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

ARCH81 · Nov 27, 2023

Hi, and thanks for any advice. After pulling over 900 lbs of dead weight in the center of my joists I had no deflection for tiling a bathroom floor ( roughly 6.5' x 6.5'). I measured from the floor below to see if there was any measurable deflection/sag, but there was not even a 1/16 of an inch. Am I good to tile?

Background: I have a renovated addition bathroom in a dormer. It's on double 2x6"s with a 12' span. The John Bridge Calculator says I'd have about an L / 287, BUT it's in a dormer. So the gable wall is about at the middle of the 12' span. It is connected to the bottom plate of course which is connected to the doubled joists. The wall is then connected to the header that spans the gap of cut rafters and connects to both sides of the double rafters. So I believe the dormer structure is not letting the joists sag as it's being held by the bottom plate to the wall 2x4"s through the header and on the side rafters. I could only measure from the ceiling of the room under it but there was no measurable sag. It would be too difficult to measure from the top of a 2x6" from one end to the other as it slops too tight on the outer wall due to the rafter pitch. Most of the weight I put right in the center of the 12' span.

I think I am ok to tile given the weight test. Thoughts? Am I missing something? Thanks again.

Tile Marble Granite · Nov 27, 2023

I admire the John Bridge Calculator as I still use it as a reference when challenged. Always keen to keep on learning timber joist deflection, there's a lot to learn.
Are you going to use uncoupling membrane, or ''Old School'' pour down mud?

ARCH81 · Nov 27, 2023

Tile Marble Granite said:
I admire the John Bridge Calculator as I still use it as a reference when I do timber joist jobs. Always keen to keep on learning timber joist deflection, there's a lot to learn.
Are you going to use uncoupling membrane, or ''Old School'' pour down mud?

Well, I was going to get the floor to 1 1/8" with 3/8 CDX ply under the 3/4" OSB. Then I was going to use decoupling with heated wiring embedded as it doesn't add much more height than one or the other separately. So a bit high but sturdy and with some crackling resiliency.

Since writing I also measured from the bath wall to the center of the span (using a laser level) in addition to from the room below with about 900+ lbs there was only 1/16th of an inch movement max using either method.

Oh, I should have mentioned the joists are 16 OC but being doubled there is maybe about a 13" gap between joists, or 16" OC from the middle of the two joists.

Tile Marble Granite · Nov 28, 2023

Send pictures of the joists, entire area

ARCH81 · Nov 28, 2023

I hope this helps. I had to shrink the normal jpeg files for them to upload.

1) Bath Area in dormer
2) Gable wall of dormer. The rafter header is visible with the insulation removed. The wall starts just past half the 12' span if measured from the opposite bath wall. I'm pretty sure the gable wall via the bottom plate to wall to rafter header to double rafters is holding the floor from deflection. The load of the gable is structurally in the double rafters. Technically the walls could be removed, but the gable wall holds the joists so not much movement.
3) Doubled 2 x 6"s. The one with three is just two with a partial non-supportive 2x4" there for some reason.
4) Joists from the left angle.
5) Joist from the right angle.

Tile Marble Granite · Nov 28, 2023

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	6"

Joist Width	4.5"

Joist Spacing	16"	on center

Joist Length	12	feet

This translates to a deflection of L / 546.

Tile Marble Granite · Nov 28, 2023

The sister joists

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

Joist Height	6"

Joist Width	3"

Joist Spacing	16"	on center

Joist Length	12	feet

This translates to a deflection of L / 364.

ARCH81 · Nov 28, 2023

Tile Marble Granite said:
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 6"

Joist Width 4.5"

Joist Spacing 16" on center

Joist Length 12 feet

This translates to a deflection of L / 546.

Maybe from the typo on Joist With. Shouldn't it be 3" from the double two-bis.

Here are pictures from the JB Calculator I am using online. I didn't buy a special app. Maybe your app is better?

Thanks for the help again. Much appreciated.

ARCH81 · Nov 28, 2023

ARCH81 said:
Maybe from the typo on Joist With. Shouldn't it be 3" from the double two-bis.

Here are pictures from the JB Calculator I am using online. I didn't buy a special app. Maybe your app is better?

Thanks for the help again. Much appreciated.

What do you get at a joist height of 5.5"? I do believe the online calculator I'm using could be different than yours. Maybe more conservative on the assumptions perhaps.

If it's very close then I believe I'd be fine as the center is basically hanging from the rafters. I was just wondering if there was anything I wasn't considering.

P.S. I have done a floor before for my other bathroom. I would have had deflection issues but was able to put in a support beam/wall in the basement to solve the issue.

Tile Marble Granite · Nov 28, 2023

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

The joists you have over there are 1.5'' x 5.5'' often called 2 x 6

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

ARCH81 · Nov 28, 2023

Tile Marble Granite said:
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 · Nov 28, 2023

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 · Nov 28, 2023

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 · Nov 28, 2023

Tile Marble Granite said:
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 · Nov 28, 2023

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?

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