Deflection check for an odd scenario. I think I am ok to tile.

[ARCH81]

Joist Height was calculated at 6'' not 5.5''

Correct. I was wondering what you would get in your app if they were calculated at 5.5" since that is their actual height.

 

[Tile Marble Granite]

oh ok, the sandwiched joist

Joist Material	SYP or Douglas Fir, in good condition Unknown wood, but in good condition Unknown wood, kind of cracky or knotty

Joist Height	5.5" (2x6)

Joist Width	4.5"

Joist Spacing	16"	on center

Joist Length	12	feet

This translates to a deflection of L / 431.

 

[Tile Marble Granite]

sister joists

Height	5.5" (2x6)

Joist Width	3"

Joist Spacing	16"	on center

Joist Length	12	feet

		This translates to a deflection of L / 287.

 

[ARCH81]

Joist Height was calculated at 6'' not 5.5''

The joists you have over there are 1.5'' x 6'' often called 2 x 6

A 2 x 8 is actually 1.5'' x 7 1/4

Sorry, I am not following you. A 2" x 6" (nominal) is 1.5" x 5.5" (actual). So it should be Joist Height 5.5" and Joist Width 3" since they are doubled. Actually, though I think you will get the same deflection number as me (L/ 287).
The real question is thoughts the lack of deflection given I weight tested it at 900+ pounds and only got 1/16" movement in the center of the joists due to it being attached to the dormer rafters. I don't think the calculator matters since I'm testing how it actually is moving under load given its structural assistance.

 

[Tile Marble Granite]

In the carpentry industry they exaggerate the numbers a 2 x 8 in fact is only 1.5'' x 7 1/4'', a 2 x 6 in fact is only 1.5'' x 5.5'' always smaller numbers

How did you 900 lbs weight tested the joist?

 

[ARCH81]

How did you 900 lbs weight tested the joist?

I piled tiles, cement, paint cans, my girlfriend, etc on the far dormer wall. That is where the halfway point of the joists are and hence the potentially deepest sag.

 

[Tile Marble Granite]

Kinda figured you did that just wasn't sure. Anyways, if I were to work on a 2 x 6 joist I would try my best to reinforce those joists to the best of my ability and keep that floor as light as possible using uncoupling membrane.

 

[Ben-gee]

Just put in a few noggins here and there, amazing how much it stiffens up a floor.