Choosing roof underlays and membranes for your roof build-up

The underlay sits beneath the external covering and helps protect the lower layers from water and wind. It should drain water in line with the roof details. On the heated interior side, by contrast, a layer that limits or regulates vapour transmission is used, selected to suit the building element.

These materials are not interchangeable. Laying a film with high diffusion resistance in the wrong place can hinder drying out. An external membrane on its own, in turn, does not provide the interior with airtightness. Before buying, you need to know the layer build-up, not just the roof surface area.

Sd describes diffusion resistance in the form of an equivalent air layer thickness. A lower value means easier vapour transmission through the material under given conditions. It is not, however, a measure of air flow through leaky joints. Diffusion and draughts through a gap are different phenomena.

High vapour permeability of the external membrane can help the assembly dry out, but it does not replace the required ventilation. The choice of vapour barrier or vapour-regulating membrane must be related to the whole building element. The greatest available resistance on the inside is not always the only correct solution.

Weight per unit area helps describe a material, but it does not cover all properties. Resistance to tearing and to damage during fixing, the watertightness class, behaviour after ageing and the temperature range should also be checked. Different membrane constructions can give different parameters at a similar weight per unit area.

The material should be approved for the given substrate. A membrane intended for use on rigid sheathing must match such conditions. It cannot be assumed that every product suits both full boarding and loose laying. It is worth comparing the label data with the current instructions.

Metal sheet heats up and cools down, and condensation can form beneath it. The underlay and the ventilation must take such conditions into account. A membrane should not be chosen on the price of the roll alone. Its approved application and its resistance in the planned roof assembly must be checked.

T18, T35 and tile-effect metal roofing do not impose one identical solution for every building. The pitch, the length of the roof slope and the way the loft is used also matter. An open canopy and a heated house require a different approach. An anti-condensation coating beneath the metal sheet is not an automatic substitute for a membrane.

Even a good material does not perform correctly if the laps and penetrations remain unsealed contrary to the system requirements. Particular points are the chimney, the window, the pipe and the junction with the wall. Water and air exploit local gaps. Buying a more expensive roll does not compensate for inaccurate workmanship at these details.

Tapes and adhesives should be compatible with the material and the substrate. The surface must offer the conditions required for adhesion. The lap must not accidentally lead water beneath the lower layer. At the eaves, the termination of the membrane must be clearly defined so that moisture does not reach the insulation or the fascia board.

The permitted exposure time to UV radiation depends on the specific product and the conditions. It should not be treated as the recommended time for leaving a roof without its covering. The membrane is usually an element that is ultimately covered. A delay in the works may require an additional assessment of its condition before it is covered.

It is worth checking both direct sunlight and the effect of light through openings. The material must be protected from dirt and from substances incompatible with its application. If it has been rubbed through or torn, the repair is carried out with a suitable system, not with a random tape from the site kit.

The area of the roll is not the same as the area of the finished covering. Laps, joints and details must be taken into account. Tapes, collars and the labour for sealing are added to the cost. For the interior layer, the junctions with walls and services are particularly important, and these often take more time than unrolling the film itself.

When comparing offers, list the same scope of application and the same set of accessories. Do not choose the thickest film without assessing its function. The best material is one that suits the calculated moisture regime and can be installed tightly. Correct installation is part of that choice.

On the order, record the function of each material: underlay on the outside or vapour control on the inside. Next to it, note the required build-up and the compatible accessories. The phrase ‘roof film’ alone leaves too much room for error.

The membrane is selected for its place of work and for the whole building element. Check the Sd value, watertightness, resistance and installation conditions, then plan the joints. A thick roll without correct detailing will not provide a tight or dry roof.