Elemental Gains

A coffered plywood roof by Tonkin Liu transforms a grade-II listed Georgian townhouse and its garden in London

Photos
Edmund Sumner

The Sun Rain Rooms is a two-level extension and refurbishment of a grade-II listed Georgian townhouse in London. Designed and constructed by Tonkin Liuin collaboration with local craftspeople, they serve as both a studio for the practice and home for the partners’ family.

The extension is intended to reframe the rear of the terrace by amplifying the characteristics found within its fabric. The perimeter walls of the old yard support a coffered plywood roof that is curved in plan and section to allow maximum light into the garden. A pipe on the roof’s edge brings rainwater down from the top of the house into a harvesting tank, which when discharged, transforms the patio into a reflecting pool.

The design intent for the prototypical roof was to create a thin and lightweight construction where structure, form and ornament become one, writes Tonkin Liu. Learning lessons from nature, including surface-structures in seashells and the geometry of bird wing bones, we developed an insulated, stressed skin comprising glued and pinned layers of laminated marine-grade plywood. The timber was CNC-cut using tool paths generated by custom-built scripts that unroll the layers in a precise digital 3D model of the geometry.

Round coffers form catenoids (a rotated catenary curve) that link an upper and lower skin, increasing structural depth and creating space for an insulated cavity within the roof build-up. The geometry was tested using physical and digital models, as well as structural analysis, in a process of optimisation that yielded the eventual organic diagrid arrangement.

The position of the plywood coffers echoes the wave pattern of raindrops landing in the pool around which the roof wraps.

Each stepped coffer culminates in a circular double-glazed rooflight. The glass is bonded to an aluminium ring that is mechanically-fixed to each coffer upstand. This also acts as a heat sink for the integral LED lightring below it. A cold-applied liquid plastic fleece is bonded to the plywood deck and butts up to the double-glazed units to form a seamless waterproof membrane.