Roof drainage failures: consider the details

Figure 1 A copper diverter flashing was installed to deflect water from the gable roof into the adjacent main roof gutter. >Photos courtesy Wiss, Janney, Elstner Associates Inc. (WJE)

When developing roof drainage details, designers need to consider not just typical conditions and geometries, but also atypical or transitional geometries that may concentrate or direct water flow in unexpected ways. A sloped roof gutter detail which works well at typical locations may be inadequate for re-entrant corners. If the gutter is cross sloped for drainage, the detail at the shallowest portion of the gutter deserves special consideration.

During review of existing exterior conditions at a 1930s institutional building in the Midwest, the detailing at a built-in cornice gutter was investigated. This building has a sloped clay tile roof and a built-in gutter lined with sheet metal. At most portions of the building, the sheet metal lined built-in gutter is performing well. However, the detail at the eaves of a lower cross-gable roof where it intersects the main roof was found to be problematic.

At this location, the sloped roof of the short cross-gable roof intersects the main roof, creating a valley.

As originally constructed and contrary to the original design drawings, the gutter did not extend along the short eaves of the perpendicular gabled roof; rather, a copper diverter flashing was installed to deflect water from the gable roof into the adjacent main roof gutter (Figure 1). Given the arrangement of downspout locations, this end of the main gutter is the high point of the gutter cross slope and, therefore, the shallowest portion of the gutter. The result of this combination of geometries is a large volume of water being funneled by the roof valley and the diverter into one area of the gutter—the location at which it has the least depth to receive the water. During heavy rains or when filled with snow in the winter, this portion of the gutter regularly overflows; exposing the painted wood cornice below to undue amount of water, resulting in deterioration (Figure 2).

Figure 2 The gutter overflow has resulted in deterioration of the painted wood cornice.

The original 1930s designers developed an effective and robust roof gutter for this building, which has performed well in draining most of the roof areas for more than 80 years. However, it appears the gutter depth, slope, and downspout configurations are inadequate to drain the water that would be collected by a valley. The inadequate gutter and downspout configuration was exacerbated by a change during original construction, which further concentrated water flow to one location. During a planned roof repair project, modification of this detail is recommended to improve the performance of the assembly. Possible strategies include extension of the gutter along the cross-gable portion of the roof, reconfiguration of the gutter drainage so a downspout and low point of the gutter are located at this transition, or installation of diverter flashings at the gutter to prevent water from overflowing.


Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation. He can be reached at

Koray Tureyen is a senior associate and licensed professional engineer with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois. He specializes in the investigation and diagnosis of structural and water infiltration problems.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier
or CSI.

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