TEP ROOF OY · MATERIAL AND INSTALLATION GUIDE

Vapour control in timber roof construction

A timber substrate may mean a structural timber board or a ventilated trussed roof. Vapour control position and installation differ between these, so timber alone does not identify the solution.

TEP-MK-06 · Version 1.0 · 19.10.2026

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Distinguish two types of construction

In a ventilated framed or trussed roof, a vapour control film is installed at the warm-side level specified in the design. Structural support and joint detailing are essential: a vapour control film or taped joint is not intended to support insulation.
In a roof with insulation above a structural timber board, the vapour control layer may be installed on the board surface. Timber boarding beneath the roof covering is a different layer. Distinguishing these prevents the vapour control layer being placed incorrectly.

Possible material options

For a well-ventilated roof, a PE vapour control film specified for the application and its sealing system can be compared. Check the film's long-term durability, joint tapes and penetration collars together. Plastic film thickness alone does not define the system or permit substitution of the specified product.
Manufacturer-approved self-adhesive bitumen-aluminium vapour control systems are available for timber boards, including plywood and OSB. In these systems, the board, primer and jointing method form a complete solution. Approval cannot automatically be transferred to another timber product, board coating or moisture condition.

Narrow the choice according to the build-up

Select water vapour resistance according to internal moisture exposure and the potential for drying. A film solution for a well-ventilated trussed roof must not be transferred directly to a poorly ventilated timber roof. The designer must assess the full layer sequence before the material is confirmed.
Movement of timber and other elements must not tear the film or open the joints. Provide the designed movement allowance at connections, support joints and match fixing to the product instructions. Sealing must not rely solely on a taut film.

Agree the work before covering

Provide a roof section, timber board product and surface information, and a penetration layout. Define vapour control continuity at walls and prefabricated element joints. Agree who seals interfaces between contracts and who inspects them.
Establish timber moisture condition, protection and working methods before starting. A timber substrate does not give general permission for flame work. Request an itemised method, junctions and inspections in the quotation; any heating and site fire safety measures must be determined separately.

Use and design responsibility

TEP Roof Oy guide cards support project preparation and the definition of contract scope. They are not project-specific construction designs. The project structural designer is always responsible for checking the suitability of the solutions presented and adapting them to the project's structures, loads and requirements. Construction requires project-specific design and approval, together with the current manufacturer's instructions for the selected products.

Sources

An editorial compilation by TEP Roof Oy. The product- and system-specific source instructions must be checked for the selected solution.