The ‘Pre-Fab’ Boom
By John Connell
Though currently the darling of residential design and construction media, “prefab” is much more than a passing fashion. Much more likely to pass is the notion that prefab necessarily implies cheap construction of an inefficient building that will not last.
Prefab is here to stay. Doubt it? Just consider today’s door or window products. Not so long ago these items were hand-built on site. And similar transformations have taken place with kitchen cabinets, fireplaces, flooring, tile, shower bases, stairs (circular and conventional), roof hatches, chimneys…these site fabricated building parts are now mostly made in factories. Today we even have custom pre-cast foundations delivered to the site (Superior Wall Inc.).
That said, it is important to understand that factory delivery of prefabricated components and structures is constantly evolving and not all prefab products are equal. While few measure up to all the media hype, most have many significant advantages. A key, as with many things, is knowing what to look for and where to find them.
Undoubtedly there are savings to be had in building prefabrication, as distinct from “stick-built” construction done on the building site. Manufacturers’ deeper buying power allows them to source materials at Home Depot prices or lower. How much of those savings are passed along to the customer depends on the manufacturer, the project and, to an extent, on the customer’s knowledge of the factory delivery process.
Additional labor savings are found in the reliable work schedule of a factory. Building indoors means no ‘down’ days due to weather. Additionally, some factories even use round-the-clock shifts.
Waste is much lower in a factory – typically 5% compared to 15% or higher on a traditional construction site.
Finally, there are significant savings that accrue from knowing the exact date of delivery. This means the builder can have all supporting trades lined up for efficient completion of the project. Again, these savings are dependent on finding trades that are experienced with factory delivery. The builder needs to clearly choreograph the supporting trades and request a price reduction for the reduced time and scope.
Incredibly, a typical three-bedroom house spends only a few weeks on the assembly line. But to really optimize that efficiency all the design decisions must be clearly made in advance. Architects and professional designers are worth their fee in this context but most manufactures are also practiced at collecting the design decisions needed to insure smooth fabrication on the assembly line.
A common prefab axiom holds that anything that can be accomplished on an assembly line should be done in the factory. This is only true up to a point. There are some finishes that can be efficiently installed in the factory only to be damaged by the site workmen connecting the mechanicals and closing up the “marriage” walls on site. It is possible to have too much done in the factory. Deciding how much to have prefabricated requires a knowledge of the factory as well as your supporting site trades.
Generally, prefab delivery is a sterling example of the builders’ motto: “prior planning prevents poor performance.” When all design decisions are addressed and understood before the project hits the assembly line, the final product will arrive at the site on schedule – usually to the day and the hour! Now that’s magic.
Most factories produce wall and floor assemblies superior to those built on site. Using dry materials, the latest fasteners, jigs and CNC (Computer Numerical Control) cutters – these building parts are square, straight and solid. But that’s just the beginning. Because modular units are considered a “product” and sold across state lines, they must be rigorously inspected before being allowed out of the factory. Law requires that designs of components and assemblies pass third-party engineering review to assure compliance with all codes, including structure, electrical, plumbing, energy, ADA (Americans with Disabilities Act) Standards, fire and safety.
Increasingly, we find factories specializing in high-performance “energy prefabs.” These houses are extremely tight and well-built. The first house in Vermont to receive the LEED residential rating was a prefabricated home.
While prefabricated delivery is still evolving, we are lucky to be in a time when these products have never been better. Builders and homeowners still need to be selective about who they work with, just like picking a general contractor or any other trade. But if you know who and what to ask, this is a building approach at the height of its game. In the last few years, I have used this approach to build energy-efficient homes, architect-designed residences, simple camps and multi-unit mixed-use residential homes. While I still use conventional stick-built methods when necessary, I always start with prefab.
John Connell teaches prefab design and construction at the Yestermorrow Design/Build School in Warren, VT — including an ongoing online course with factory tours and one-on-one project reviews. You can reach him at 2morrow Studio at 802.496.5546 or 508.477.5400. His website is: prefab2morrow.com.