Pet Policy Addition - designated waste areas

[John Intfen]

So I have a 2 story duplex in Houston, TX and each unit has a balcony. During my last visit to the property I noticed one of the upstairs tenants dog was going to the bathroom on their balcony which is right above the downstairs tenants balcony. This is obviously very gross, but when I went to my lease document and pet policy I couldn't find anything that explicitly said this was wrong. 

I am wondering if anyone has an article in their pet policy that prohibits this kind of action and designates certain areas for dogs to go to the bathroom?