When the weather is cold I smell it the most. Last winter I had a roofer come over to rplace a vent pipe that snapped off because of the ice sliding off my metal roof. I thought the same thing happened again but another roofer checked it out and said the vent pipe is fine but that methane gas sinls lower then air when it's cold out. I need to do somehting because it is so bad in the master bedroom and master bathroom. If the vents pipes are't clogged or broken should I still have more things checked?"
My house is 11 years old and on a septic system but the leach field is located a long distance from the house.

I'd first check in the attic to make sure the vent stack isn't broken or cracked up there. When it is cold your furnace and hot water heater run more and they pull outside air into the home. They could be pulling in sewer gas. This is just a start because there are many ways your problem could come up including a failed wax ring under your toilet

Pipe checked in the attic no break. The room that I am getting the smell in protrudes out from the rest of the house and it has many windows, could it be just that the sewer gas is accummulating in some nooks and crannys outside and then comes in?