Engineered Wood I Joists

Engineered Wood Rafters and Ceiling JoistsAs 1960s architecture introduced open plan living spaces the need for longer and longer wooden joists became apparent, resulting in the development of engineered wood I joists in 1969.

Although conventional lumber joists over 16 feet long were available they were both cumbersome to handle and, for most domestic buildings, prohibitively expensive. With more and more people wanting to build homes based on large open plan designs the market was ripe for the introduction of a cheaper joist to support floors, without compromising the integrity of the floor.

Engineered Wood Joists

Not surprisingly, part of the name for engineered wood I joists derives from the cross-section through the joist, which is that of a capital I. However, rather than being a piece of lumber that has been shaped into an ‘I’ cross-section, engineered wood I joists were originally created by making a sandwich of plywood layers, forming a web; which is then edged, or covered, with a strong lumber flange.

The flange will be finger jointed, presenting a greater surface area for the glue to be applied to, thereby ensuring maximum strength to the engineered wood I joist. Since the 1990s orientated strand wood has also been used to make the web; another alternative to plywood is using actual veneers of lumber. However, whilst veneers of lumber might be most aesthetically pleasing; they will also make a very expensive engineered wood I joist.


Creating a joist out of a single length of lumber would mean that the wood grain, and therefore the natural alignment of the wood fibers, is running along the length of the joist. As weight is added to the joist it will bend, compressing fibers on its top and stretching fibers on its top. This creates unevenness in the overall strength of the joist.

When an engineered wood I joist is manufactured using oriented strand wood, stronger fibers will be used for the top and bottom layers - maintaining an equal strength throughout the cross-section. So, roughly speaking, with an engineered wood I joist - choosing the correct orientated strand boards can double the strength of the joist and, subsequently, its stiffness.

Advantages of Engineered Wood I Joists

The first advantage is that by using orientated strand board, or plywood, engineered I joist will be significantly less expensive that an ‘I’ joist sawn from a solid piece of lumber. However, when constructing your dream home - cost isn’t everything.

Other advantages of engineered wood I joists include: increased spanning potential, easier handling due to their lighter weight, easier to drill through for installing service pipes and lines; as well as better performance in overall strength. Don’t forget also that the greater design flexibility that engineered wood I joists will give you.

Conventional lumber joists can be bought at up to 20 feet in length, although the average is nearer to 16 feet. Although it would need supporting with a central cross-beam - you can buy engineered wood I joists up to a massive 60 feet in length. Imagine what that means to the design capability of your property?

Furthermore, and here’s another money saving aspect of engineered wood I joists, they can be fitted at 19 or even 24 inch on-center spacings; compared to the regular 16 inch ones. So, you’ll need fewer joists, saving you even more money.

Photo of the Forest Science Centre by Roddy, Creative Commons Attribution License