Mass Timber Insights for Supportive and Affordable Housing

Supportive housing typology in NYC and mass timber

Mass timber construction is well-suited suited to the development of supportive housing in New York City. Often smaller in scale that other new construction residential projects, the supportive housing typology is usually a mid-rise building, between 5-8 stories, and clad in brick to blend in with the surrounding fabric of the city. With the 2022 NYCBC revisions, a mass timber building can be built up to 7 stories as-of-right, with non-combustible exterior walls. The adaptability of mass timber allows for field adjustment in ways that concrete does not, including ease of trade coordination. Aesthetically, mass timber can be exposed, offering a pleasing finish material in the common spaces, program offices, and residences, without the added cost of applied finishes. Mass timber also aids in compliance with carbon emission regulations, like NYC’s local law 97. Mass timber construction lowers the embodied carbon of a building with 25%-40% less emissions created in fabrication than using steel or concrete, and the wood sequesters carbon in its fibers. A combined structure of mass timber interior slabs, columns, bearing and sheer walls, with the pervasive and highly valued brick facade, is an opportunity to build a commonly needed, aesthetically pleasing building type more sustainably, efficiently, and affordably.

Improving on “heavy” construction – where mass timber can supersede block & plank

One of the most common construction methods for building affordable & supportive housing is block & plank, primarily for the ease and speed of laying up the load-bearing block walls, and trucking in the precast concrete slabs to be craned into place. A CLT (CLT = Cross-Laminated Timber, one example of Mass Timber construction) floor slab will typically span 20’, and up to 35’ – similar enough to plank spans to offer an attractive alternative. CLT slabs, load bearing panels and members are all fabricated offsite. These lighter CLT members can be craned into place and assembled more quickly than the block & plank sequence, leaving only the non-combustible exterior walls the be field-built, or prefabricated separately. In either case, construction time is cut down significantly. Some of the drawbacks to block & plank construction are also addressed using mass timber, like handling on-site trade coordination. A typical CLT member is laser cut for precise coordination of chases, shafts, and penetrations, but can usually be adapted with simple tools when needed, whereas adjustments in concrete slabs are difficult if not impossible.

The lighter structure also requires the foundations to carry less load, adding structural efficiency to the building overall, as well as potential cost savings in material. The similarity to block & plank approach makes the use of mass timber a quicker adaptation for the builder, and a more direct method for comparing cost and schedule benefits for developers.

Rendering of 120 Liberty Ave and a CLT frame under construction by others

Several of our projects, offer immediate conceptual comparisons.  Our Brownsville Crossing project is currently in process and part of HPD’s Neighborhood Construction Program. Comprised of three separate buildings on non-contiguous vacant lots, the tight & irregular sites make for a difficult construction sequence with block and plank. A modular CLT alternative, with the shorter delivery and craning time for structure, could have a more efficient process, reducing the disruption to neighborhood traffic and cutting the costs associated, letting the multiple sites approach an economy of scale. 935 Bruckner, an unbuilt project that received HPD design approval, is a proposal for supportive housing with all studio units on a typical 100' x100' lot. It was designed both as a block and plank building and as a modular building with a steel frame. The modular approach would be a good fit for CLT construction and could me a model prototype for the next generation of modular housing.

Rendering of 935 Bruckner proposal