It should also be recognized that the waterproofing membrane may be exposed to damage during the installation of the terrace surfacing materials and that temporary protection may be required. Plywood sheets may be acceptable in some cases, but mineral surfaced roofing felt, asphalt board, or the material required for free drainage can provide the desired protection and be left in place. The amount of protection required is influenced by the surfacing system, the workmanship to be expected, and the bearing characteristics of the materials below the membrane. Adequate drainage of roof terraces is of the utmost importance because most terrace surfacing materials can be damaged or displaced by water or frost action. In addition to sloping the membrane, it is necessary to provide a subsurface drainage or percolation layer through which water can run freely.
Ventilation can also accelerate removal of water and drying of surface materials.
The subsurface drainage layer immediately above the waterproof membrane should be continuous under planting and paved areas, pools, fountains, stairways and even under garden walls or other nonstructural surface constructions. Stone or precast surfacing units can be supported by beams, corner posts or sleepers to provide the required space. It is also possible to groove, corrugate or contour the bottom of such units to produce a network of voids. Where the surfacing is not preformed or is made up of small units a no-fines concrete or granular base may have to lie employed. A granular base is normally used under planting areas to provide the required soil drainage.