Once you have eliminated the non-paying furry tenants patch up all cracks & holes... take your time and inspect the whole house, anything the size of a dime or bigger is a large enough opening for vermin.. so get your chicken wire, get your spay foam sealer and building materials and patch up that residence

I have shaved quite a bit of my mortgage in the last one year by using this strategy.

Generally I take it as the landlord's responsibility because you are sealing up the property with foam chicken wire etc. and you don't want your tenant doing that on their own.

My partner can handle most common repairs (plumbing/electrical/etc) so can shave off a % or two, but if you will be relying on an outside repairman for all, then you have to be honest and bump it up.

My purchase price would reflect that - I'd probably offer them $1000 and shave my offer price to seller by another few thousand. 

Yelling at the tenant will do no good as he's suffering from depression and moving home to mom, and on top of which he's too tired to shave or bath apparently.

I wouldn't own a painted building - put foam board insulation and vinyl siding over it and forget it.However, for flips I don't usually buy old buildings. 

People in their 30’s have already been in the workforce for 8-15 years.

I do think that the small percentile that wants to do things themselves to shave a penny might be the target customers for automation to get price of services down even further.

If it were a major problem lots of people would be in trouble .Most houses built in the 1960's and 70's have it .