Check all the vent lines in the house for obstructions. If one vent is blocked, it may be channeling sewer gas through another one.
An easy way to check a vent is to flush the nearest toilet and listen for a sucking sound coming from the shower or sink drain. If you hear this, it means air is not being sucked in through the vent.
Visually inspect the vent outlet for the obstruction, which could be a ball, leaves or even a dead bird. If this visual inspection isn't conclusive, there may be debris collecting in a 90-degree angle fitting somewhere in the line.
Spray water into the vent opening, or use a plumbing snake to clear this debris A roof vent smell can be the result of improper installation of the venting system in your house. If you can't find any evidence of obstructions in any vents, you can buy a roof vent filter.
When you attach this to the problem vent, it will abate the odor.
Vent lines can develop leaks, and these can be difficult to pinpoint. A leaking vent line near the air intake for the central heating system can spread sewer gases throughout the house.
Since vent leaks are not accompanied by dripping water, you have to find them with a special machine that blows pressurized smoke into the line. This is not a commonly used machine, and you will probably have to call a plumber who has one.
If there is a leak in the vent line, he should be able to find it in less than an hour.
Dry Fixture Traps
If a vent pipe is blocked, a large amount of water deposited into the drain lines by a toilet or washing machine can suck air out of the P-traps of nearby drains or fixtures. This water normally blocks sewer smells, so if the trap goes dry, you will be able to smell sewage from the drain or fixture.
This sucking action will be accompanied by a gurgling sound which will help you identify the problem. The remedy is to clear the vent line, either by spraying water or inserting a plumbing snake through the vent opening.