How soon can the vent from a roof drain trap go horizontal?

I am replacing a storm line running to a roof drain, and it currently has no trap and connects directly to my main sewer line (combined sewer and storm lines are very common in my area). As long as I'm replacing it, I figure that I should do it properly and add a trap to it. How soon can the vent from this trap go horizontal? I assume that the "6 inches above flood rim fixture" standard doesn't apply here, since the "fixture" in question is presumably the roof, and if you were 6 inches above it, you'd already be vented.

I'm specifically thinking of having the roof drain drop directly into a trap, then having the vent rise briefly and then jog horizontally to meet up with the main vent.

If it's on your roof, there is no reason to trap and vent it. The vent would be open air right next to it.

UPC 2009 states that all storm drains connected to a sewer line should be trapped:

1103.0 Traps on Storm Drains and Leaders.
1103.1 Where Required. Leaders and storm drains, when connected to a combined sewer, shall be trapped. Floor and area drains connected to a storm drain shall be trapped.
Exception: Traps shall not be required where roof drains, rain leaders, and other inlets are at locations allowed under Section 906.0, Vent Termination.
1103.2 Where Not Required. No trap shall be required for leaders or conductors that are connected to a sewer carrying storm water exclusively.

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And its definition of "Storm Drain" is:

Building Drain (Storm) - A building drain that conveys storm water or other drainage, but no sewage.

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Granted, I take your point that it's basically being vented anyway. But a roof drain does not extend 6 inches above the roof, nor is there any guarantee that it's at least 3 feet from any window opening. So by my reading of the code, it should be trapped.

If your roof is not at the top of the building, then are you talking about a "Deck" drain?

Vents can go horizontal when they are 6" above the flood lever of the fixture, and any other fixtures it's connected to.

Terry said:

If your roof is not at the top of the building, then are you talking about a "Deck" drain?

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This particular drain is for the roof of a 2 story extension off the main house (which is 4 stories). But it's part an attached townhouse, so there are neighboring buildings whose windows could be within 3 feet (even at the topmost roof - buildings are all different heights).

Any vent for a roof drain, which is something I have never installed, would have to rise ABOVE the roof, otherwise when the drain gets plugged the water would simply rise in the pipe until it could flow into the drain system through the vent.