The last two times it has rained, our basement has had a very strong odor, it seems to be a sewer smell. I checked in the crawl space where we have a hole that we drain our dehumidifier into it and that doesn't seem to smell. The water and toilets don't stink either. IT is a very overpowering smell that permeates even the second floor. Any idea what it could be? We have a septic tank behind our house about ten feet from the basement. The last time we had come home from someplace and couldn't figure out where it was coming from, eventually it went away.

Do you know where your vent pipe exits through the roof? Check the vent for blockage and try rodding about 20-25 feet into the vent. Also, how long has it been since your septic tank was pumped?

No, we do not know where it is vented. We are new home owners. A roof repairman mentioned that the original vent for the toilet was blocked purposely. He said we should check with a plumber to see where it was vented.

No, we do not know where it is vented. We are new home owners. A roof repairman said that the vent for our toilet was purposely blocked and to check with a plumber to see where it is vented. We haven't done that yet.

Purposely blocking a vent pipe is a strang situation. The vent handles your whole house probably, not just the toilet. You do need to get a plumber to check for proper venting. Seems/smells like you are getting sewer gas in your home because the vent is blocked. Hopefully you will not find that someone vented to just your attic.

What do you mean when you say, "vented to just your attic?" If they did, is this a problem?

The vent pipe must go through the roof. If someone cut it off inside the attic it would let sewer gas collect in your home. The attic isn't 100% air tight and separarte from your interior. Sewer gas is heavier than air and will settle through cracks and opening in the ceiling and or attic door. Take a peek in the attic and see if you can locate the vent pipe. It should be directly below the capped roof vent. Did you use a home insopector when you bought your home? This should have been caught before the purchase.