Panelization Takes Command

TEXT Lloyd Hunt
PHOTOS Brockport Home Systems Ltd. unless otherwise noted

In 1956, my father built the house where he is still living, on the top of the Niagara Escarpment in a snow belt. At that time, everything from custom-sized windows down to kitchen cabinets was built on site. In an era when roofs were usually stick-framed, he made a radical move in deciding to construct roof trusses. He ordered drawing templates from Practical Builder magazine, laid the trusses out on the main-level subfloor, and fastened them together with 2” galvanized concrete nails and 3/8” plywood gussets, single shear on each face. The trusses were then set aside while the exterior walls were constructed. Since its completion, the trusses have been repeatedly tested for strength and deflection by many a winter storm. 

On-site truss assembly was an exciting new method of construction for the time. Today, it is assumed that residential trusses–along with stairs, doors, windows and kitchens–are all factory-made. Designing kitchens is as easy as a visit to IKEA where “your dream kitchen [is] coming right up” and “you do not have to be handy.” Truss designers use Mitek Sapphire Structure software to create three-dimensional truss layouts from scratch or from imported Revit or SoftPlan drawings, and then send the files to the factory floor for construction. These high-tech tools allow for advanced assemblies and therefore require less expertise on site. Timber construction has also advanced in the last 30 years with the introduction of manufactured wood products, notably wood I-joists, laminated veneer lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL) and oriented strand board (OSB). 

While at first it may sound radical, the complete factory construction of wood-frame buildings using timber panelization is a logical development from these advances. This technology functions by building all floors and walls in panels on the factory floor and assembling them on site. The logic is similar to using precast hollow-core concrete floor slabs or precast concrete walls, but offers the advantages and economies of wood-frame construction.

In the most advanced panelization facilities in Canada, the process is to a large part automated, ensuring a high level of precision. Brockport Home Systems in Toronto, for instance, uses software similar to Mitek Sapphire Structure to create 3D drawings for entire wood-frame buildings, which are subsequently fed into computer-controlled cutting and assembly machinery. Going beyond regular construction drawings, panelized shop drawings allow for scrutiny of every timber component in a project. The detailed review of 3D drawings allows for potential conflicts between systems to be detected and accommodated early on. For instance, at the design stage, an extra I-joist may be added to coordinate with 4” plumbing drains. This is the kind of change that, with traditional construction, requires foresight, planning and communication on site–a challenge with the highly compartmentalized trades found on present-day construction sites. High-precision 3D models also double as excellent record drawings at the end of the project. 

Price-wise, factory-constructed panelized buildings are not necessarily cheaper than conventional on-site wood constructions, but the advantages in terms of quality are numerous. To start with, the source materials can be controlled in a more rigorous manner. Brockport’s director of research and development, Robert Kok, explains that the factory’s high construction volumes allow them to demand premium-quality materials from suppliers. Materials are sorted before they enter the manufacturing process and any defects are rejected, including badly warped or knotted studs. This is in stark contrast to the “no picking, no sorting” notices typically seen at lumberyards. The environmentally controlled factory environment allows for wood to be properly acclimatized and not subject to the elements as they would be on a construction site, where materials are often unprotected from rain or subjected to freezing. 

The assembly process itself is also highly controlled and incorporates testing. For instance, floors are constructed with 4’ x 12’ x 7/8” OSB and assembled with high-quality low-VOC elastomeric glue. Machine-controlled nailing ensures that the specified nails and spacing are used, and each nail is precisely centered in its joist or stud. Samples are tested after 24 hours, seven days and 30 days, ensuring a solid, no-squeak floor. Since the glue can cure in a controlled environment free of water penetration, the panels are unlikely to develop “freeze crack” once the heat is turned on in a building, and there is no need for surface sanding to correct edge expansion of the OSB. The floor panels are all designed to the highest building code standards for deflection, allowing porcelain tiles to be installed on any OSB floor surface without concern that the grout will crack. 

Factory-built wall panels, for their part, are also higher-quality products. Machines that control the spacing and depth of nailing make a noticeable difference in shear-wall construction–averting on-site arguments with builders due to common mistakes that affect shear-wall performance, such as over-nailing. Two pre-drilled holes in each stud allow electrical wiring to be quickly installed. Finally, wall panels are equipped with a high-quality air barrier that will experience no wind loads greater than the wind from delivery trucks travelling at posted speed limits. In the final building, further air leakage is reduced due to the precision fit of the panels. 

For both wall and floor panels, efficiencies in terms of material and time are notable. Wood LVL, PSL, LSL and I-joists are all delivered to the factory within 1/16” of specified length. Other materials are cut in a planned way to minimize waste. For clients seeking extra LEED points, wood can be specified from certified forests. 

Factory construction ensures safer and more efficient labour conditions. David Moses of Moses Structural Engineers notes that, according to an Arizona State University study, homebuilders spend the majority of their time waiting, whether for trades, weather, deliveries or other delays. “Imagine you own a factory manufacturing a product,” he says. “You would never have your people and equipment sitting around idling for 50% of the time. You’d be out of business.” In a plant, semi-skilled labour can work on construction in two shifts, rather than the typical single shift of skilled labour on site. When complete, the panels are numbered and systematically packaged for quick installation. Typical assembly time for a 4,000-square-foot house is two days. This is fast enough that neighbours do not have time to file complaints about construction noise–nor can clients or contractors make last-minute changes to the design. 

With our woodsmen history, Canadians have been stereotyped as “hewers of wood and drawers of water.” This manifests in Part 9 of the National Building Code, where, regardless of professional engineering or architectural credentials, all are permitted to construct simple buildings less that 600 square metres in floor area and three storeys in height. The tacit prerequisite for the use of the rafter tables referenced in National Building Code Article is a firm grasp of the imperial framer’s square. Today, the articles of Part 9 referring to timber structural design are becoming quickly obsolete with declines in common skill, changes in timber technology, improved timber products, consumer demand for much larger homes (and spans that exceed the building code design tables), and the requirement by many building departments for stamped engineering drawings regardless of building size. 

Panelization with it
s sophisticated construction and longer spans makes timber construction securely a family member in Part 4 of the National Building Code, taking its place alongside steel and concrete in terms of use and spans. Changing building codes are encouraging timber construction up to four storeys across Canada and up to six storeys in British Columbia. The HOT development in Mississauga by Quadrangle Architects provides a hopeful example of the potential results. Working in concert with an enlightened developer, the architects are employing panelized timber to construct a series of four-storey mid-rise residential buildings. Similar to modern European designs, HOT takes shape as a simple block with added modules. The long deck spans available with panelized construction allow for open plans and deep light penetration into the residential units. The use of renewable wood construction accrues measurable environmental benefits. 

Moreover, by building in wood, significant cost savings are achieved relative to a comparable concrete construction. In 2010, Grant Roughley of RHC Design Build assessed cost savings when constructing two buildings of identical size and occupancy, one with a steel structure and the second with wood construction. Grant concluded that the cost difference was 16.3% lower on the overall project budget for the wood-framed system.

Timber construction has evolved radically from the time when entire buildings were crafted on site. Over the past decades, individual building components have come to be assembled in factories and only installed on site. We are not far from a moment when, as Kent Larson of MIT’s Open Source Building Alliance puts it, “building homes on site makes as much sense as building a car in your driveway.” As its efficiencies and advantages emerge, it is clear that we are preparing for an era where panelization will take command. CA

Architect Lloyd Hunt has a practice based in Glen Huron, Ontario. He teaches timber design and building codes at the University of Waterloo School of Architecture.