commented that it appears to be working fine. Within a few days we received an invoice and letter from the property manager advising us that no fault was found with the toilet and we would have to pay the bill. We challenged this as her spouse is not a licensed plumber and we advised that the toilet was inadequate in size. She contacted the homeowner who agreed to send out a plumbing service at this expense. The plumbing service came to the house and agreed that the toilet is a low end toilet with a small throat and this problem could potentially come back.
A week later, we received another letter from the property manager, stating that the plumber’s findings were consistent with their initial findings that we are "at fault" and need to pay the invoice while also using a comment in the letter which was made when the plumbers were here and taking it completely out of context. In the second letter she does recognize that the toilet is low end with a small throat and that it could cause a problem for some users in that the toilet will clog with heavy use.
In addition, she highlighted and sent a copy of the original rules signed by us which states the following:
"Expense or damage caused by the stopping of waste pipes or overflow from bathtubs, showers, toilets or wash basins must be paid by residents. Tenant is responsible for all clogged drains after the first 30 days from move in. Tenant is responsible for all clogged toilet drains that they or their guests cause".
Now we understand that gross negligence on our part would cause us to have to pay for repairs, but if it has been stated that the toilet is "low end with a small throat", by the management team themselves, does this not negate the rule above. At this time we are not sure what "heavy use" other than normal use would be as no one can be expected to monitor certain bodily functions when using a toilet.
In this same letter, it is also stated that the owner of the house is willing to “consider” installing a toilet with a larger throat in order to reduce the likelihood that we will have clogs again.
At this point, we are not sure if we have to pay the $470.00 for the cleanup. At no time were we advised that we would be billed for this. Any assistance would be greatly appreciated. - Is this your question? Add additional information