Myths and Magic Surrounding the PreFab Boom

Though currently the darling of residential design & construction media, prefab is much more than a passing fashion.  Prefab is here to stay.  Doubt it?  Just consider today’s door or window products.  Only a generation 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.  We even have custom pre-cast foundations delivered to the site (Superior Wall Inc.).

That said, it is important to understand that factory delivery is constantly evolving and not all prefab products are equal.  While few measure up to all the media hype most have many significant advantages…if you know where to find them.

Cost Savings

Undoubtedly there are savings to be had in prefab delivery.  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 project and the customer’s knowledge of the manufacturing 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.

Schedule Advantages

While it is true that a typical 3-bedroom house spends only a few weeks on the assembly line, that efficiency can only be realized if all the design decisions are clearly made in advance.  Most manufactures are well-practiced at collecting the information needed from their customers 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 walls on site.  It is possible to have too much done in the factory.  Knowing what to have prefabbed requires a knowledge of the factory and 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 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 they pass additional third party engineering review to assure compliance with all codes, including: structure, electrical, plumbing, energy, ADA, fire & 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.

First LEED Home in Vermont

While prefabricated delivery is still evolving, we are lucky to be in a time when the 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.  While I still use conventional stick-built methods when necessary, I always start with prefab.

Connecticut Cottage

Vermont Modern

John Connell teaches prefab design and construction at the Yestermorrow Design/build School in Warren, VT.  The online course starts on Jan. 26, 2017 and includes a week on campus from May 27 – June 2, during which factory tours and one-on-one project reviews are held.