Under every tiled or slated roof is a second line of defence — the underlay — and its technology quietly changed a generation ago. Understanding the change explains both why old lofts drip in winter and what you're buying when a re-roof quote says "breathable membrane".
The old way: bitumen felt
Twentieth-century roofs got bitumen felt: a waterproof sheet that catches whatever rain gets past the covering and sheds it to the gutter. It works — until it ages. Felt embrittles over 40–60 years, tearing at laps and crumbling at the eaves (what failure looks like). And it has a design flaw: it's waterproof in both directions. Moisture rising from the house gets trapped in the loft, which is a large part of why older roofs suffer loft condensation.
The new way: vapour-permeable membranes
A breathable membrane is clever material science in a dull-looking roll: microscopic structure that blocks liquid water coming down while letting water vapour escape upwards. Rain out, breath through. The loft stays drier, the insulation stays effective, and the membrane itself remains flexible for decades rather than embrittling. Fitted with proper ventilation detailing, it largely retires the condensation problem that plagues felt-era roofs.
When it matters to you
Re-roofing: a breathable membrane comes as standard with any re-roof we do — it's a major part of what makes the new roof better than the old one, beyond fresh tiles. Buying a house: the underlay's condition (visible from the loft) tells you more about the roof's remaining life than the tiles do. Chronic loft condensation: a membrane swap is the at-source cure, though it means lifting the covering — ventilation retrofits are the interim fix. One caution: a membrane is not a roof. Tiles off and "the membrane will hold it for now" is construction-phase thinking, not a repair strategy.