Building paper is a reinforced, weather-resistant sheet material used as a secondary barrier against wind and moisture in walls and roofs. It is installed behind cladding or beneath roof tiles to stop wind-driven rain from penetrating while still allowing water vapour to escape from inside the structure.
Application
Building paper sits between the structural frame (or insulation) and the external finish. It acts as a safety net: if rain gets past the outer cladding, the paper directs it downward and out of the wall or roof assembly before it can reach the insulation or timber frame.
Where it is used
- Timber-frame walls — Wrapped around the outside of the frame before brick or timber cladding is applied.
- Pitched roofs — Laid over rafters or sarking boards beneath the battens and tiles or slates. Often referred to as roofing felt or underlay in this context.
- Facades with ventilated cavities — Placed on the warm side of the cavity to keep insulation dry and block wind infiltration.
Types of building paper
- Bituminous felt — Traditional option, relatively inexpensive. Less breathable than modern alternatives.
- Breathable membrane — A microporous film that blocks liquid water but allows moisture vapour to pass through. This is now the standard choice for most new builds because it reduces condensation risk.
- Reflective foil membrane — Combines moisture resistance with radiant-heat reflection, useful in roof applications.
Installation tips for DIY builders
- Always overlap sheets by at least 100 to 150 mm and tape or staple the seams to prevent gaps.
- Install the paper with the printed (or correct) side facing outward — orientation matters for breathable membranes.
- Avoid leaving building paper exposed to sunlight for extended periods before cladding, as UV can degrade it.
- Make sure the membrane is taut but not stretched tight — a slight drape helps water run off rather than pool.
Related terms
- Building materials
- Timber frame
- Insulation
- Roofing
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