What goes beneath a roof covering, and how do you choose the right layers?

The word “underlay” is sometimes used loosely when talking about roofs. A roof drawing may call for a rigid deck of boards or panels, a membrane or bituminous felt. Each has a specific role. Name the material and its function on the order so that an unintended layer is not fitted on site.

Rigid decking can transmit loads to the frame and provide a base for the layers above. A flexible membrane needs support appropriate to its own role. Deck boards in turn need a suitable water-management detail. Review the joints, penetrations and drainage at the eaves as part of the whole assembly.

The covering and the selected system determine whether a continuous stable base is required. Many profiled steel sheets can span battens or purlins set at the specified spacing. Before adding or removing a full deck, check both structural loads and the moisture conditions in the altered roof.

Where decking is specified, check thickness, grade, supports and joint layout. Provide gaps and movement joints as required for the selected material. A wood-based panel may differ from boards of the same thickness in stiffness and vapour resistance. Agree substitutions in the design before delivery.

Wind can drive small amounts of rain or snow under the outer covering. Condensation can also form under certain conditions. The secondary drainage layer collects this moisture and directs it to an outlet. Arrange laps in the direction of drainage and seal the joints as required by the system.

The outer covering remains the primary weather protection. Check how long the underlay may be left exposed before covering, its temperature rating and whether it can lie on the proposed substrate. If building work stops for longer than the permitted exposure period, provide suitable temporary protection.

Where the design provides a ventilation channel above the underlay, counterbattens help maintain its depth. Keep the inlet at the eaves and the outlet higher up open. Trace the air path around roof windows, chimneys and valleys, which can interrupt a straight channel and require separate details.

Check whether the specific membrane is approved for direct contact with insulation. Bituminous felt on decking may require a different ventilation arrangement from a highly vapour-permeable membrane. Use the declared material properties and the complete roof section to decide where gaps are needed.

Above-rafter PIR boards form a continuous thermal layer when fitted with the specified fixings. Their facings and joints have particular vapour-resistance characteristics. Confirm whether the exact system can also perform an underlay or vapour-control function. Allow for indoor conditions, joint airtightness and the layers on both sides.

A roof sandwich panel combines an insulating core and steel facings. Installation follows panel-specific details for supports, joints and seals. Before adding layers beneath it, consult the panel documentation and the building design. The build-up for PIR boards on timber rafters cannot simply be transferred to a sandwich panel roof.

Specify the exact material and application. Include tapes, seals and penetration accessories. For roll materials compare the usable area after laps. For rigid decking allow for cutting and edge support. A missing tape or collar can hold up installation even when all the rolls have arrived.

Before laying the covering, inspect continuity, connections and the outlet for water. Photograph locations that will become concealed. Treat fastener penetrations, tears and contamination as directed for the product. Reliable secondary drainage depends on the underlay being properly integrated with the rest of the roof.