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Old 03-03-2018, 04:10 PM   #1
sanjosetiler
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Deflecto with I joist

Good afternoon all. I have a quick question regarding deflection. I have I joists in my home which I know are not covered by the deflecto calculation. They are 9.5 inches tall and have 2x4 material on top and bottom of webs as typical. I understand that the only way to get a definitive answer on deflection is to contact the manufacturer, but I'm asking here for a very rough and non binding guess as I am confident they are overbuilt. If I do the calculation for 2x10 dimensional lumber of the same size I get a deflecto result North of 800 (2x10 DF with a ten foot span on 16 inch centers). Since I joists are supposed to be stronger than DL of the same size is it safe to at least assume that I have a minimum of 800 then? Is there any scenario where a perfect condition I joist is not as strong deflection wise as DL of similar dimension? If my assumption is incorrect I guess I'll have some voice mail trees to navigate next week with the manufacturer.
Thanks in advance.

Ron
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Old 03-03-2018, 04:30 PM   #2
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I'm sure that you are aware of this but subfloor material & thickness must also be considered in determining suitability of the "floor system".

Is the actual span of these joist 10'? Is the floor assembly on the 1st or 2nd floor? Any bearing partition walls below the joist you plan on tiling?
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Old 03-03-2018, 04:47 PM   #3
sanjosetiler
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Yes I am aware that the floor material is an important part of the equation, thanks. Yes the span is only 10 ft. Foundation wall to wall in that part of my house is 20ft. so I assume they didn't want to go with joists for that span and just split it down the middle. This is on the ground floor of my home and my home has a crawlspace so nothing underneath except air and a bit of mouse poop.
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Old 03-03-2018, 04:52 PM   #4
sanjosetiler
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As far as the floor is concerned, it is 3/4 OSB. Planned to put 3/8 ACX over the OSB followed by Ditra.
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Old 03-03-2018, 05:02 PM   #5
jadnashua
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You won't know the deflection rating of the trusses unless you call the manufacturer or you have the blueprints. Generally, they build to L/480 unless more is requested, but you cannot be certain. ONe thing missing was their spacing which will affect what you need for the subflooring. A second layer of ply may be required, depending on the spacing and the type of tile you wish to install, but it may not.
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Old 03-03-2018, 05:09 PM   #6
sanjosetiler
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Jim, they are engineered joists, not trusses. Spacing is 16 inches on center which give a pretty narrow spacing with the 3 1/2 inch top plates.
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Old 03-03-2018, 07:06 PM   #7
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That's good...regardless of the type, the wider the spacing, the more difficult it is for the subflooring to provide sufficient strength between them.

Without knowing the brand and exact model, none of the generic deflection calculators will be able to determine their actual deflection...you need the manufacturer involved to tell.

You only are required to use a second layer of subflooring when you are using a natural stone tile or the spacing is greater than 19.2", so 16" is good.
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Old 03-03-2018, 10:57 PM   #8
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Only the manufacturer can give you a definitive answer on stiffness. Often they publish a technical guide, what is the manufacturer and model number?

We can get an approximation with a little work:

An I-joist consisting of 3.5" wide by 1.5" thick flanges, with an overall height of 9.5", will be less stiff than a solid sawn joist that is 3.5" x 9.5". There is, after all, less material present in the I-joist.

The I-joist (ignoring the web) can be approximated as the difference between two solid sawn joists, one of the same outside dimensions, and the other representing the void space between the flanges. In the example above, that would be a 3.5" x 9.5" joist and 3.5" x 6.5" void space. The deflectolator for those sizes (well, 9.25" in the first case), 16" o.c., a 10' span, and SYP or DF in good condition gives deflections of L/1999 and L/850. The way deflection arithmetic works out means the difference would be L/1149.

That would pretty much be an upper bound for the stiffness. The actual stiffness may be less, perhaps due to shear deflection in the thin web, which is something I don't understand. If you're looking to install ceramic tile, I think it would be very likely the true stiffness is at least L/360. If you're looking to install natural stone, I wouldn't be as confident that it exceeds L/720, so further research is called for.

Cheers, Wayne
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