A loft that cannot breathe gets wet on the inside.

The call usually starts as a leak. There is water in the loft, the insulation is sodden in patches, there is black mould on the underside of the felt and sometimes on the rafters themselves. But it has not rained for a week, or it drips worst on cold still mornings, or the wet is spread evenly rather than running from one point. That is condensation, and no amount of work on the outside of the roof will touch it.

What has usually happened is that somebody topped the insulation up to modern depth and pushed it hard into the eaves, closing off the gap that let air in. On a house with old bituminous felt — which does not breathe at all — that gap is the only ventilation the roof has. Close it and the loft becomes a cold sealed box with warm wet air arriving from below all winter.

The other half of it comes from inside the house. A bathroom or kitchen extractor that vents into the loft instead of through the wall puts litres of water up there every week, and it is startling how many do. That one is not a roofing job at all, but it is worth finding out before you pay anybody for anything.