The underconstruction must be dimensioned in such a way that it can withstand any deformation during swelling and shrinkage during the year. It has to be stable for the long term use. Therefore, only wood of the same or similar density as your decking should be used for the underconstruction. So for exotic wooden terrace you have to use only exotic wood underconstruction, for larch terrace use larch underconstruction, for thermo pine terraces use thermo wood underconstruction etc. For sufficient installation, only stainless steel screws should be used. The choice of steel grade depends on the specific wood type. The distance between underconstructions should not exceed 40-50 cm (depending on the thickness of the boards). E.g. th. 21 -22 mm = max. 40 cm and 25-28 mm = max. 50 cm.