Drain or manhole cover smelling outside the house

A sewage smell from outside drains is usually caused by a blockage in the underground drainage system causing waste to back up and gas to escape, a cracked or broken drain allowing effluent to seep into surrounding soil, or a dry trap in a rarely used gully (a floor-level drain grille).

Locate your manholes — lift the cover using a flat-bladed tool (they are heavy: use two hands and stand to one side). Look inside from above at ground level. If the chamber is full of sewage, there is a blockage downstream (between this chamber and the next one, or between it and the sewer connection).

Check all gullies (the ground-level drain grilles around the property) — these are where downpipes, kitchen waste, and yard drainage connect to the system. Each has a water seal in the trap below. If a gully has not been used recently, the water seal may have evaporated, allowing sewer gas up. Pour a bucket of water down each gully to refresh the seal.

If a chamber is dry (no water in it) but smelling, the vent pipe may be blocked or missing — sewer gases are venting back through the property rather than up the vent stack.

If chambers are backed up: try clearing with drain rods (available to hire, or a drainage company can jet the drain). Rod from the highest chamber downstream — push rods in, attaching additional sections, and rotate clockwise only (anti-clockwise unscrews rods and they can be lost in the drain).

If you cannot clear the blockage with rods, or if the drain appears cracked or collapsed, a drainage engineer with CCTV survey equipment is needed. Many councils and water companies offer free drain surveys for blockages that affect their main sewer.