Every now and then we get a homeowner contacting us about nasty smells coming from their septic system and what could be causing it. It’s poo of course! (well poo gases to be exact).
Usually this odor emanates from the vent stack on the roof. Each drain in your house has a vent stack associated with it. The vent stack is the small pipe(s) you see sticking out of your roof. These pipes are designed to vent the gases from the septic system out through the roof. They also help drainage of wastewater through the plumbing system by preventing air-locks from forming in the pipes.
Depending on where the vent pipe comes out of the roof; the height of the pipe; the height and shape of the roof; plus the direction and speed of wind, the gases could end up back down on your patio due to downdraft. See diagram below.
This problem can be alleviated by simply raising the vent pipe above the height of the roof.