When modeling the slab grid, the upper and lower chord planes are both horizontal (that is, the arching and drainage slope of the grid is not considered). This approach is very unfavorable to the vertical deformation (deflection) of the grid. Use support to find slope?

       There are usually two ways to find slopes. What you are talking about is to find slopes through small supports, which are only used for grids with small spans, and the second is to find slopes through structures. Slope, this method is required for large-span grid structures.
       There is also the combination of support and structural slope finding, especially when the slope finding on the structure is restricted (the structure is too high to find the slope and the angle of the adjacent members is too small), so it cannot be achieved If the slope of the building is required, the slope can only be found on the basis of the structure!
      In addition, if the slope is too high for the support alone, the combination of the two can also be used when the stability of the support is unfavorable!
       It is not right to say that it is very unfavorable. It depends on whether your grid is drained internally or externally. If it is drained outward, the purlin in the middle area must be very high, and the weight of the middle area will also increase, just its own weight. The deflection is large, which is not the case if it is drained to the middle.