There are much straighter and lighter trees that grow faster available for dimensional lumber: firs, spruces, and various other pine species.The rafters are not the biggest issue here -- a sturdy rafter structure is to be expected in a pre-truss structure, where simple solid rafters in line hold up the roof, not a complicated structure of thinner sticks taking up lots of floor space in the attic.You need to look at the attic floor from below carefully.

The ceiling joists will not be strong enough to put additional weight on.

I worked mostly with middle aged clients who struggled with general health and weight loss.

I had a listing with Chinese Sheet Rock and when I would lean against the wall, it would bow under my weight (I am not a big person).

A good property manager with solid vendors is literally worth their weight in gold! 

Paying a proficient short sale negotiator is worth it's weight in gold to increase the chances of a short sale being completed efficiently and in keeping a strong line of communication with all parties.

However, after some brain training, the positivity and encouragement presented will carry much more weight than the negative. 

My understanding is that you cannot simply throw tile onto a shingle roof without reinforcing the trusses or roof framing for the added weight. 

I think if you looked at your balance sheet you've already more than dipped your toes :)  You may not have generated massive cash flow but depending on when you bought - the Keys, Brickell, Miami and any land out west suitable for a summer home have appreciated massively and probably shifted your asset/liability balance more than 99% of cash flow investors working at real estate.So another option feeds right into this without having to go with a management weighted DST or syndication. 

The risks here have the most weight in determining success or failure.