Parapet and Roof Upstand System

A parapet is the part of an external wall that carries on past the roof surface. Because it stands above the roof it is weathered on its top and on both of its faces, and because it continues past the insulated envelope while remaining bonded into the wall below, the construction it puts above the roof line lies outside that envelope and stays continuous with what is inside it. It is a wall, a roof edge and an obstruction to drainage at once.

The roof's waterproofing does not stop at the parapet's base. It turns up the inner face and is mechanically held near the top of that turn-up. Above the termination, the coping and its own concealed damp-proofing take over. Those have to be considered together because anything getting past the coping arrives behind the roof waterproofing, on the wrong side of everything intended to keep it out.

A parapet also changes how the roof drains. Water that would otherwise leave over an edge is now retained by a wall, so outlets through or over the parapet become part of the assembly, and so does a route for water when a primary outlet is obstructed. Without a considered exceedance route, a blocked outlet turns into a retained body of water bearing against the junction.

The distinction from a rooflight kerb and upstand system is legible from the ground. A parapet is a perimeter element with weather on both faces and above, which is why it needs a coping and damp-proofing of its own. A rooflight kerb is an aperture inside the roof plane, surrounded by roof on every side, so water reaches it by running across the roof rather than by falling on it, and its top is closed by a manufactured unit rather than by a capping.

A parapet works from the top down, and the coping is the first thing water meets. Every joint in that capping is a potential entry, which is why the damp-proofing beneath it exists at all: not to make the coping redundant, but to catch what the joints let through and return it outside before it reaches the wall or the head of the roof waterproofing. Where that concealed layer is discontinuous at a corner or a joint, the coping stops being a weathering component and becomes a funnel.

The waterproofing termination and the coping define a zone between them that nothing else protects. Anything entering above the termination and below the coping's damp-proofing runs down inside the parapet, behind the roof covering, and reappears at the ceiling near the perimeter. That is the classic parapet leak, which looks like a roof failure and is not one. Raising the coping's damp-proofing or lowering the waterproofing termination each enlarges the band of construction that neither layer protects, which is why the two levels are set in relation to one another rather than independently.

Insulation and drainage pull the design in opposite directions at the same corner. Returning insulation up the inner face reduces the cold construction projecting outward, but it thickens the upstand exactly where the waterproofing has to turn and where an outlet has to pass through. Meanwhile the parapet has removed the roof's ability to overflow at its edge, so an obstructed outlet no longer produces a wet edge but standing water bearing on the turn-up. The insulation return, the outlet positions and the exceedance route are the same problem under different names.