Property risk
Dry traps and sewer gas
The most common reason an empty house smells wrong, and among the easiest to prevent.
What happens
Every plumbing fixture has a P-trap holding a small volume of water that seals the drain against sewer gas entering the building.
That water evaporates. In an unused fixture a trap can dry within a few weeks, faster in dry winter air with the heating running.
Once dry, the seal is gone and sewer gas enters the building freely. Floor drains, basement showers, laundry sinks, and bathrooms in unused parts of the house dry first because they are never used.
The result is the characteristic stale, faintly unpleasant smell in long-empty houses — which matters commercially as well as practically, because a buyer walking into a listing reads it as a plumbing problem.
Why vacancy makes this worse
Traps only dry when fixtures go unused, which by definition only happens in a vacant property. Occupancy solves this automatically.
What a visit can observe
- Sewer or sulphur odour, particularly near floor drains or unused bathrooms
- Odour strongest in the basement or in unused areas
- Visible dry trap at accessible floor drains
- General stale smell throughout a long-empty property
Reducing the risk
- Run every fixture briefly at each visit — this is the entire solution and takes minutes.
- Pour water into floor drains, which are the most commonly forgotten.
- Add a small amount of mineral oil to traps in fixtures that will not be used at all, to slow evaporation.
- Close toilet lids to reduce evaporation from the bowl.
Related questions
How long can a house sit empty?
Physically, a house can sit empty indefinitely if it is maintained — heated, water managed, monitored and looked after. The limiting factors are not structural, they are practical: insurance, deterioration and the accumulation of unnoticed problems.
The insurance constraint is usually the binding one. Many policies begin restricting coverage after roughly thirty days of vacancy, though the threshold and the consequences vary. Beyond that point some owners need a specialised vacant-property policy, which is a different product with different terms and typically a higher premium.
The deterioration constraint is gradual and cumulative. Traps dry within weeks. Pests establish over a season. Moisture problems compound. Systems that sit idle behave unpredictably when restarted. None of these are dramatic on their own; together over a year they produce a property that needs significant work.
The real answer is that a house can sit empty for a long time safely, provided somebody is attending to it. It is unattended vacancy rather than vacancy itself that causes the damage.
What actually happens to a house when it sits empty?
Less dramatically than people imagine in the first month, and more than they expect over a year.
Within weeks: plumbing traps dry out and sewer gas enters the building, producing the characteristic stale smell of an empty house. Dust settles undisturbed. Air becomes stale without the ventilation that ordinary occupancy provides.
Within months: pests find their way in, particularly through the fall. Small leaks that would have been noticed immediately run continuously. Moisture accumulates in unventilated spaces. Landscaping deteriorates visibly, and the property starts announcing its own vacancy.
Over a year or more: systems left idle can seize or fail on restart. Moisture problems develop into mould. Rodent populations establish. Exterior maintenance issues compound — a small roof problem becomes a leak, a drainage issue becomes a basement problem.
Almost all of this is prevented by ordinary attention: heat maintained, water managed, fixtures run, the property visited, and problems caught while they are still small.
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