Condensation has a pattern
Loft condensation often shows over a broad area. Nail points may bead with moisture, the underside of felt can feel damp on cold mornings and mould may appear on stored items or timbers. It tends to be worse during cold weather when warm moist air from the house reaches the loft.
A roof leak is more often localised, though a long-running leak can spread a surprising distance.
Look for sources from the house
Bathroom extract ducts should discharge outdoors, not into the loft. Loft hatches need a reasonable seal. Gaps around pipes and cables can allow warm air to rise. Large amounts of wet washing dried indoors can increase moisture throughout the house.
None of those checks removes the need to inspect the roof if staining is concentrated below a chimney, valley or damaged covering.
Photograph the area before work starts if you can do so from a safe position. It gives you something concrete to compare with the finished repair.
Ventilation needs to be continuous
Roof-space ventilation works by giving moisture a route out. Blocking eaves vents with insulation can reduce airflow. Some roofs rely on other vent positions or breathable membranes, so copying a vent detail from another house is not always sensible.
If insulation is moved for inspection, put it back without packing it hard against ventilation paths.
Do not guess from one cold morning
A loft can be damp during a particular combination of cold weather and high indoor humidity. Photograph what you see and recheck after different weather. If droplets appear without rain, condensation becomes more likely. If one rafter becomes wet immediately after rain, concentrate on the roof above it.
Persistent timber wetness deserves attention either way. The remedy is different, so the distinction matters.
Need somebody to look at the roof?
If you want to ask local contractors about the repair, describe the symptom and include anything you have noticed about the weather or the part of the roof involved.
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