The short answer
A loft that cannot breathe collects condensation, which drips onto insulation and timbers and looks like a leak. Guidance based on BS 5250 sets minimum ventilation at the eaves, so insulation must never block the gap. Low, damp ground beside the Avon makes the point more pressing.
Christian Malford lies on Oxford Clay, with the River Avon forming most of its northern and eastern boundaries. Damp air indoors is not unusual in houses on low ground, and where that moisture meets a cold roof, it condenses. People often mistake the result for a slow leak, and spend money chasing a fault that is not there.
Q3.1What does the ventilation guidance say?
Marley's explanation of BS 5250 for a cold roof with a non-breathable (HR) underlay is to provide 25 mm of ventilation along the eaves for pitches of 15 degrees or less, and 10 mm for steeper roofs. Roofs above 35 degrees, or spanning more than 10 m, call for extra ventilation high up. Breather membranes (LR underlay) follow different rules, so the underlay matters.
Q3.2Where does loft ventilation go wrong?
Loft insulation laid right into the eaves can block the gap. Bathroom extractors that vent into the loft add moisture. Over decades, old roofs gather partial repairs that close off vents. We look at all of these, with the roof and the loft together, before suggesting anything, and we photograph what we find so you can see it too.
Q3.3What can be done about it?
Pull insulation back from the eaves, fit proper vent products where the felt allows, and send extractor air outside. Our roof inspections and surveys page describes what a survey covers. If the problem turns out to be a real leak, making the roof safe comes first, then the lasting fix once the structure has dried out enough to repair.