The building envelope concept maintains that in order to effectively keep water out of a structure one must design and construct continuity between the vertical and horizontal components of the building's waterproofing systems. Accordingly one must consider walls when dealing with roofs and vise versa.

Examine figure 1, Typical Roof Cross Section, and note that the roof membrane is only part of the system which keeps water out of a building. Perimeter flashings make the transition between the roof and the side walls. Thru-wall flashings make the transition between the roof and intersecting walls such as is found at penthouses and similar structures.

Figure 2, Cavity Wall Cross Section, indicates that the building envelope concept must be followed through in the design of walls. Thru-wall flashings must be incorporated in cavity wall systems above relieving angles and at deck intersections.

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