Choosing an underlay for the planned roof covering

In a conversation about a roof, the word substrate can mean different layers. One person means rigid sheathing, another a membrane, and a third battens under the metal sheet. Their tasks are different. Sheathing or a substructure provide support, while the underlay primarily helps protect against water that gets under the covering.

Therefore, in a quote it is worth specifying the names of all elements and their sequence. The entry substrate in the price alone is too general. It does not say whether it includes boarding, underlay material, tapes and the execution of details. When comparing contractors, the lack of such a breakdown can lead to significant differences in the later scope of work.

In many pitched roofs, a high vapour-permeable membrane is located above the insulation, under the counter-battens and battens. It protects against accidental water and limits draughts through the layers. However, its usefulness depends on the product’s properties and the roof assembly. The permitted substrate, temperature resistance and rules for direct contact with the insulation must be checked.

The membrane does not replace the load-bearing structure and is not a working platform. It requires correct overlaps, sealing of penetrations and water discharge at the eaves. In a typical assembly, a ventilation gap formed by the counter-battens remains above it. Its performance is also determined by the inlets and outlets, not just the material unrolled on the rafters.

Boarding or suitable boards create a continuous substrate. It may be required by the selected technology or result from the design assumptions. However, the additional weight and the method of moisture drainage must be taken into account. Rigid sheathing does not automatically mean that any underlay material can be laid on it without further checks.

If a layer with high diffusion resistance is present on the sheathing, the insulation and ventilation assembly requires particular attention. Replacing one material with another can change the roof’s ability to dry out. Moist wood should not be enclosed between impermeable layers. During renovation, it is worth establishing what is actually under the existing covering.

Tile-effect metal roofing requires support in accordance with the module geometry and the instructions. The first batten and subsequent spacings may have different significance. Trapezoidal steel sheet is selected for the support spacing based on structural data. One cannot assume a single scheme just because both coverings are steel.

Similar tile-effect metal roofing profiles may also require different settings. Before ordering battens and starting work, you need specific instructions. The design must also account for the support of flashings at the roof valley, chimney and edges. A sheet that has proper support in the middle of the roof slope may require an additional solution at a short offcut.

With above-rafter insulation, PIR boards become part of the system above the structure. The fixings must transfer loads through the layers in accordance with the design. The airtightness of the joints and the support of the counter-battens must be resolved. It is not enough to add a rigid board under the old installation pattern and use a longer random screw.

A roof sandwich panel works differently. Its facings and core form a ready-made lightweight enclosure element supported on purlins. It does not automatically require the same membrane and batten arrangement as tile-effect metal roofing. In both technologies the final solution follows from the documentation, but their layers and fixing principles must not be mixed.

Start with the design of the entire building envelope and the requirements of the specific covering. Only then compare the cost of the boarding, membrane and installation components. A cheaper material may require additional work or may not suit the intended substrate. Conversely, an expensive substrate will not improve the roof if its joints remain leaky.

At handover, check the continuity of the layers, the condition of the support and the execution of the details before they are covered. Keep photos and material markings. This way, during future installation of a window or services, it will be clear which arrangement must be restored. A good substrate is part of a coherent roof in which each layer has a clearly defined task.