TPO Roofs in Vienna, VA: What Maple Avenue’s Zoning Says About a Flat Roof

Section 18-212 measures a principal building from the average finished grade at its front to the highest point of the roof. Along Maple Avenue the limit depends on the district. The Avenue Center District allows 42 ft. and drops to 35 ft. within 95 ft. of a residential property, across a street included. The Avenue East and Avenue West Gateway Districts allow 35 ft. Every one of them asks for a 16 ft. minimum ground floor, floor to floor.

The code does not say how added roof thickness is counted. On a building close to its limit, ask staff whether a tapered insulation build-up that raises the roof surface changes the measured height.

Section 18-536 says rooftop equipment such as air handlers and exhaust fans should sit as close to the middle of the roof as possible, so it is not visible from the right-of-way or from neighbors. It then says all rooftop equipment must be fully screened from both by material consistent with the building’s facade. Section 18-501 applies the whole article to new construction, additions and alterations of commercial, industrial and mixed uses in any district.

A screen is a structure standing on the roof, so in our reading it brings supports and curbs that the membrane has to be flashed around. The Asphalt Roofing Manufacturers Association adds a drainage point for any low-slope roof: slope or cricket the roof upslope of equipment, and pipe HVAC condensate to a proper drain (ponding).

The Avenue Center Amenity Overlay exists to accommodate rooftop amenities above the base height. A publicly accessible rooftop use, rooftop dining for one, gets an extra 12 ft. for an elevator or stair penthouse only, which must sit centered on the roof, 10 ft. from the sides and 20 ft. from the front and rear. Chapter 18 is silent on what lies under that use. The model building code does require low-slope coverings to resist impact, by one of three named tests including a foot-traffic test, so ask how any rooftop use is built over the membrane.

The Church Street overlay lists first-story roofs at a minimum slope of 6:12 and bars mansard roofs, while its conceptual renderings name “flat with cornice” among roof forms. The model code’s design minimum for single-ply is one-quarter unit in 12, so ask a manufacturer about a product’s slope range before a steep front roof is priced.