Folded Plate Roofs Are Back, and Now in Mass Timber

Perkins&Will and StructureCraft reinvent a midcentury classic concept.

Library in Washington

James Steinkamp Photography

Folded plate structures were invented in the 1920s and became all the rage in mid-century architecture around the world. They are capable of very long spans and have their own wonderful aesthetics, but they can be fussy and difficult to engineer and build.

In his 1964 essay The Aesthetics of Folded Plates, Clovis B. Heimsath of Rice University wrote:

"Who is really responsible when a good folded plate design is produced — the architect or the engineer? Both should take credit. The architect is not technically familiar with folded plates though he seeks to use them properly. The architect, in turning to the engineer, should look to him for direction not only in the calculation of a structure but in its significance as well. ..A fine building is a tribute to all who worked on it, and real teamwork between engineer and architect is too seldom achieved but an end greatly to be desired. When overall design and structure come together in good work, it is always worth the extra thought invested in it."

Take the new Southwest Library in Washington, D.C., dominated by its stunning folded plate roof made out of dowel-laminated timber (DLT). It is truly, as Heimsath described, a testament to the architect, Carl Knutson of Perkins&Will, and the engineer, Lucas Epp of StructureCraft in Abbotsford, British Columbia. And indeed, Epp tells Treehugger that he and Knutson "collaborated super closely throughout the project from the concept through to construction." StructureCraft is the Engineer of Record for the project and did the engineer-build of the structure.

Epp tells Treehugger there were numerous technical challenges and complications.

"The 'crinkled' folded plate roof presented engineering challenges in how it would be detailed, fabricated, installed, and erected. Connection detailing, prefabrication, the assembly jig, and erection planning were keys to its speed of erection and successful installation. The unique shape of the long-span folded plate roofs created a particular challenge for both the structural engineering and construction of these complex elements. The use of dowel-laminated timber in a folded-plate structure was a world first.
panel in shop in abbotsford
Panel in shop, Abbotsford, British Columbia.


The panels were assembled in the StructureCraft factory in Abbotsford (see Treehugger's tour of it here), in a process where lumber is assembled on a giant custom-built rig, holes are drilled, and then dried hardwood dowels pushed in. As the hardwood absorbs moisture from the surrounding softwood, it swells a bit and locks it all together. In this case, the panels were "sheathed in glued plywood to create the necessary diaphragm stiffness. The plywood was fastened and glued in the StructureCraft shop to the folded plate glulam chords at the ridge and trough."

Assembly in Washington
Assembly in Washington, DC.


Epp continues:

"For shipping, the chords were split in half. Once on site, tension rods connected the four trough corners, as the two halves of each gable were assembled into self-supporting “trusses” of the folded plate which weighed more than 15,000lb each. These up to 70ft long and 20ft wide gables were then hoisted into position. From initial form and design to engineering, prefabrication, and installation, the entire team came together around this complex challenge and delivered successfully on the unique architectural form."
Roof structure and columns of overhang
Roof structure and columns.

James Steinkamp Photography

Writing in 1964, Heimsath noted "support for the plate roof can be effective or disastrous. In buildings in which it is effective, it is honest and clear. The roof itself should "read," so the structure supporting it should be made known." Here, the support is certainly honest and clear, especially outside with those exposed wood tilted columns.

Roof from another angle

James Steinkamp Photography

The exterior overhang is dramatic, but also serves an important function of shading. As Heimsath notes,

"It is interesting that in the application of so many of the well-designed folded plate roofs the roofs are cantilevered well beyond the enclosing walls in front and rear. Visually this accomplishes two things: it lets the viewer know what is going on and prepares him for the space within. The roof overhangs cast shadows on the enclosing walls making them less of a visual barrier. If the enclosing walls are of glass, it is well that there is a cantilever because glass in the daytime reflects the sunlight and can appear more impenetrable than marble. A roof which seems to float at night when the lights are on inside causing the glass to disappear can seem an ominous block when the glass reads as a wall during the day."
Interior of Library

James Steinkamp Photography

And that is exactly what Knutson and Perkins&Will have done here–extending the roof way beyond the front wall because it looks so lovely, but also makes the glass disappear from both inside and out.

Heimsath even had something to say about lighting, noting:

"In many buildings, the simplicity of the interior form is destroyed by the lighting system, whether it is plastered onto the form itself like little warts, or hung in a random fashion with little regard for the overall effect, A well-chosen lighting fixture can setoff the roof to advantage, and the few dollars invested in the fixtures might be the difference between chaos and a noteworthy space."
Lighting inside library

James Steinkamp Photography

Lighting is certainly one area of building design that has evolved significantly since 1964, and here it does two jobs–lighting the work surfaces in the library, but also highlighting the wood ceiling above.

skylights built into roof

Clovis Heimsath via Internet Archive

Back in the mid-20th century, glazing was often integrated into the folded plate roof design. light from skylights is hard to control, but the angled panels on the roof are put to very good use with the 21st-century version: solar panels. The 100-year-old roof concept plays nicely with modern technology.

view of roof with solar panels
Roof with solar panels.

Jeff Allen

There are many reasons to get excited about this building. Folded plate structures are efficient in their use of materials and can cover very long spans, being basically very deep beams on a tilt, leaning against other very deep beams. They are complicated to engineer and build, but StructureCraft put their modern tools to work, with "complex non-linear finite element analysis to enable prediction of stresses and structural behavior of the folded plate." They used BIM (Building Information Modeling) and "a 3D fabrication model with a high level of detail to be used by both designers and constructors – the onsite team used it extensively."

Epp tells Treehugger:

 "Also, BIM allows proactive clash detection and penetration coordination amongst all trades, which is particularly important in a prefabricated highly exposed timber structure. The BIM model drove the CNC manufacturing process of all Glulam, DLT, and steel and produced detailed piece shop drawings for each element."
DC Library side


Perkins&Will and StructureCraft have designed and engineered a glorious hat on top of a lovely building, a demonstration of the principle that Clovis Heimsath wrote about in 1964, about what can happen when talented architects and engineers actually work together as a team. His conclusion, and ours:

"Folded plates are in themselves so handsome that every effort should be taken not to destroy their integrity, a job which requires the best both professionals have to offer."