Expert: Richard Taylor, AIA - 4/28/2004

Question
I recently had an addition built as per plans from an architect. The addition size is 32' X 33". The original plans called for 2X8 floor joists spanning 14' 2" 16" On center, the building inspector rejected the plans due to the span of the 2X8's. The architect re did the plans and made the 2X8's 12" On center, and this was accepted by the inspector. Now the addition is done and in the rooms where the 2X8's span that 14' 2" (12" OC) when someone walks across the room things shake on the tables! The addition was built over a crawal space 3 courses of block high. Is there something that I can do to help eliminate this problem? Besides re-building the addition?

Answer
Ralph:

Thanks for your question!

Oops!  There are a number of solutions for this problem, but first let me explain a bit about floor structures.

I don't know what lumber species was used for your floor joists, but according to the International Residential Code (IRC), you'd need Southern Pine #2 as an absolute minimum for a 14'-2" span.  But even at that, although that's strong enough to support the loads, it's not stiff enough to prevent the problem you're having.

"Deflection" is the name for how much a structural member "flexes" when it's loaded, and that can create "bounce" in the floor.  The code minimum for deflection is the length of the joist divided by 360 (called l/360) which means the maximum allowable deflection for your 14'-2" span is about half an inch.

An while that's permissible by code, it's not good design.  l/480 is a better standard, reducing the maximum deflection on your floor to about 3/8".  But in my opinion, even that's not enough.  I can't imagine what 16" o.c. would have been like!

Whenever a floor structure is designed, it's important to know what the room is going to be used for.  For some uses, some deflection is tolerable while for others, it ruins the room.  It's the responsibility of the design professional to ask about the use of the room, your level of tolerance for deflection, and design the structure to suit.  Just going by the code books isn't enough - you don't need an architect for that!

In our area, we very rarely use 2 x 8s, and then only when the span is close to 12' - to avoid exactly the problems you're experiencing.

There are a number of different solutions, however:

1) Double some of the floor joists
2) Install 3 rows of solid full-depth blocking between all joists
3) reduce the span of the joists by adding additional beams on either side of the existing beam
4) install another layer of plywood decking, perpendicular to the first, glued and screwed

These are generic possible solutions.  Without specific knowledge of your situation, I can't do a "real" structural analysis to assure you that a solution will work.  You'll need someone on site to do that.

Here's what I suggest.  Contact the appropriate wood products association for your region (I've listed some below) and ask them for help.  Most associations have local reps that will be happy to come out to your house (no charge) and examine your structure and suggest solutions.  The association you contact depends on where you live:

Southern Forest Product Assn: www.sfpa.org (All Eastern US)
Western Wood Products Assn: www.wwpa.org (West of Rockies)
Northwest Wood Products Assn: www.nwpa.org

If you can't find a solution at any of those, try the lumberyard where the joists were purchased.  They can direct you to other resources.

Finally, if you don't get free help, call a local structural engineer.  If you're in a metro area, there are firms around that do residential work for reasonable prices.  Have them do a structural analysis and design a solution.

Hope this helps - let me know how it turns out!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtaylorarchitects.com