An earlier practice was cancelled after bad weather prevented the medical helicopter taking off and drivers then lost 30 minutes of the final session because of further delays.
Practice finally got going when the persistent rain abated but it was halted one minute and 52 seconds before its scheduled end when Williams driver Antonio Pizzonia and Minardi's Gianmaria Bruni skidded off the wet track.
Brazilian Barrichello set the pace for Ferrari with a lap time of one minute 57.085 seconds ahead of German team mate Michael Schumacher, who will wrap up a seventh drivers' crown if he wins for the 13th time this season on Sunday.
Brazilian Pizzonia slid off the track and into the tyre barriers at the Rivage corner before Italian Bruni crashed out at the high-speed Eau Rouge corner.
Bruni had a neck brace fitted by marshals at the side of the track as a precaution before being taken back to the Minardi garage.
Toyota driver Olivier Panis also had a trip into the gravel at Eau Rouge but the Frenchman managed to negotiate his way back on the track.
McLaren's Kimi Raikkonen, the quickest driver on a dry track in Friday's practice, was third in the times with his teammate David Coulthard fifth behind Sauber's Giancarlo Fisichella.
The Spa-Francorchamps circuit, a drivers' favourite, returns to the Formula One calendar this weekend after a one-year absence because of a ban on tobacco advertising.
