Wednesday, June 02, 2010

The Opportunities of Constraints: Washington's Building Height Limits and Rooftop Landscapes


Part II of our series on height limits in DC
By Sacha Rosen, AIA

Imagine a vista of green rooftops stretching as far as the eye can see – floating slightly above the tallest trees lining the public streets below, and fading off into a horizon of forested hills to the north and the river valley to the south. The laws regulating Washington's building heights – the 1910 Height Act in particular – have created a unique and recognizable urban skyline which underscores a local culture of history, democracy, and respect for the institutions of government. But the prevailing building heights in Washington also create the opportunity for a system of vegetated roofs which is environmentally sustainable, compatible with the best of contemporary architectural design, and adds yet another “green” dimension to the unique character of the city.

Design Challenges

Practically speaking, the L’Enfant Plan, 1910 Height Act, and other local zoning ordinances cause most buildings in Washington to have broad floor plates, shallow floor-to-floor depths, and a single principal facade on the property line facing the street. Mid-block buildings are fully built to the property lines at either side, and corner buildings fill their lots and abut all street frontages. Few buildings are seen in the round, and if they are, their side and rear facades are typically designed to be secondary in nature. Because most developers want to maximize the building volume, very little sculpting is possible.

Architects and critics complain that the building forms indigenous to Washington are retrograde, boxy, and uncool. Contemporary architecture provides few tools for designing the flat, horizontally-proportioned street facades of the typical buildings here. It can be difficult to make a building appear to soar when it’s as broad as it is high, and it is challenging to create a plastic, sculptural facade while also striving to achieve the absolute maximum enclosed volume. So many designers rely on traditional styles, or end up with something that looks like half a building from somewhere else.

Despite the challenges of designing Washington facades in a contemporary fashion, leaders of the local architectural community have produced some nice buildings. Shalom Baranes’ 22 West Condominiums is a nice geometrical composition in glass and dark metal; HOK’s International Brotherhood of Electrical Workers (pictured, at left) has the elegance of much taller glass boxes; Phil Esocoff’s curved brick and ornamental cast stone at 400 Mass Ave bring these materials into the 21st century. Although not yet in this city, international (star)chitects such as Herzog de Meuron, Renzo Piano, and Williams + Tsien have embellished other cities with beautiful, contemporary or avant-garde stand-alone facades which could stand as inspiration for future projects here.

Design Opportunities

Architects experienced with the Washington context are familiar with a number of design opportunities within the local regulations and traditions. The first is the concept of “spires, towers, domes, minarets, [and] pinnacles”, otherwise known as “rooftop embellishments” which are specifically allowed in the 1910 Height Act as exceptions to the height limits. Originally conceived to permit significant prominent features of governmental and institutional buildings (the Capitol dome and church spires, for example) to rise above the balance of the cityscape, the design community has capitalized on this exception for contemporary commercial buildings as much as civic ones. Unfortunately, some of these look like spiky halos (1980’s), luggage racks (1990’s), or the now-ubiquitous folded metal sunshade (2000’s).

The second form-based design opportunity is the concept of “projections into public space,” permitted by the building code, which includes bay windows, balconies, canopies and marquees, cornices, beltcourses, and pilasters. Architects rely on these to add richness and depth to otherwise flat facades. We all love looking into the bay windows when we’re strolling among Capitol Hill’s townhouses; we don’t often notice how important these elements are on large buildings as well. Although these features are often criticized as overly traditional, good designers are able to stretch and reinterpret the rules in the name of plasticity and drama.

What people don’t often recognize is that Washington’s zoning parameters and building height requirements also happen to embody a number of design- and form-generating features which are consistent with the fundamental principles of Modernist design, as formulated by Gropius, Le Corbusier and other masters beloved of the Dwell-reading set. Although these architects made grievous errors in urban planning (including Corbu’s demands to raze most of Paris in favor of a few highrise towers), their work in aesthetics and building design is still relevant and forward-looking.

Horizontality, and not verticality, is one of the principal hallmarks of Modern design, in contrast to traditionalist styles. This is clear to anyone who has laid eyes upon the dramatic roof overhangs of Frank Lloyd Wright’s prairie style houses or the crisp geometry of Mies’ design for the iconic Barcelona Pavilion. In downtown Washington, the overall horizontality and comparatively short building facades are accentuated by the shallowness of the floor-to-floor heights, which results from stuffing a maximum number of floors under the height limit. To reduce the amount of height required for floor structure, almost all buildings tend to be constructed of flat-plate concrete slabs and columns, setting the stage for the realization of several other tenets of Modernism: “free-floating” interior columns unbound by the constraints of load bearing walls and exterior curtain walls free of structural elements.

Rooftop Opportunities

One of the most significant design opportunities seized upon recently by local architects and the development community is the building rooftop. This is not a Washington invention; modernists as early as Gropius regarded the flat roof as a “fifth fa├žade,” a part of the building that would be increasingly visible from ever taller adjacent buildings and passing airplanes (or Google Earth, these days). To Le Corbusier, his urban plans notwithstanding, the roof was the focus of one of his canonical “five points” of modern architecture, and almost prophetically, he called for the roof to be planted as an elevated garden to replace the landscape claimed by the building’s footprint. Today, investment in time and effort required to improve a rooftop is justified by the provision of additional building amenities, or simply an architectural gesture that will differentiate the building in the marketplace.

A number of factors contribute to the design opportunities afforded by Washington rooftops. First, the general building massing determined by the height limits in conjunction with the L’Enfant plan produces relatively large floorplates, with the roof level similar in size to the typical floor. Although building mechanical equipment is typically located on the roof, this equipment is not overly large, since it serves a relatively short building. In fact, the size of the rooftop mechanical enclosure on most buildings is limited to 37% of the total site area, and 1/3 of the total roof area. Such enclosures must be set back from the edges of the building by a distance equal to their heights, and must be of a consistent height and of a material compatible with the main building exterior. Therefore, at least 2/3 of the roof area, and typically the entire perimeter, is open to the sky. This amount of open area presents a veritable creative playground to a design-minded architect.

Until several years ago, the zoning regulations required residential buildings to provide a certain amount of residential recreational space, a significant portion of which had to be located outdoors. Because most buildings almost completely cover their lots in the downtown core, this outdoor recreation space was typically located on the roof in buildings. At the same time, the interior space of the mechanical penthouse was specifically prohibited from including any space for “human occupancy.” Recently however, the recreational space requirement has been lifted, coinciding with an easing of the strict rules governing the permitted uses within rooftop structures. First modified to allow uses appurtenant to outdoor recreational facilities (such as showers near outdoor swimming pools), the regulations now permit a wider range of recreational facilities on the roofs of apartment houses and hotels, including fitness centers, bars, and party rooms.

To enhance the quality of rooftop spaces, some designers have included plantings and gardens on building roofs for many years. Today, the more stringent requirements for stormwater filtration and retention, as well as requirements and market demands for environmentally sustainable projects induce the industry to provide substantial vegetated roofs on many projects. Buildings with minimal occupant roof access typically have extensive roof plantings (think sedum or turf), while buildings with significant occupied roof terraces are provided with a combination of intensive (think trees and shrubbery) and extensive vegetated areas – the latter also provided on the tops of mechanical penthouses.

Rooftop Panorama

Many of the rooftops in the downtown core are within one or two stories of each other in height. They are quite large, and increasingly provide substantial vegetation. A few roofs have significant access for residents, workers, and guests. As a result, there is a potential for beautiful vistas from each rooftop to neighboring buildings, an overall urban amenity created by the prevailing height limit and related planning context. This elevated garden plane is not so high as to be disconnected from the extensive system of street trees in the city, which already helps make Washington one of the greenest cities in the world.

In addition to the visual and social aspects of this rooftop garden plane, these green roofs have an increasingly significant impact on the environmental sustainability of the city. Such roofs mitigate heat island effects, manage stormwater runoff, provide building insulation, and present a use for treated grey- and blackwater systems within buildings. The potential for such an extensive system of rooftop plantings suggests that there may be additional opportunities for a true ecological impact, as flora and fauna adapt to this unique elevated landscape.

What could be greener or more sustainable than this rooftop panorama? Let’s embrace the opportunities provided by our building height limits, and continue to develop our own unique architectural language and urban design sensibility. No other city has the potential for the reintegration of plants and landscapes into a dense urban environment on such a grand scale. And this sustainable vision is only possible as a result of our building height limits and urban plan.

Sacha Rosen is a Principal of R2L:Architects, a firm specializing in the architectural design of multi-family residential, commercial interiors, and institutional/higher education projects in the Washington DC area. Sacha has ten years experience in architectural design, including eight years of project management experience on multifamily residential, urban mixed-use, commercial office, chancery, hospitality, and campus planning projects. He has a Master of Architecture degree from the University of Oklahoma, where he also taught courses in the history of architecture.

1 comments:

Jose said...

Wouldn't it be great to have a series of green roofs that were somehow coordinated, so that there was some kind of underlying design element between different buildings, to accentuate the contiguous line of green roofs?

 

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