Whether it is a shower or an exterior deck or balcony, the waterproof membrane surface must be sloped to drain or away from the building. Shower pans or receptors are supposed to have a pre-sloped mortar bed installed over the base substrate before installing the waterproof membrane. The pre-slope needs to have a minimum slope of 6 per 305 mm (1/4 per 12 in.) and be flat within that plane with no more variation out of plane than 6 in 3049 mm (1/4 in. in 10 ft), or 2 in 305 mm (1/16 in 12 in.). Exterior decks and balconies should also have a pre-sloped surface to meet those standards before applying the waterproof membrane.
On exterior decks and balconies over occupied spaces, the waterproof membrane should be a primary roofing membrane. Achieving an adequate slope to drain is a critical step to achieving a successful installation that will properly manage the water.
This author worked on a high-rise condo a number of years ago where there was no pre-slope base under the shower pan waterproof membrane of the shower installation. The waterproof membrane was laid directly atop the flat structural slab. It so happens the shower area was at a high point of the structural slab, so the membrane was actually negatively sloped away from the shower drain.
In this case, the waterproof membrane was not correctly flashed or seamed—when the water reached the waterproof membrane under the tile assembly, it sloped to and through the breached seam, and then traveled through an open-cell acoustical sound board under the bathroom tile floor. The water traveled through the water-sensitive sound board out under a hardwood floor in the adjacent rooms, resulting in the warping of an expensive hardwood floor, deterioration of the sound control board, and massive microbial growth on the wood underlayment under the flooring.
Ironically, when all the floors were removed, the only dry spot in the condo was where the shower pan was located, since it was negatively sloped away from the drain. The entire condo had to go into containment with HVAC air filters, et cetera. To say the least, it was a tremendously expensive remediation.