Assess new tenant for damage

[Debbie W.]

My new tenants are gainfully employed and very nice. They provided security upon lease signing. 

This past week, the basement became partially flooded, damaging family stuff I had stored down there and building products we had left over. THe waste line spewed waste onto the driveway outside through the vent. We vacuumed up the 2" water in half the basement. THe plumber charged $240 to respond and clear the line of paper towels. THe tenants admitted disposing paper towels into the toilet when cleaning 2 days prior. It took 3 people over 12 man hours to clean up. I was so annoyed as we did this in 95 degree heat. 

I had discussed the rules about what goes in the toilet when we signed the lease. I estimate a total of $500 for the event since we did the cleanup. What should the letter say to bill them? Do I have recourse if they don't pay? Can I save the bill and put it against their security. This is their 2nd month and I'm hoping it was just an accident. Thanks for reading.