lots of people have did the blow it out the sewer roof vent and been caught, i must stress that the area around the pipe where it exits the roof gets warm for some reason, it is only a few degrees but looks white under the thermal imaging camera from a helicopter, and they have thought it was the start of a house fire and had the front door kicked in by the fire brigade and police, you are better blowing it directly in to the sewer and any hot air and methane will be blown out divided by all your neighbours roof vents, dont worry about that as all houses have water traps on sinks and toilets to protect the house occupants so they wont get gassed, by the time it gets to their exit pipe it will be stone cold.
although any houses that have not been occupied for a while will have methane in them as the water traps lower with evaporation.
so you may kill a few vagrants or squatters in the process.
nice.