Installation of Vapour Permeable Underlay

The membrane is a secondary water barrier. The roof covering is the first defence against rain, and the membrane is the second. In ordinary weather it hardly comes into play. In a storm, in driving rain or when snow melts, water can still get through the joints in the covering. Then the membrane decides whether the timber and the insulation stay dry.

It matters most over a habitable or insulated loft. Wet mineral wool insulates far less well, and timber that stays damp for a long time can grow mould or start to rot. Such problems often show up only after several seasons, when stains appear on the ceiling.

People often confuse this membrane with a vapour barrier. The breathable membrane goes on the outer side of the insulation, under the roof covering, and lets vapour out. The vapour control layer goes on the room side, behind the plasterboard, and stops moisture from the rooms getting into the insulation. Each layer has a different job, so one cannot replace the other.

Contact or non-contact

A contact membrane is highly vapour permeable, so it can lie directly on the insulation or on the boarding. It is used on insulated roofs where there is no room for an air gap between the insulation and the membrane.

A non-contact membrane needs a ventilated gap underneath. It suits older roof build-ups and roofs without living space below. If someone lays it straight onto the insulation, moisture can stay trapped in the structure.

Under metal sheets or under tiles

Metal heats up and cools down quickly, so condensation forms easily underneath it. Under metal roofing a tougher membrane is usually chosen, and ventilation is planned with care. The membrane also has to withstand roofers walking on it while the sheets go on.

Under clay or concrete tiles, the membrane mainly protects against wind-driven rain and snow. It works together with counter battens, tiling battens and the ventilated gap. The right choice therefore depends on the type of covering, the roof pitch, the insulation and the way the roof is ventilated. A simpler membrane may be enough over an unheated garage, while a better quality membrane makes sense over a living space. The manufacturer's technical data sheet always states where the product can be used.

This is work at height, carried out by a carpenter or roofer with proper fall protection. The usual sequence looks like this:

1. Check the roof structure: how straight the rafters are, the condition of the timber, and the junctions at the eaves, chimney and roof windows. Repair damp or damaged timber first.
2. Prepare the eaves so that the membrane can run onto an eaves tray or into the gutter and water drains clear of the building.
3. Roll out the first strip horizontally along the eaves, printed side facing out as the manufacturer marks it.
4. Continue with further strips towards the ridge, each higher strip lapping over the one below. The overlap is roughly 10 to 20 cm, more on lower pitches; the exact figure comes from the manufacturer.
5. Fix the membrane temporarily with staples. With a non-contact membrane, leave a slight drape between the rafters so any water runs down to the eaves.
6. Tape the laps and seal the details with the system tapes, collars and sealants.
7. Nail counter battens along the rafters. On low pitches a sealing tape is often placed under them to close the nail holes.
8. Fit the tiling battens and the roof covering as soon as possible so the membrane is not exposed to sunlight for long.

The counter battens create a gap where air flows from the eaves to the ridge and carries moisture away. Without this gap, water would condense under the covering. So the membrane is part of the whole roof, not just a separate sheet of material.

If the membrane goes on as part of re-roofing or other roof alterations, check with your local building authority what the building regulations require for your property. In a block of flats, the first contact is the building management or the owners' association.

The simplest case is a plain gable roof with no penetrations, sound timber and good access. The membrane is rolled out, fixed and covered with little cutting.

The job takes longer and costs more mainly because of:

• Dormers, valleys and hips, where the membrane has to be cut and lapped more
• Chimneys, roof windows, aerial and soil pipe penetrations
• A low roof pitch with higher demands on watertightness
• Taping every lap over an insulated loft
• Repairing rafters or adding boarding or battens before installation
• Poor access to the roof and bad weather

When you compare quotes, ask for a breakdown showing whether they include materials, tapes, counter battens, transport and preparation of the base. A quote without these items can look attractive, but the details are then missing.

A finished roof can look fine until the first strong wind with rain. In practice these faults come up again and again:

• Overlaps that are too small, or strips laid from the top down so water runs under the lap
• The membrane laid the wrong way up, so it lets vapour through less well
• Ordinary general purpose tape instead of system tape, which loses its grip over time
• Penetrations simply cut through without a collar or taping
• Membrane not taken onto the eaves tray, so water runs into the wall or insulation
• A missing or blocked ventilation gap
• Holes from tools or roofing work that nobody repaired
• Membrane left uncovered longer than the manufacturer allows, so UV light damages it

Repairs are usually difficult, because the covering and battens have to come off and wet insulation sometimes has to be replaced. A small fault in one detail can mean work on the whole roof.

Before the membrane disappears under battens and roof covering, it pays to walk the roof with the tradesperson and check:

• The strips run horizontally from the bottom up and each higher strip laps over the lower one
• The overlap matches the data sheet and the roof pitch
• The membrane faces the right way according to the printing
• Laps and details are sealed with tapes made for this membrane
• Chimney, roof windows and penetrations are sealed with a collar or tape and no gaps are left open
• At the eaves the membrane runs onto the tray or into the gutter
• There are no tears or holes in the surface
• The counter battens form a continuous ventilated gap from eaves to ridge
• The roof covering follows within the time the membrane manufacturer allows