Can concrete appear as thin as paper and defy gravity?
A cantilevered, ultra-thin concrete roof challenges gravity—not as an artificially generated image that ignores physical laws, but as a real built structure in an ordinary garden. With this 53-square-meter garden pavilion, FACE and DUCON bring ultra-high-performance concrete from the lab into practice, allowing it to float delicately. An 11 by 5 meter, just 90 millimeter thick roof slab hovers over a long table with an integrated fire pit. Along the line of maximum bending moment, a circular oculus frames the sky—and on clear nights, outlines Orion's Belt. The span is vast, the gesture minimal, the message clear: Concrete can be lightweight. White steel cantilevers lift the slab and subtly reverse the flow of loads.
The garden-facing partition of rotatable wooden slats defines the outdoor space of the pavilion, while a Kambala wood lounge—comprising bench and table—seamlessly extends the living area into the terrace. The materials remain honest: UHPC roof, natural stone floor, hardwood slats, Corten planters, and a discreet drainage channel. The details are meticulously composed, subtly contrasting and alienated to create tension. Thus, the transition between house and garden now functions year-round.
This terrace tells an architectural story spanning from ancient origins to modern follies. It evokes Gottfried Semper's concept of the "primitive hut"—that primal dwelling where fire, wall, roof, and floor fulfill humanity's basic needs. Here, fire is not merely symbolic but literally integrated into the table, serving as the heart of social life or for Korean BBQ. At the same time, the structure conjures the idea of a folly, acting as a function-free spatial installation that structures the garden and prompts reflection. Depending on use and season, this pavilion has a clear role as an outdoor seating area—a second living room in the garden. Yet in quiet moments, viewed from inside the house, it becomes a sculptural installation: a frameless composition of lines, shadows, and light that transcends everyday life. Images of its current state illustrate this duality: Vibrant with plants and furniture by day, illuminated by flames and indirect light that highlights the floating slab by night.
The Context: From an Aging Wooden Terrace to Living Space Expansion
It all started with an aging wooden terrace that needed replacement. No conservatory, no tent or sunshade—instead, a blend of indoors and outdoors, more outside in the inside and more inside in the outside. The constructive intervention organizes the outdoor space spatially and expands it atmospherically: A thin roof transforms the existing threshold from living room to garden. The existing structure remains legible—plaster, wooden facade, window bands—while the new terrace attaches as a precise, floating construction. Three steps of pre-weathered Corten steel lead into the garden, Solnhofen natural stone slabs connect interior and exterior, and lateral Corten planters with herb gardens enclose the space. The floating slab frames the garden view, while wooden slats filter the area and plants create a green backdrop.
The theoretical value lies in its reference to the folly: The pavilion serves as a point of reflection in the private garden—not useless, but conceptual. It invites contemplation of boundaries: Is the construct shelter or sculpture? Utility or installation? At dusk, the pavilion appears as a glowing island, redefining the garden.
The Floating Roof: Reversing Loads and Innovating Materials
At its core is the UHPC slab, drawing strength from the combination of material, form, and load transfer. Reinforced concrete isn't just for massive components with large cross-sections; it also has a delicate side. DUCON combines self-compacting high-performance concrete with spatial micro-reinforcement into an innovative building material, ideal for the floating roof: Architectural concrete with the highest aesthetic demands. The slab, suspended at only six support points, has a thickness of just 9 centimeters (slenderness l/h=120). It is held by anchored metal plates connected to three steel cantilever beams running above the slab. Special flat steel anchors, combined with the micro-reinforcement, absorb vertical loads despite the minimal thickness.
The challenge was in the slenderness and cutouts: Three rectangular openings for the beams, plus a 1.30-meter circular oculus in a cantilevered end, reduced the cross-section for bending moments. Yet DUCON's load-bearing capacity allowed assembly without additional bracing. Particularly critical was the deformation behavior: DUCON exhibits high early strength (≥ C50/60 after 24 hours), but the modulus of elasticity develops more slowly. Thus, production and installation were timed to avoid deformations.
The construction breaks down into these elements: Geometry and reinforcement make the slab function as a biaxially spanned disk, with negative moments at the supports handling field forces; the oculus sits in the area of highest moment to test the material and capture sunlight—an echo of the existing window. The supports, heavy HEA beams, contrast the paper-thin roof: Clad in wooden slats, they penetrate the slab and create the floating impression. As cantilevers, they hold the roof at three points and reverse loads—from the free edge back into the system, against gravity. Fastenings remain invisible via anchors cast in concrete. At the edge, the slab appears more filigree through setback and drip edge; below, shadow forms, and a slim stainless steel channel drains water. The eye perceives the wafer-thin space, enhancing the levitation.
The construction becomes atmosphere: Under the roof, white concrete acts like a canopy, light and smooth. In evening light, shadow lines emphasize the structure, the artistry of craftsmanship, and precision. Wood adds tactile closeness and warmth. A lens-shaped wall of rotatable Kambala slats articulates the back wall and dissolves boundaries to the garden: Vertical divisions allow air and light through gaps, modulating views. The slats, guided on a wooden frame, adapt to season, wind, and sun. In front lies the built-in bench with seat slats, shadow joints, and invisible fastenings—it "floats" on metal arms that mirror the large cantilevers as smaller miniatures. The table is robust, with a fine frame and a leg-free top.
Here, the design ties into Semper's primitive hut: Fire as the central element. A gas line forms the fire slot with BBQ module, enabling direct grilling and serving at the table—archaic in modernity.
Water, Portal, and Light: Elements in Dialogue
The roof slopes gently, directing rain to the central V-channel, from which it flows into a granite trough—throttled for planting. Corten troughs form a water cycle: Excess nourishes herbs and buffers dryness. Visible and understandable, it turns the worn word "sustainability" into habit.
Toward the street, the portal marks the transition: Two rough-cast concrete pillars hold Corten wings. At night, a light line traces the frame without glare—a subtle sign of "entry into home."
Light becomes material: By day, the sun casts a light cone throughout; LEDs in the oculus frame the sky like a halo. At evening, indirect ceiling lighting shifts to warm ambiance. Wall sconces target table and bench, while the table glows archaically with flames and slats cast shadows.
Construction as Character: Details and Use
The project thrives on a sum of details: Drip edge with setback, visible screws, spacers, joints, stone offsets, and rust patina. Each serves the character, uniting high-quality building art and craftsmanship from concrete, wood, and metal work in the design. Foremost is the honesty of materials: Concrete provides calm, steel precision, wood intimacy, Corten patina, water movement, and fire warmth. Everything collaborates; the terrace becomes the house's best room.
Time and use create the image: The materials withstand heat, moisture, and cold. Time leaves traces. Yet poetry arises in the patina and usage—the red wine stain on wood, Corten as surface, leaves on natural stone. The pavilion is no showpiece but a tool of time, improving with years.
The Seasons Define the Rhythm of the Place
Spring: Slats open, herbs scent the air, the roof wards off showers.
Summer: Shade under the overhang, the basin drains slowly, evenings stretch in light. Autumn: Fire becomes routine, the grill works with lid, the bench gets blankets. Winter: The roof sketches snow, the oculus shows a clear halo at Christmas.
Images embody this cycle: From sunny days with open slats to long evening fires on winter nights.
In this terrace, theory and practice merge: From Semper's primitive hut to Tschumi's folly—a modern garden space that entertains, relaxes, and inspires.
