Who should pay for leak damage?

[Marci Stein]

So my tenants are moving after paying on time for 6 years . 

However, after they came back from a months vacation last week , 

they informed me the roof leaked into the walls and the wall panels are falling down from mold and wetness 

This couldn’t have happened in a month . 

I will now replace the roof over their room ( an add on room ) 

And the wall  for $ 3,000. 

I have a month security deposit for $850 which I’m not sure about returning. 

I feel if they had told me sooner it would not be such a costly mess . 

Please help me decide on how much to deduct , if anything. 

Thank you !!