Home inspector Paul Wilson says there are particular signs to look out for that indicate you might be living in a former marijuana grow-op.

Ottawa Police dismantle an average of 23 grow operations each year. A City of Ottawa by-law passed in November 2012 lays out how these properties must be re-mediated before they can be re-occupied. But police estimate there are dozens more grow-ops in operation each year that are never dismantled.

Wilson said it's these properties that can hide problems.

Marijuana grow-ops can cause lasting damage inside homes due to the moisture from marijuana plants, which create mould and spores in walls, ceilings and floors, according to Wilson. The air inside the homes can also endanger residents' health.

Those who run grow-ops also often re-wire homes to bypass an expensive hydro bill that can cost about $1,500 per month.

Patchy electrical work also makes a fire 40 times more likely in the home, according to a City of Ottawa staff report released in November.

Here are 12 physical signs your home may once have housed a marijuana grow operation:

• Modified ductwork that doesn’t seem to make sense.
• Circular holes in floor joists or roof trusses from venting (look for holes that have been patched).
• Chunks of brickwork on the exterior that have been replaced
• Brown stains in soffits (the exposed undersurface of a roof eave) from the grow-op venting to the outside, or brand-new soffits.
• In the winter, bald spots without snow on the roof from heat loss.
• Stains on basement floors from pots that sat there for long periods of time, or stains in laundry tubs.
• Modified wiring and electrical panel. Sometimes live wires can still be in the insulation.
• New plumbing for water supply and drains.
• Foundations and concrete walls cored or breached to get wiring around the hydro meter.
• Warped/rotted wooden structures (staircases, floors and window cases) due to moisture damage.
• Toxic moulds form/spread poisonous spores in the open, within wall cavities, inside window frames, ventilation systems and attic spaces.
• A musty smell or, in the case of meth labs, odour that smells like cat urine.

If there are no obvious physical signs that indicate you live in a former grow-op, there are three things you can do to double-check:

• Check for any electrical changes with the Ontario Electrical Safety Authority and/or police.
• Get an indoor air quality test.
• Check if any permits were drawn for that address.

Ottawa police also have tips for any neighbours who might suspect they live near a marijuana grow operation. Staff Sgt. Mike Laviolette said there are four basic observations that tell him a home is housing a grow-op:

• Condensation in windows.
• No activity coming to and from home.
• A pungent odour.
• In the winter, no snow on the roof.

Poll question

Interactive map

View Marijuana grow-ops dismantled by Ottawa police since 2011 in a larger map

The information for this map was obtained through an Ottawa police website list of former grow-ops as of March 7, 2013. For a better view of the details for each former grow-op — including when it was busted and how much marijuana it contained — we recommend viewing the full version of this map.