Construction defects: dampness

What happens when windows have no drip edge ("leaking tooth")

A missing drip edge in a window is a common cause of dampness, and damp walls are an especially stubborn problem. Often all it takes is one missing groove to produce the defect many people recognize on sight: a damp exterior wall. A home inspection that looks for this kind of defect will catch it, and spare you dampness you can't otherwise explain.

What is a "drip edge"?

A drip edge keeps rainwater away from the walls. In professional terms it is also called a "leaking tooth" (shen madlef): a groove cut into the underside of a stone window sill. Reach under the part of the sill that projects outward and you can feel this groove running along its length.

What the drip edge does

Rainwater running down a building's exterior walls also reaches the window sills, flows over them, and creeps along the underside of the stone, in the part that projects past the wall, toward the wall beneath the sill. The drip edge stops that creep: when the water reaches the groove, it drips off instead of continuing along the underside of the sill until it reaches the wall below.

Where else a drip edge is used

A drip edge is also built into coping, the stone installed along the top of a wall, typically on walls that separate the balconies of adjacent apartments, or on roof parapets. Here is an example of a drip edge in a roof parapet, built to the standard:

Without a drip edge, rainwater behaves as described above: it runs to the underside of the sill and creeps to the wall beneath it. That lets dampness into the joint where the sill meets the wall, and from there into the apartment, and it also stains the exterior wall.

Aesthetic damage as well

On walls without a stone drip edge, the water simply runs down the wall and stains it. The photo above shows a wall clad in stone at the top, but the stone has no drip edge, and you can see the line where water has seeped between the stone pieces.

This aesthetic defect gets worse every winter and forces the wall to be repainted every year.

One difficulty facing home inspection engineers is that the standard itself is not clear on this point. It does require a drip edge on roof parapet walls, as in the illustration above, but sets no direct requirement for window sills.

This is where an engineer's experience and rigor matter: checking not only against the standard, but against good practice, under which a drip edge is unambiguously required, and which is reflected in the general specifications that call for it.