Flat Roof - Over Joist

Flat roof insulation installed over the joists creates a continuous thermal layer across the structural frame, helping to reduce thermal bridging and improve overall energy efficiency.

The SuperFOIL multifoil system is designed for flat roof applications in both new build and refurbishment projects, where insulation is positioned above the joists and beneath the external waterproof finish.

This system can form part of a warm roof or hybrid build-up depending on project specification.

How the Over Joist System Works

Installation begins at one edge of the roof.

The insulation is:

Stapled directly into the joists to secure the start of the roll
Rolled out across the flat roof surface
Installed under gentle tension to remove slack
Stapled at maximum 300 mm intervals
Overlapped by a minimum of 50 mm on each run
Fully sealed at all overlaps using SuperFOIL MultiFOIL Tape

At the opposite edge, the insulation is cut flush with the joist line, sealed, and stapled in place.

This creates a continuous reflective insulation layer across the roof structure.

Joists are typically constructed from timber or steel, depending on the structural design of the building. The method of fixing insulation must be adapted to the substrate, with timber joists typically allowing mechanical fixing using staples or battens, while steel structures require appropriate self-drilling screws or other mechanical fixings.

Counter Battens / Firrings & External Finish

To complete the installation:

Timber counter battens or firrings are fixed through the SuperFOIL into the joists
Battens must be installed at maximum 600 mm centres
Batten/firring depth must match the selected SuperFOIL product
External decking and waterproof membrane are then installed over the firrings

Important technical note:

Firrings must not fall to zero depth.
They must maintain sufficient depth to allow SuperFOIL to expand and create the required low-emissivity airspace.

Timber battens or firrings must be appropriately sized (typically 25–90mm depending on product thickness) to create and maintain the required airspace and structural support for the external roof finish.

Sealing & Airtightness

To ensure long-term performance:

All overlaps must be taped
All cuts must be sealed
All penetrations must be airtight
Roof-to-wall junctions must be fully sealed

Failure to seal correctly may result in air leakage, reduced thermal performance and condensation risk.

Roof-to-Wall & Cavity Wall Junctions

Flat roof installations require careful detailing at junctions.

Roof-to-Wall Junction

Overlap minimum 50 mm onto wall insulation
Fix with mastic or double-sided tape (optional)
Seal with reflective tape
Secure with counter battens

Cavity Wall Junction (critical detail)

Where the flat roof covers an external cavity wall:

SuperFOIL must only extend to the inner leaf of the cavity wall
It must NOT pass over the cavity
The airspace above SuperFOIL must be isolated from the wall cavity

This prevents interstitial condensation issues.

Penetrations & Services

Where services pass through insulation:

Carefully trim insulation
Avoid gaps
Seal fully using SuperFOIL MultiFOIL Tape
Double-sided tape or mastic can enhance the seal

Copper pipes must be wrapped in non-metallic insulation before contact with aluminium foil to prevent reaction.

Hybrid Roof Solutions

The guide specifically mentions hybrid systems.

If insulation is installed both:

Above joists
And below joists

Special consideration must be given to condensation risk.

SuperFOIL strongly advises technical consultation before specifying hybrid build-ups.

Where SuperFOIL is specified as part of a hybrid roof solution, condensation risk analysis and professional technical guidance are strongly recommended.

Importance of Airspaces

Airspaces are critical for multifoil performance.

To maximise efficiency:

Correct batten depth must be used
Airspace must be maintained within the roof build-up
Remaining joist depth must allow for a 50mm ventilated cavity if additional insulation is used

Without airspace:

Radiant energy becomes conductive heat
Insulation effectiveness is reduced

Maintaining airspace allows SuperFOIL to reflect up to 95% of radiant energy.

Installation Checklist (Professional Section)

You can include this contractor-focused block:

Before completion ensure:

Product cut and trimmed correctly
Staples spaced no more than 300 mm
All overlaps taped
Correctly sized counter battens installed (max 600 mm centres)
Ventilated cavity maintained (minimum 50mm where required)
Suitable finishing material installed

Recommended Products for Flat Roof – Over Joist

For flat roof over-joist systems, SuperFOIL multifoil products are selected based on the overall build-up design, required U-value and moisture management strategy.

Commonly used products include:

SF19
SF40
SF60

These are used within standard multifoil build-ups with correctly designed airspaces.

Technical note: these products act as an airtight reflective insulation layer with integrated vapour control (VCL), making them suitable for constructions where vapour movement is restricted and moisture is controlled within the build-up.

Where a breathable construction or enhanced moisture management is required, breathable products may be specified:

SF19BB
SF40BB

Such solutions must form part of a fully designed system and should always be supported by condensation risk analysis.

Technical note: breathable (BB) products allow water vapour to pass through the construction while remaining watertight, enabling outward drying and reducing the risk of moisture accumulation within the roof build-up.

The final product selection must always be verified through U-value calculation.

Installation Guide

For full installation procedures, detailing requirements and technical specifications, please refer to the official SuperFOIL Installation Guide.

Download Flat Roof - Over Joist Installation Guide (PDF)