You can use the existing DirecTV wiring to convey the Over-The-Air Tv signals to your TV sets without any major modifications: we would substitute an amplifier splitter for the satellite signal divider that is currently in the system, so you will not have to run any new cables to your TV sets.
As far as antennas go, Sea-Tac has one peculiar situation: most of the TV major TV transmitters are located in one area near downtown, however, KCPQ, the FOX staton, lies about 25 miles West of the city, and we usually need to use a two-antenna system to insure a good signal on FOX. Since you are quite close to most of the transmitters, we can use a small antenna. like a Winegard HD-7694P mounted on the roof and aimed at 210 degrees by your compass. that will pull all the major stations except FOX, which will require a separate VHF high-band yagi and a Join-Tenna to combine the signal from the channel 13 antenna with the main antenna. I usually use either a
Winegard YA-6713 or an AntennaCraft Y-5-7-13 yagi, aimed at 230 degrees, to insure good reception of FOX.
http://www.winegard.comhttp://www.antennacraft.nethttp://manuals.solidsignal.com/AntInstallGuide.pdfhttp://www.amazon.com/Jointenna-Model-0 ... 874&sr=8-4