Flounder houses and flat city roofs
Drive through the city’s brick neighborhoods and look up: a large share of the housing has no shingles at all. Flat roofs draining behind parapet walls, and the flounder houses. That St. Louis oddity with one tall wall, one short one, and a single roof plane sloping between them. These roofs work on different physics from a suburban gable, and they fail differently too.
- 1 Ridge caps. First to fatigue in a climate that swings hard in both directions.
- 2 The field. Thaw cycles work the covering harder than the calendar suggests.
- 3 Valleys. Litter holds water against the covering exactly where it should be leaving.
- 4 Flashing. The chimney, and where a lower roof runs into a brick wall.
- 5 The vent boot. A rubber part with a life measured in years, not decades.
- 6 Eaves and gutters. Refreezing and a plugged box gutter back water up under the edge.
A flat roof is a shallow pool that must never act like one
A pitched roof sheds water; a low-slope roof disposes of it: down a gentle fall, to a scupper or an internal drain, often toward the alley. Everything depends on that fall staying true and those outlets staying open. A membrane with a low spot holds a pond after every rain, and ponded water ages any membrane fast: it magnifies summer heat, freezes in the winter oscillation, and probes every seam it covers.
The perimeter does the rest of the failing. Where the membrane turns up the parapet, the joint endures the most movement on the roof, masonry and membrane expanding on different schedules, and the coping on top of the wall is the detail that keeps water out of the brick’s open core. A city roof leak traced honestly very often ends at a parapet corner or a plugged scupper, not in the middle of the field.
The flounder’s single plane
The flounder house concentrates all of this on one sloping plane, usually clad in membrane or roll roofing at a pitch too shallow for shingles. Its tall side wall works like a parapet along the entire length of the house, so the roof-to-wall flashing there is effectively half the roof system. Where a later owner added shingles at a pitch below what shingles tolerate, the covering sheds rain but loses to slow meltwater : the covering is wrong for the geometry, and no repair changes the geometry.
Replacement on these buildings is a different craft from suburban work: membrane selection, tapered insulation to rebuild the fall, parapet and coping detail, drain hardware. It is worth asking any bidder what share of their work is low-slope; a crew that lives on suburban gables treats a city flat roof as an oversized patch, and the alley side of the block is full of the results.