Staad pro doesn't support for modelling of rigid diaphragm for inclined floor/inclined slab/inclined roof, so that response spectrum analysis can not be achieved in this case, so it is necessary to implement feature of modelling rigid diaphragm/semi rigid diaphragm for inclined floor/inclined roof....
Thank you for posting this idea. However if you have inclined planes then you can use the control/dependant feature to define a control node and make one or more others dependent. Use the rid option to link all degrees of freedom.