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Unread 11-13-2007, 06:54 AM   #1
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Join Date: Dec 2006
Location: Northeast Florida
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I am trying to figure out the deflection of a floor that has engineered floor joists. According to the Deflecto listed on this site, it is not intended to be used with this kind of joist.

These joists are made of a (2) 2x4's laid flat with 2x4 bracing between them run on alternating angle's about every 16" or so. Overall they are a little over 12" tall and are on 24" centers.

Anyone have an idea how to figure this out?
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Unread 11-13-2007, 07:19 AM   #2
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Join Date: Oct 2006
Location: Near DC
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You need to call the mfr, or find the plans, or have a PE come out and do the calculations. Mfr sometimes has a stamp or tag on one or more opf the joists. Custom home mayy even have the plans attached to one of the joists.

In my experience since they usually span longer distances they get designed to limit total deflection, and usually end up in the 4-500 range but thats just a guess.

If you are determining suitability for tile - I would say for ceramic you will be OK. If you want stone then an engineer and some work are going to be involved. Not impossible to stiffen them up, but cost will be the concern.

Have Fun
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Unread 11-13-2007, 08:25 AM   #3
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Location: Calgary
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Failing information from your truss manufacturer, the basic deflection response can be calculated if you tell me the following:

a) size of top and bottom chords (you indicate 2x4s)

b) the distance between top of top chord and bottom of bottom chord (you suggest ~12"). Chords are the top & bottom horizontal pieces

c) the species & grade of wood used (e.g. spruce/pine/fir "select", "structural", "no.1", etc., but is usually stamped on the wood at periodic intervals)

d) confirm that the top and bottom chords are not cut, notched, or otherwise altered to receive the web members (the web is the portion between top and bottom chord - in your case a series of diagonal elements)

e) the total span involved (we already know spacing = 24")

If possible a photo or two showing the typical connection detail(s)
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