This fixed head with 23mm x 4mm tabs wont self-level when installed with Harmer Modulock deck supports and the 23mm x 4mm tabs will support the material being laid over the system. What is Harmer Modulock deck support system? Harmer Modulock deck supports allows you to adjust the height of paving slabs, timber or plywood sheet decking to level out irregular, uneven or sloping surfaces so the spaces are more usable. Harmer Modulock is a raised pedestal system which also lends itself to concealed drainage, pipework, and services beneath a substrate and application for terraces, walkways, balconies and ballasted flat roofs. Complete with self-levelling heads that allow for a 5% gradient change, the heads have anti-noise and anti-slip rubber layers on their heads. Using Harmer Modulock deck supports to adjust the levelling of surfaces is extremely easy.