A cross-plan layout combined with a butterfly roof creates one of the more distinctive forms in residential architecture. The Butterfly House in Ubud, Bali, demonstrates how four bedroom wings extending from a central core can frame views in multiple directions while a butterfly roof ties the composition together. The house sits elevated on stilts, with a 420 m² floor area organized around a circular site plan. This article examines the structural strategies behind cross-plan houses, butterfly roof drainage and form, elevated construction methods, and the material choices that make butterfly-friendly landscaping integration possible in tropical modern architecture.
The Cross-Plan Layout: Organizing Space Around a Central Core
A cross-plan house distributes its program along four axes radiating from a central intersection point. Each wing of the cross points in a different direction, framing specific views of the surrounding landscape while maintaining visual connection to the heart of the home. In the Butterfly House, the cross measures approximately 20 meters along each axis, with the four bedrooms occupying the extremities. The central intersection houses the staircore and a sunken seating area that anchors the social life of the building.
Structural Requirements for Cross-Plan Framing
The cross-plan geometry demands a structural system that can transfer loads from each wing back to the central core. This is typically achieved through a combination of perimeter columns at the wing tips and a central column cluster or shear wall at the intersection point. The Butterfly House uses reinforced concrete columns at each wing end and a central concrete core that houses the staircases. This arrangement allows each wing to be structurally independent while the central core provides lateral stability against wind and seismic forces.
The connection between each wing and the central core is the most structurally demanding part of the frame. Engineers must detail the reinforcement at this junction to handle moment transfer from the wings to the core. Steel dowels extending from the core into each wing slab, combined with continuous top and bottom reinforcement, create a monolithic connection that distributes loads evenly. Projects that explore small studio architecture design strategies often use similar central-core approaches to maximize floor area efficiency within a compact footprint.
Wing Dimensions and Span Calculations
Each wing of a cross-plan house acts as a cantilevered beam extending from the central core. The span of each wing determines the structural depth required. For a 5-meter wing span, a reinforced concrete slab thickness of 200 mm is typical. Increasing the span to 8 meters requires a slab thickness of approximately 300 mm or the inclusion of post-tensioned tendons to control deflection. Engineers calculate the reinforcement ratio based on the dead load of the slab itself plus the live load from occupants and furnishings, which residential codes typically set at 1.5 to 2.0 kN/m².
| Wing Span | Slab Thickness | Reinforcement Ratio | Deflection at Tip |
|---|---|---|---|
| 4.0 m | 180 mm | 0.8% | 6 mm |
| 5.5 m | 220 mm | 1.0% | 11 mm |
| 7.0 m | 280 mm | 1.2% | 18 mm |
| 8.5 m | 350 mm | 1.5% | 28 mm |
Elevated Construction on Stilts: Foundations and Site Integration
Building a house on stilts lifts the living spaces above the ground plane, creating benefits for drainage, ventilation, and site preservation. The Butterfly House rests on reinforced concrete columns that raise the main floor approximately 3 meters above grade. This elevation allows the ground-level space to function as a continuous landscape that flows under the building, connecting the garden on one side to the pool and atelier on the other. The elevated approach suits sloped sites, flood-prone areas, and locations where preserving existing terrain is a priority.
Column Design and Load Distribution
Columns supporting an elevated structure must handle both vertical loads from the building weight and lateral loads from wind and seismic activity. For a 420 m² house on stilts, engineers typically specify columns spaced 4 to 6 meters apart, with each column sized according to the tributary area it supports. A column supporting 25 m² of floor area at two levels carries approximately 300 kN of vertical load, requiring a reinforced concrete section of about 300 mm by 300 mm. The column foundations must extend to competent bearing soil or be designed as pile foundations where surface soils are weak.
The gap beneath an elevated house creates opportunities for landscape integration. Ground-level spaces under the structure can accommodate parking, shaded walkways, outdoor dining areas, or planted gardens. The Butterfly House uses this undercroft space for the living room, which becomes a continuous landscape flowing through the site. Property owners considering a similar approach should also evaluate butterfly bush pruning and maintenance for the surrounding garden plantings to keep vegetation from encroaching on the structural columns over time.
Butterfly Roof Design: Form, Drainage, and Passive Performance
The butterfly roof inverts the traditional gable form. Instead of a peak at the ridge with slopes running downward on both sides, the butterfly roof has a central valley with slopes rising to the eaves on each side. This V-shaped profile creates a dramatic architectural silhouette while channeling rainwater to a central collection point. The Butterfly House uses this roof form over each of its four wings, with the roof shapes blending together at the central core to create a unified sculptural form.
Structural Framing of the Butterfly Roof
Framing a butterfly roof requires structural members that can resist the outward thrust generated by the inverted slope. Traditional gable roofs push downward and outward onto the supporting walls; butterfly roofs push downward and inward toward the central valley. This means the valley beam at the center of the roof must be designed to handle concentrated water loads and the compressive forces from the rafters. Steel or engineered timber beams are common choices for the valley beam, with galvanized steel connectors at each rafter-to-beam junction.
The butterfly roof for passive solar and rainwater harvesting offers additional environmental benefits beyond its visual appeal. The central valley acts as a natural drainage channel, directing rainwater to downpipes that can feed storage tanks or irrigation systems. In hot climates, the raised eaves of a butterfly roof provide deep shading on the walls below, reducing solar heat gain during peak sun hours. The roof slope angles typically range between 15 and 30 degrees from horizontal, balancing drainage efficiency with visual effect.
Waterproofing the Central Valley
The central valley of a butterfly roof concentrates all runoff at a single low point, making waterproofing critical. A typical butterfly roof valley uses a formed metal gutter or a built-up membrane system with a minimum width of 300 mm. The waterproofing membrane should extend at least 150 mm up the roof slope on each side, with mechanical fastening at the top edge to prevent water migration behind the membrane. Regular cleaning of the valley gutter prevents leaf buildup that could block drainage and cause ponding.
| Roof Form | Drainage Pattern | Solar Exposure | Wind Performance |
|---|---|---|---|
| Butterfly (inverted) | Central valley | Low (shaded walls) | Moderate |
| Gable (standard) | Two side eaves | Moderate | Good |
| Shed (mono-pitch) | One side eave | Variable | Good |
| Flat roof | Internal drains | High | Excellent |
Reclaimed Wood as a Primary Building Material
The Butterfly House uses reclaimed wood of different types as the dominant material across walls, ceilings, and decorative surfaces. Reclaimed timber brings texture, history, and environmental benefits that newly milled lumber cannot match. The wood in this project came from deconstructed buildings and industrial structures, giving each plank distinct weathering patterns and surface characteristics. Using reclaimed materials avoids the carbon cost of harvesting new timber while diverting waste from landfills.
Sourcing and Preparing Reclaimed Timber for Construction
- Source verification: confirm the timber’s origin and ensure it was not treated with banned chemicals such as CCA (chromated copper arsenate) before 2004
- Metal detection: run each plank through a metal detector and remove nails, screws, and embedded hardware before milling
- Moisture conditioning: air-dry reclaimed timber to a moisture content below 19% before installation to prevent shrinkage and checking
- Surface preparation: sand or plane the timber to remove surface contaminants while preserving the natural patina
- Insect treatment: apply borate-based preservatives to eliminate any wood-boring insects that may be present in older lumber
Builders working with reclaimed wood should budget for approximately 15 to 20 percent waste factor, since some planks will be unusable after inspection and processing. The structural performance of reclaimed timber varies widely depending on its species, original grade, and condition. Consulting a structural engineer to verify load capacities before using reclaimed wood in structural applications is recommended. The butterfly roof structure drainage and modern applications often incorporate reclaimed timber as ceiling decking, where the visual texture of aged wood enhances the interior atmosphere.
Wall Finishes and Decorative Texture
Beyond structural applications, reclaimed wood creates distinctive wall finishes through varied installation patterns. Horizontal board-and-batten installations emphasize the length of a room, while vertical arrangements draw the eye upward and make ceilings feel higher. The Butterfly House combines different wood textures with intricately patterned walls, sculptures, and plants to create an artistic environment. Dark-stained accent walls contrast with lighter ceiling planes, and the irregular grain patterns of aged wood add visual interest that new materials cannot replicate.
Visual Relationships and Spatial Connections in Cross-Plan Layouts
The cross-plan layout creates unique opportunities for visual connections between spaces. Each bedroom wing faces inward toward the central atrium while also framing outward views through its terminal window wall. The Butterfly House achieves this through operable partition walls that allow each bedroom to open visually to the central space. When the partitions are open, the entire house reads as a single volume with views passing through the core from one wing to another across the central void.
The two staircases at the center of the house spiral upward in a helix pattern, completing each other visually from different angles. A sunken sofa area at the base of the stairs defines the social heart of the home while maintaining sight lines to the garden through the elevated underside. The butterfly roof design that transforms residential architecture contributes to this spatial experience by creating varied ceiling heights across the different wings, with each bedroom crowned by a distinct roof silhouette that blends into the overall composition.
The material palette of reclaimed wood, textured walls, and carefully placed artwork creates an environment that feels curated rather than constructed. Plants placed at key visual junctions soften the transition between indoor and outdoor spaces while improving indoor air quality. The tropical modernism approach visible in projects like the modern barnhouse vision for residential design shares this emphasis on blending built form with landscape, using material honesty and spatial openness as guiding principles rather than decorative excess.