The spacing of pedestals under terrace joists should comply with the recommendations of the installed terrace system's manufacturer. Installation that deviates from the manufacturer's instructions may result in voiding the warranty.
Generally, the spacing of pedestals depends on the material of the joists and their stiffness. Typically, joists are placed at a distance of about 50 cm, and in such cases, pedestals are often set at the same interval along the joists (resulting in 5-6 pedestals per square meter). However, the final spacing of the pedestals depends on the intended use of the upper surface and the loads it will be subjected to. If there are no specific instructions from the supplier of terrace joists regarding the appropriate spacing of pedestals, it is best to perform a test by setting the pedestals at a distance where there is no deflection under the weight of an adult person. Consult your joist supplier for the recommended spacing of terrace pedestals. Detailed information on the number of terrace pedestals per square meter in a ventilated terrace can be found in this article - How many pedestals per m2 for a ventilated terrace.
