Our duo overlay board packs. Compromises of two layers of 3mm thick hardboard. Making the total height only 6mm. The rise in the floor really isn’t much. each covering a square meter-age of 0.72 sqm. The overlay board is laid in a brick work staggered fashion. This improves the overall strength, and the rigidity of the combination of overlay boards. And it also reduces the, minimal chances of any liquids. Might find there way through the flooring. And onto your underfloor heating system.