Roofing membranes: which properties matter when choosing an underlay?

In a typical ventilated pitched roof, the membrane lies beneath the batten grid that supports the covering. It shields the structure and insulation from the outside. Water that appears beneath the sheets should drain along it to a properly constructed eaves.

The outer membrane is not the same as the vapour barrier on the room side. Both layers may look similar, but they perform different tasks. Swapping them can disrupt the moisture conditions of the building envelope. The material should be identified by its marking and technical data sheet, not just by colour or surface texture.

The Sd value expresses the resistance to vapour diffusion as an equivalent air layer thickness. A low value means vapour passes through the material more easily. When choosing, however, the whole roof must be assessed, including the covering, the insulation and the airtightness on the interior side.

High vapour permeability alone does not ensure a dry structure. Moisture can enter through leaky air connections, not only diffuse through the surface of the film. Overlaps and details are therefore important. Nor should it be assumed that every membrane with a low Sd can be used in any assembly without a gap or on any substrate.

The product data sheet should be checked for the declared watertightness class and behaviour after ageing tests. Resistance to temperature and radiation during the intended exposure is important. The parameters should match the conditions beneath the selected covering.

The membrane is not the final roof. The manufacturer specifies the permissible time for leaving it uncovered and the conditions for such use. Once this time has been exceeded, the material must be assessed before covering. It is not worth covering a layer whose condition is in doubt with sheet metal. Any replacement after the works are complete will be considerably more difficult.

In case of doubt, it is advisable to request a declaration of performance and instructions valid for the specific batch. A trade name does not always make it possible to recognise a change in the material version. Keeping the label from the roll will make it easier later to identify the product and determine its requirements.

Grammage indicates how much a square metre of the material weighs. It may be linked to resistance to damage, but on its own it does not describe all properties. Two membranes of similar weight can differ in construction, service temperature and strength after ageing.

It is worth checking tensile strength and tear resistance at fixings. On a large, complex roof slope the material is exposed to more installation work. Compatibility of tapes and seals is also important. The best technical data sheet will not help if the joints are made with a random tape that loses adhesion after a season.

Membrane strips are laid with overlaps and in the direction specified in the instructions. The details depend among other things on the roof pitch. Locations of service penetrations, chimneys and windows require matched connections. A cut should not be left without directing water over the obstacle.

The method of fixing under the counter-battens must also be checked. Holes from fasteners may require sealing as provided for in the system. At the eaves the membrane must terminate on the correct flashing. A small sag or folded edge can retain water. A ventilation path under the covering must be maintained above the membrane.

The roof surface is not equal to the quantity of membrane to purchase. An allowance is needed for overlaps, turn-ups and details. Its size depends on the roll width and the shape of the roof slope. It is worth drawing the laying direction, especially with many roof valleys and dormers.

Tapes, seals and penetration elements must be added to the order. When comparing offers, check not only the price of the roll but also its area and the complete cost of connections. The material should have clear marking and documentation that will be retained after installation.

Before purchase, provide the seller with the type of covering, substrate and insulation. Also state the planned date of covering. If the roof is to remain without steel sheet for a period, the selection must take this situation into account. A sensible choice is based on service conditions, not on a promise that the thickest membrane will cope everywhere.