Last updated: August 2026
The roofing felt vs breathable membrane comparison matters more than almost anything else on an older Dublin roof, and it is the part nobody ever sees. On most mid-century houses the original bitumen felt has already failed, which means the tiles above it are working alone whether the owner knows it or not.
What the Old Felt Was Doing Up There
A tiled roof is not watertight in the way people assume. Tiles shed the great majority of water, and in wind-driven rain a certain amount gets past them. The layer underneath is what catches that and carries it back out to the gutter, which is why it exists at all.
On a roof from the 1930s to the 1970s, that layer is bitumen felt, a sanded, tar-impregnated sheet laid over the rafters before the battens went on. It did the job well when it was new. What it was not designed for was seventy years of Irish weather.
Bitumen goes brittle with age and with heat cycling. It stops being a flexible sheet and becomes something closer to dry paper, and it tears along the rafter lines under its own weight. You can usually see it for yourself, from inside the attic on a bright day, hanging in strips or gone entirely between the timbers.
What a Breathable Membrane Does Differently
A modern breathable membrane is a light, tough synthetic sheet that does the same water-shedding job far better, and adds one property the old felt never had. It lets water vapour pass outward while keeping liquid water out.
That one-way behaviour is the whole point. Moisture is always being produced inside a house, from cooking, washing, drying clothes and simply breathing, and some of it finds its way up into the roof space. Old bitumen felt trapped it there. A breathable membrane lets it out.
It is also far tougher physically. It does not go brittle the way bitumen does, it resists tearing, and it holds up to being walked on during the build in a way old felt did not.
Why This Matters in a Mid-Century Attic
Here is where it becomes a practical problem rather than a technical one. Houses of this era were built with no real thought given to roof ventilation, because with a draughty attic and modest insulation it was not needed. Over the years most of them have had thick insulation laid across the attic floor, which is a good thing for heating bills.
What that does, though, is make the roof space colder while the moist air from the house is still finding its way up. That moisture condenses on cold timbers and on the underside of the felt, and it shows up as damp patches, dark staining on rafters and sometimes water dripping onto the insulation. It looks exactly like a leaking roof.
Diagnosing that correctly matters, because re-roofing to solve a condensation problem is an expensive way to fix the wrong thing. Equally, a re-roof that puts in a breathable membrane and ventilates the ridge does genuinely solve it, which is a real benefit of the work beyond just keeping rain out.
Can the Felt Be Replaced on Its Own
Not really, and it is worth understanding why. The felt sits under the battens, which sit under the tiles. Getting at it means taking the tiles and battens off, which is most of the work of a re-roof already.
That is why the felt and the tiles tend to get dealt with together, and why we look at both when assessing a roof. It is also why a roof whose tiles are near the end is the right moment to act, because you get the membrane, new battens and a ventilated ridge as part of a job you were going to do anyway.
There is a partial option worth knowing about. Where the felt has failed only at the eaves, which is common, felt support trays can sometimes be fitted from outside with limited disruption. It is a repair rather than a solution, and we will tell you honestly whether it buys enough time to be worth it.
