Traditional Sawn Lumber vs Engineered I-Joists vs Wood Trusses

Posted by in Wood | February 03, 2014

What’s best for floor joists in residential construction: traditional sawn lumber, engineered I-Joists, or wood trusses?

The answer to this is more complicated than “which is the best.”  The answer is subjective.  That is, it depends not only on the factors of the framing circumstances, but it also involves personal preferences.

Sawn lumber is a tried and true method of framing that carpenters of little experience and ability can install adequately and modify as required for special framing situations.  The materials are readily available.  It is somewhat forgiving in that it is often able to function well even with minor installation errors.  It is relatively easily reinforced or repaired.

Sawn lumber performs relatively well under moderate exposure to fire.  Sawn lumber is not easily burned through, and thus, a fire in a floor space may be contained or slowed in its spreading.  Often fire damaged lumber requires only scraping, cleaning, and sealing.  It performs even better when exposed to excessive amounts of water usually requiring only drying to its original moisture content.

Engineered I-joists provide the most efficient use of material since the web is commonly comprised of an oriented strand board (OSB) or a plywood.  These are also very good over longer spans and since they have a low moisture content after production, they shrink very little and result in few loose fasteners and subsequent floor squeaking.  However, the installation of these requires more experience and training than sawn lumber.  Field modification or repairs usually require consultation with the manufacturer.

Engineered lumber often does not perform as well when exposed to fire or water.  The thinner material that comprises the web of the I-joist can be relatively easily damaged or burned through by fire.  The OSB web of the I-joists can be swelled by absorption of excessive moisture causing weakening of the web.  The two by four (2×4) flanges can exhibit cupping, warping, or splitting from excessive swelling from moisture absorption.

Wood trusses provide a very efficient use of materials and are excellent over long spans.  Like the I-joists they have a low moisture content after production, so they shrink very little and result in few loose fasteners and subsequent floor squeaking.  However, the installation of these requires more experience and training than sawn lumber.  Field modification or repairs almost always require consultation with the truss manufacturer.  Trusses are poor at containing a fire and due to their open web can allow a fire to spread even within a floor assembly, unless fire barriers are installed within the floor system.

For a common residential structure such as ranches, bungalows, or simple colonial style I still prefer sawn lumber accompanied by engineered LVL’s.  But when the floor framing is special, long span, or complex, the engineered I-joist and wood trusses are probably preferable.

If you disagree, please note that I did write earlier that there are subjective factors.  We will just have to agree to disagree, as they say.

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