Materials compared
Breathable Membranes Under a Roof: What Replaced Bitumen Felt
A breathable roofing membrane lets water vapour escape a roof void while blocking wind-driven rain from entering, which is what distinguishes it from the impermeable bitumen felt it replaced. Search the terms below, compare the figures for breathable membrane, and book a survey if one applies. It covers: Underlay · Breathable membrane · 1F bitumen felt.
The underlay is the secondary barrier beneath the covering, catching anything that passes the tiles or slates. It cannot be renewed without stripping, so the choice made at recovering lasts as long as the roof does.
Two details decide whether either one works
- Drape The sheet is laid with a controlled sag between rafters so water runs to the gutter. A taut sheet does not.
- Eaves termination Underlay cut short discharges behind the gutter into the fascia — a defect invisible from anywhere.
Underlay left exposed by a slipped unit degrades quickly under ultraviolet light, which is why a slipped slate on a roof with no underlay at all admits water immediately.
4 related pages cover this in more depth: full re roof birmingham, Roof Ventilation, flat roof deck warm and cold construction and welsh slate roof.
What does roofing underlay actually do?
Underlay is the secondary barrier beneath the covering. It catches wind-driven rain and snow that pass the tiles or slates, and carries that water down the slope to the gutter rather than into the roof structure.
Breathable membrane is a vapour-permeable sheet that passes moisture outward while resisting liquid water inward; the comparison of 1F bitumen felt is the impermeable predecessor, which traps vapour in the void.
How is a breathable membrane different from felt?
A breathable membrane passes water vapour outward from the void while resisting liquid water inward. Bitumen felt blocks both, so moisture generated inside the building collects beneath it and condenses on the underside.
Reduces reliance on ventilation but does not always remove the requirement. The sheet is laid with a controlled sag between rafters so water runs to the gutter. Underlay cut short discharges behind the gutter into the fascia.
Does a breathable membrane remove the need for vents?
No, not always. Vapour permeability reduces the ventilation requirement without automatically eliminating it, and membranes differ in whether they pass air as well as vapour. The requirement is assessed for the specific roof.
Underlay cannot be renewed without stripping the covering. Underlay is modern membranes outlast the bitumen felt they replaced, though exposure shortens both.
Why does the membrane sag between the rafters?
A controlled sag carries any water that reaches the membrane down the slope and out at the gutter. A sheet pulled taut across the rafters holds water at the batten line instead of draining it away.
Underlay left exposed by a slipped unit degrades quickly. 1930s bitumen sarking perishes and tears above the wall plate.
- Breathable membrane passes vapour outward, while resisting a liquid water inward.
Where does underlay usually fail?
Underlay fails at three points: the eaves termination cut short of the gutter, the area exposed to sunlight beneath a slipped unit, and the perished bitumen sarking on interwar roofs that tears above the wall plate.
- Impermeable felt traps vapour which a breathable membrane releases.
- An underlay cut short at the eaves discharges behind the gutter.
- Exposed underlay degrades quickly under an ultraviolet light.
You only get one chance to change it.
Why underlay is a recovering decision
Vapour permeability reduces the ventilation requirement but does not automatically remove it, and membranes differ in whether they pass air as well as vapour. Underlay cut short of the gutter discharges into the fascia — a defect invisible from anywhere and common on modern estate roofs.
Getting the underlay assessed
The controlled sag between rafters is what carries water to the gutter; a taut sheet does not. Underlay cannot be renewed without stripping the covering, so the choice made at recovering lasts as long as the roof does.
Underlay, and 6 more
- underlay
- breathable membrane
- vapour permeable
- air permeable
- drape
- eaves termination
- sarking
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