Modular Mass Timber Hybrid Construction for Affordable Housing in the United States

Modular Mass Timber Hybrid Construction for Affordable Housing in the United States

This article is part of a series discussing the current research activities of the Affordable Housing Research and Technology Division within the Office of Policy Development and Research.

As part of the Office of Policy Development and Research's (PD&R's) ongoing support of building technology research, modular construction has emerged as a promising technology for increasing production and supply of affordable housing. Research is needed to identify ways to continuously improve the design and manufacture of modular components (modules) to ensure efficient transportation and less labor-intensive assembly at their destination. PD&R is funding a 24-month project with Washington State University (WSU) to research ways of using a mass timber hybrid construction concept to improve the design of modules to ease both transportability (time and cost) and site assembly to increase housing affordability in the United States. This article describes the scope of the research project, its objectives, the study plan, and the status of the project.

Background

One of the factors contributing to declining rates of affordable housing production and supply in the United States is the continued predominance of traditional onsite construction practices. Unlike alternatives such as modular construction, which can be done year round in a controlled factory environment, traditional onsite construction is weather dependent, inefficient, and costly, requiring storage for the construction materials, temporary access roads to the site for transporting construction equipment, and temporary housing for workers.

Objectives and Scope of the Research Project

The research project involves a comprehensive literature review and survey of the current state of the practice of modular construction, the design and demonstration of hybrid modules using mass timber components and light wood frames, and the development of a design guide for hybrid modular mass timber buildings aimed at construction workers and engineers. The project, which will incorporate advice from an expert panel of stakeholders, aims to effectively combine mass timber and light wood frames to build mass timber hybrid modules for modular housing construction.

The project's main objective is to identify more affordable and durable options for producing modular residential single- and multifamily buildings in the United States using mass timber components and light wood frames. The goal is to lower the cost of production, transportation, and assembly of modules by reducing the design and production time and easing the transportation and assembly of the modules at building sites.

Study Plan

To address the high transportation cost of modular building modules, WSU will design and test dual-purpose modules that can serve as part of the shipping, delivery, and construction sequence (such as incorporating mass timber components and their connections into the design of the parts of the trucks and/or flatbeds that transport the modules from the factory to the building site), making the transportation phase more efficient and economical. WSU will also design and test modules that can be repurposed to accommodate different occupancies and usages (such as converting from residential to office use, or residential to hotel use) over their lifetimes. WSU will design and test by integrating mass timber components, such as cross-laminated timber (CLT), mass plywood panels (MPP), and light wood frame (LWF) elements, to develop hybrid structures (mass timber floor systems with light wood frames) that are efficient and cost effective to transport. If successful, this hybrid approach will result in structurally sound and durable modules that cost significantly less than currently available modules.

A stakeholder-based panel of experts will review and coordinate the development of the design guide for mass timber modular building based on the results of the research study. In addition, this panel will provide guidance and feedback throughout the course of the project, ensuring that it aligns with the construction industry's best practices, regulatory requirements, and existing research.

Status of Research Project

Conceptual Design of a Modular Hybrid Mass Timber Building System

As of the quarter ending March 31, 2025, WSU had completed its literature and practice review of the current state of the practice in modular construction and had begun assembling the panel of experts, which will consist of representatives from industry, academia, and code and design standard committees. 

WSU has also finished developing conceptual designs for modular hybrid mass timber buildings based on the results of the literature and state-of-the-practice reviews of modular construction. These designs include different combinations of mass timber and LWF structures for modular construction, focusing on the structural performance and load carrying capacity of the hybrid system. Although the design of both systems (mass timber and LWF) is fairly developed in current building codes and design standards, combining and optimizing the two to meet compatibility and strength-limit states requires detailed design considerations. WSU has also started preliminary design investigations for vertical elements (such as walls) across various occupancy categories (for example, residential, office, and hotel buildings). 

Conceptual Modules and Transportation System Design

WSU has completed the design of the panels for the modules (combining CLT for the floors and roof, with LWF interior walls for flexibility and structural support) and the transportation system that fits within the standard flatbed truck. The panels are designed to be used as part of the transport system as well as components of the modular house. This dual-purpose functionality ensures that the units can be transported cost-effectively and efficiently under current transportation regulations (because they become part of the truck's structural system when loaded) while eliminating packaging waste.

The design incorporates preestablished and aligned connections among the various components of the modules, and the modules themselves are all integrated into the flatbed of the transport system. This design enables faster and more cost-effective placement and assembly of the modules at the construction site. To ensure stability (preventing bending and shifting) of the modules during transport, WSU has incorporated posttensioning into the design of the module components using high-strength steel tendons that run along the entire length of the modules. Through simulations, WSU has also optimized the loading sequence to enhance stability during transportation, with each module arranged to balance the total weight on the flatbed.

Next Steps

WSU plans to meet with the advisory board (researchers, industry representatives, and manufacturers) to present progress and solicit feedback for future research directions as well as finalize the design and testing of several hybrid modular buildings that can withstand seismic and wind pressures to understand how mass timber hybrid modular buildings will behave under varying earthquake intensities. Finally, WSU plans to conduct a life-cycle analysis of the whole process, publish and disseminate the guidebook after the expert panel's review, and present the results at the 2026 HUD Innovative Housing Showcase.