The math would say that if I want 120,000 of rental income, I just have to scale what I've done to 1.5mm worth of properties.1.

But rectangle tiles are manufactured, they are more noticeable.

And you'll learn how to scale it up (Imagine buying a 24 unit property that cash flows at $200 per month, per unit... with almost no money down.

I've seen a lot of bad work too, which I usually attribute to ignorance, but never on this scale.

Anil, I would be very careful for many reasons stated above, but most importantly:1) property management is crucial so take your problems with your duplex and scale that up to 25 units2) $100/mo per unit return is not very compelling for the issues you will deal with in C/D rental areas.  

The best thing you can do is research the window manufacturer(s) to see if there have been problems with any of their product lines, and of course make sure they've been in business for a long time and provide an outstanding warranty.

In my case, I wanted to build a business that could scale.

I have heard talk of some of the REITS scaling back but don't know if they are or not.The successful multi-family investors that I know do not simply buy for cashflow. 

At a certain point I'd imagine It is also hard to scale a business off the books.

Toledo has many old industries and considered the world capital of glass manufacture.