The signal strengths change very little , 10 , 20 , 30 , 40 , feet.
Roof top antenna height will be Ok , 10 , 20 , feet or so will be Ok. If nothing is blocking the signal.
Install a Winegard HD7698P antenna with a Antennas Direct CPA-19 preamp , aimed at about 286 degree magnetic compass direction.
Here is how to aim antennas ,
http://www.kyes.com/antenna/pointing/pointing.html.
For 1 Tv connected use no splitter.
For 2 Tv's connected use a , HFS-2D , 2 way splitter.
For 3 Tv's connected use a , HFS-3D , 3 way splitter.
Buy the HFS splitters at ,
http://www.hollandelectronics.com , or ,
http://www.solidsignal.com.
Here are some roof top antenna mounts ,
http://www.ronard.com/909911.html ,
http://www.ronard.com/34424560.html ,
http://www.ronard.com/ychim.html ,
http://www.ronard.com.
Buy the ronard antenna mounts at solidsignal by typing the word ronard in the solidsignal search box.
Here are some places to buy antennas and etc. ,
http://www.solidsignal.com ,
http://www.amazon.com ,
http://www.winegardirect.com.
As always trees and tree leaves do a real fine job of reducing or blocking Tv reception and so do buildings and other obstructions.
It is best install the antenna at a location that has the least amount to no amount of obstructions of any type or kind in the directions of reception.
The Tv/s Must Channel Scan for the Broadcast Tv Channels , sometimes named the 'Air Channels' or 'Antenna Channels' in the Tv setup menu becauce the Tv transmissions travel through the air from the transmitting antenna to the receiving antenna.
DO NOT channel scan for cable tv channels.