If you are on the top floor, you may not need a mast nearly as tall as you think. A lot of people just starting out look at where the dish is pointing and think that is the elevation of the satellite. However the lnb is actually offset and the look angle is about 20 deg higher.
I don't know where you live, but in a lot of locations in the continental US, if you can get the dish to clear the roofline, you are home free in seeing the satellite. Look around at other houses in your area that have dishes on the north side and see if they are simply mounting to the eaves and pointing at the peak of the roof. I would use the azmith calculator for your zipcode and then take a look at the elevation setting with an inclinometer.