UNDERSTAND THE PROBLEM
Why is it damp in here?
Tell us what you can see. Get a short list of checks for your shed, garage or garden room.
YOUR ACTION LIST
Start with what you can see
Answer the five questions to get relevant checks. More than one moisture source can be present.
This is a checklist, not a diagnosis or a mould-safety assessment. Keep away from wet electrics. Significant, recurring or unexplained damp needs an appropriate professional assessment.
Condensation or a leak?
Condensation forms when moist air reaches a sufficiently cold surface. Rain can also enter through defects in a building. The timing and location of moisture give you clues, but neither proves a cause on its own. This checker uses those clues to organise your next checks; it does not score your building or predict damage.
A cold-morning example
Droplets under a roof after a cold night make condensation worth investigating. Look at moisture brought inside and the existing airflow, while still checking for defects that could admit rain.
An after-rain example
A patch that grows after rain makes water entry worth checking first. Compare photos taken in dry weather and after rain, and inspect accessible details from ground level.
What should I buy first?
Start by identifying the moisture source. A dehumidifier, coating or extra insulation will not repair a leak. Follow the checks before deciding on a product or repair.
Can I use this for a garage or garden room?
Yes, as an initial checklist. Wall, roof and floor construction vary, especially in insulated or occupied rooms. Use a suitably experienced professional when the cause or appropriate remedy is unclear.
Background: RICS: damp and mould and Met Office: understanding humidity. The checklist is our own interpretation of your answers.