Ask about roof run and rise (pitch) which determines the height above roof the metal vent pipe must achieve to co-incide with the btu of your furnace which also determines the size in diameter of the flue pipe needed.
An 80% furnace with a draft induced exhaust does not pressurize the exhaust through the vent pipe. From the time the gas makes it past the draft inducer motor it is natural draft meaning hot air rises on it's own so the angle of the vent pipe, length and heigth, all needs to be determined so the gas can freely escape.
If the angle is not correct, well, all gas products burned produce moisture and that moisture (and dirt in the air) can accumulate in the pipe and trickle back down and eventually block the pressure switch and then you won't have any heat and you'll be spending money on a service tech to so nip this in the bud correctly the first time.
Not too mention if the gas collects in the exhaust pipe as it isn't allowed to flow freely you can have it travel back and flow into your system. Flame roll out or possible CO poisoning can then occur. Two very harmful, potentially fatal, situations.