If the installation of 4.8mm thick hardboard flooring above the line of existing flooring within a SUPALUX® timber joisted floor system isn't practical, our suggested solution would be to apply it below the line of the existing flooring. It should be mechanically fixed, tightly between the joists, and sealed with a bead of FSi PRYOCOUSTIC® fire resistant sealant between the boards and the joists. As long as the board joints between the existing and new flooring layers aren't coincident, this will effectively mimic the standard T&G jointing method for the required flooring within our tested systems.

Note: As this represents a departure from our fully tested and certified system details, we recommend speaking with your building control officer or responsible person to sign off this recommendation.