Is my loft damp caused by condensation or a roof leak?

Note what the weather was doing when the damp appeared. Rain-driven damp means ingress. Damp that arrives after a cold, still night with the heating on, and that eases as the day warms, means condensation.

Recording this over a fortnight resolves the question in most cases without anyone going on the roof.

Ingress produces a route: staining down one rafter, a wet patch on insulation directly beneath a defect, or a tide mark on underlay. Condensation produces a field: droplets on nail points and underlay across the coldest part of the roof space, sometimes with mould on the timber surface.

The feel matters too. Ingress staining is often dry between rainfall events but permanently discoloured. Condensation surfaces feel wet without any discolouration behind them.

This distinction decides whether you need roof work at all. Condensation is usually resolved by clearing eaves ventilation, correcting insulation and redirecting extract fans. Being sold a repair or a new roof for a condensation problem is one of the more common ways homeowners lose money on roofing.

A contractor who will not commit to which cause is present should be asked how they intend to establish it.

On a cold, still morning, look at whether droplets sit evenly across nail points and the underside of the underlay, or whether the wet area is concentrated in one place with a defined edge. An even, widespread pattern across the coldest parts of the roof space leans towards condensation; a defined, localised patch leans towards a specific point of entry.

Checking whether extractor fans from bathrooms or kitchens vent into the roof space rather than outside is worth doing at the same time, since this is one of the most common and easily corrected causes of loft condensation.

Ventilation and insulation corrections are relatively inexpensive and quick to carry out once condensation is confirmed. Ingress repairs, by contrast, need the specific entry point identified before any work is worthwhile. Treating the wrong cause not only wastes the initial spend, it also leaves the roof space getting wetter while the real problem continues unaddressed.

This is one of the clearer cases where a short period of observation, done properly, saves money rather than delaying a fix. It also avoids the awkward position of having paid for roof repairs, found the loft still damp afterwards, and only then being told the actual cause was ventilation all along.