Floor Joists: Use Manufactured or 2x’s?

Manufactured Floor Joists

I had an owner-builder recently ask, “what are the best floor joists to use on my new addition and why?”  First, let’s get the options out there.  You can go the traditional method with dimensional lumber, otherwise known as 2×10 or 2×12 floor joists.  Or, you can use a manufactured floor truss, that is typically made up of 2×3 or 2×4 members and are designed and built by your local truss plant.  The third option would be to use manufactured floor joists that are made up of OSB (oriented strand board or similar material) webs and top and bottom flanges of dimensional or ply material.  There are other variations but these are the top three that I have used.  Let’s go through them one at a time.

Floor Trusses

Dimensional Floor Joists (a.k.a. 2×10’s or 2×12’s)

Pros: Easy to get, no special drawing/design needed, installation methods are known by all carpenters.  Cons:  Prices can fluctuate big time over a small period, span is less than the other alternatives, smaller width makes miss nailing more prevalent.

Floor Trusses

Pros:  span greater distance that milled floor joists, incorporate mechanical chases to run HVAC through, large flange width makes nailing a no brainer.  Cons:  Typically more expensive, heavier:  depending on span may require two carpenters to move, greater lead time to get, requires special design/layout, and typically require bracing material.

Manufactured Floor Joists

Pros:  Light, span greater distances than dimensional joists, easier to run mechanicals through than dimensional, lower cost.  Cons:  Requires special layout and install.

Verdict

In my experience, manufactured floor joists are the best alternative in most cases.  Most rough carpenters now have experience installing these.  And, the supplier typically will provide the layout at no extra charge if you buy from them.  These manufactured floor joists can provide a solid floor if you go with a L/480 deflection criteria.  Basically, the deflection criteria is used to determine the joist size and spacing.  The “L” indicates the span and as the bottom number gets bigger, the deflection goes down.  L/360 is typically the building code minimum.  You will notice the floor bounce some if you are a heavier person walking across a floor designed at L/360.  And who can argue with the price for the manufactured product…plus, we may save a few trees while we are at it.

For more tips on saving money with engineered products, check out our post titled…Old Growth Lumber:  Reduce and Save Money When Building.  Have you ever thought about building your own home?  Be sure to stop by the Armchair Builder How to Build Your Own Home resource page.

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