How to Identify What Animal Died in Your Wall or Roof

One of the most frustrating things about a dead animal smell is not knowing exactly what it is or where it is coming from. However, different animals leave different clues. By paying attention to the details, you can often narrow down the suspect.

1. The Smell "Profile"

While all decomposing animals smell bad, the intensity and character of the odor vary by size and species.

• Mice: A musty, stale urine smell mixed with decay. Because mice are small, the smell might be faint or intermittent depending on airflow. It often smells "cheesy" or like rotten corn.
• Rats: A sharper, more pungent, and sickly-sweet odor. Because rats are larger than mice, the smell is significantly stronger and can permeate a room quickly.
• Possums: A very heavy, overwhelming, and nauseating stench. Possums are large animals (up to 4-5kg). The smell is often described as "thick" or "meaty" and is impossible to ignore. It will likely force you to leave the room.
• Cats/Dogs: Unfortunately, pets sometimes crawl under houses to pass away. The smell is similar to a possum—very strong and persistent.

2. Location Clues

Where is the smell strongest? This is your best hint.

• Wall Cavities: Often indicates a rodent (rat or mouse). They travel through wall voids via pipe penetrations. If the smell is focused on a power point or light switch, the animal may have been electrocuted or trapped near the wiring.
• Ceiling/Roof Space: Could be anything, but possums are the most likely culprit if you heard heavy thumping noises at night prior to the smell. Rats are also common nesters in insulation.
• Subfloor/Under House: Often cats, stray dogs, or foxes that sought shelter. It can also be a possum that fell down a wall void to the bottom plate.
• Chimney: Birds and occasionally possums fall down chimneys and get stuck.

3. The Timeline

How long has the smell been there?

• Small Rodents (Mice): The smell typically peaks after 3-5 days and may dissipate within 2 weeks as the tiny body dries out (though it is still unhygienic).
• Rats: The smell lasts longer, usually 3-4 weeks if untreated.
• Possums/Larger Animals: The smell can persist for months. A large carcass takes a long time to decompose and can seep fluids into ceiling plaster, causing long-term damage.

4. Visual Signs

Look for stains. A decomposing body releases fluids. If you see a damp patch appearing on your ceiling or a discoloration on a plasterboard wall that grows over a few days, that is often directly below or next to the carcass.

Fly Activity: A sudden appearance of large blowflies in a clean house often means they have found a way in from a carcass nearby. Follow the flies—they are experts at locating the source.