During the first half of the 2nd century AD, one of the most important buildings in the history of Western architecture was erected in Rome: the Pantheon. Its main and most impressive feature is its coffered concrete dome, which ends in a perfectly round central opening. This oculus kicked off a series of later projects that noted the value of circular openings, which were replicated as glazed skylights and as compositional elements on facades. This eventually evolved, for example, towards the detailed and colorful rose windows of the Gothic basilicas. In all its configurations, the oculus (eye in Latin) holds a symbolism that goes beyond the traditional window: its luminous projection gracefully marks the passage of time, solemnly highlighting an architectural space.
Measuring almost 30 feet in diameter, the spectacular oculus of the Pantheon is completely open; the rain enters the building and slides down to reach the 22 drains located in its pavement. However, most of the circular openings that succeeded it in history–from Palladian houses and Baroque buildings to palaces in the Middle East–incorporated glass to let only light through and avoid unwanted infiltration, a tendency that has continued to this day.
The oculi located on modern roofs require a series of special measures to avoid problems during the use of the building, such as applying waterproof and hermetic sealing at its perimeter, smoothing the transition between pieces to allow for unobstructed drainage, and choosing glass with adequate thermal, acoustic, and visual performance, including a high level of protection against UV rays. Obviously, the location of the skylight must not interfere with or weaken the roof structure, and its arrangement with respect to the solar path must be carefully considered. Resistance to impacts, burglary, and fire are also important issues to consider.
Currently, there are prefabricated circular skylights that are easy to install and use, which can even be opened (manually or electronically), contributing to the ventilation of the space. Although the natural slope of the roof should aid in the self-cleaning of the element–particularly when using flat glass–many circular skylights are clad in acrylic (or Plexiglass) domes, which allow light to spread evenly through space and let the water runoff as it falls on them. Generally, these elements are not located at the same level as the ceiling; on the contrary, they appear with a certain thickness that depends on the structure and materiality of the roof, and some models even rise between 30 and 70 cm above the roof level. This thickness can amplify the effects of the circular skylight; if it is white, the light reflection will be even stronger.
Let's review some recent examples. In the renovation of a 1960 building in Melbourne, the team consisting of Nervegna Reed Architecture and ph Architects used circular shapes to create a special environment for worship for the local Islamic community: "The space connects with the sky through the skylight which punctures the tent form, one of the oldest architectures of worship, forming a quiet oasis on the edge of the city." In MNMA studio's Selo Store, the oculus provides "a clear view of the sky, not only for illumination but also for a playful contact with nature, weather, the passage of time and to have some human scale perspective."
Designed by Fragmenture, the L&W House is configured around a central module of straight and vanishing lines. Here, the circular skylights covered by transparent domes provide abundant overhead light to the dining room and kitchen, working as a counter to the orthogonality of the furniture. In the case of the Malan Vorster Architecture Interior Design pavilion, the "porthole" is used to highlight a distinctive all-wood clad staircase.
In the United States, the Mill Valley Guesthouse by Turnbull Griffin Haesloop Architects is "dotted" by circular skylights, filling the interiors with light filtered by the surrounding trees. In Lebanon, the Carl Gerges Architects-designed guesthouse includes a burgundy underground cellar, which is "lit by a circular skylight, and hosts an elemental and synesthetic ambiance. A primitive silence with solemn lighting pairs intuitively well with the cool and moist air which smells of toast, oats, and musk."
Check out more selected projects here.