I'm an engineer, but not an antenna engineer or industry member. Any advice I give is as a fellow OTA user and homeowner.
JMO - if there is any doubt that you might be able to receive the stations that you want, buy the biggest antenna that you can afford.
What are you using for reception now? Likely you have an indoors antenna and you are getting a few stations. If I were in your shoes, I'd buy a long cable and go into your living room, the attic, and then go up on the roof with whateve antenna you have now. Run a channel scan with the antenna in each location, to see the difference in reception at each location. There is no substitute for real data.
The trees surrounding your property may be a big issue, depending on how far off and how high they are. Realize that the attic installation will be affected by the trees even more than the roof installation, as well as being attenuated by the siding and roofing materials, surrounding buildings, etc. Your TVFool report does not take the trees into account - so I would not believe that going higher will not make a difference in actual reception. You have lots of options for mounting on the roof, depending on what your building and property looks like - tripods, chimney mount, eave mount, wall mount. Again if it were mine, I would use a chain link top rail for a mast (10') and put it wherever I could get the most height. It is possible to put a 10' mast in a 10' tripod and get nearly 20' above the roof peak (I would also use guy wires for that). Height is good.
Look here at the "antenna mast mounting" parts.
http://www.antennapartsoutlet.com/ - Not an endorsement, but I have bought frm them, they sell commodity parts for antenna mounting (Rohn, Easy Up) and have reasonable shipping. Solid Signal is another source
http://www.solidsignal.com/cview.asp...ing%20Supplies and Ronard seems to be recommended a lot, but seems like 70% of Ronard products are out of stock everywhere.