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Unread 04-17-2011, 11:37 AM   #1
dlongan
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Subfloor

Hi Folks,

First tile job!
I am in process of updating a small 1/2 bath located on our 2nd floor.
The house was built in 1942.
Joists are 1.5 X 7.5 16"OC good condition (unknown wood type)
Current subfloor:
5.5" X 1" wood planks on 45 deg
5/8" plywood on top in good condition (unknown type)
Room is small - 7.5' X 3.5'

Tile will be ceramic 4x4's

So the question is should I add any more to the subfloor?
The current subfloor is even witht the wood flooring outside of the bath, it would be nice not to have a high transistion.

Thanks in advance for all your advice!
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Unread 04-17-2011, 12:27 PM   #2
cx
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Welcome, dlongan. Please change that permanent signature line to a first name for us to use.

Without an unsupported span, we can't help with the joist evaluation, and you need to do that.

The subflooring, if in good condition and well fastened is very good.

You must still add a tiling substrate of some sort, a CBU or uncoupling membrane or some equivalent. Then a ceramic tile suitable for a floor installation.

My opinion; worth price charged.
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Unread 04-17-2011, 12:32 PM   #3
dlongan
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Hi CX,

Sorry, I actually thought I had all the specs.
The joists are 11.5" long
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Unread 04-17-2011, 12:47 PM   #4
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If that dimension is on that drawing, Don, it's waaaay too small for some of us to read.

You're overspanned with those joists, even if they're good wood type. You can tile over it if you like, of course.

The between-joist deflection is actually more critical and you appear to be in good shape there. But you really wanna meet both specs.

My opinion; worth price charged.
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Unread 04-17-2011, 01:17 PM   #5
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cx,

I'm curious - what do you do with joist sizes between the options available on the Deflecto? For example, this poster has 7.5" joists, but the Deflecto goes from 7.25" straight to 8".

I had the same measurements for my install - 11.5' span, with 16 OC 7.5" joists (it's a 1946 house, and I guess lumber sizing was a little different back then?).

I took the two Deflecto values - I got a 337 and a 433; difference of 96 for a joist size difference of 3/4". My joist is 1/4" bigger than the 7.25". Dividing 96/3 I get 32. Adding this to 337 I get 369, which would be acceptable for ceramic, so I proceeded with a ceramic floor on Ditra.

So my main question - What do you do with a joist system that's not Deflecto-compliant?

Sorry to hijack your thread, dlongan; seemed like a relevant place to ask since our situations regarding joists are so similar.

And about my floor - I did 13" ceramic on Ditra over 1/2" CDX screwed to diagonally-laid T&G; it's been down for about 8 months with no cracked grout or tiles so far.

EDIT - I see, I made a mistake. OP has 'Unknown wood' but I used SYP when I ran my numbers. But, still the question - is it valid to extrapolate from two Deflecto runs for in-between joist sizes?
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Unread 04-17-2011, 01:43 PM   #6
dlongan
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@Sean12
I would assume dimensional lumber size of 2X8
en.wikipedia.org/wiki/Lumber#Dimensional_lumber

@CX

I did'nt have the span on the drawing, so please don't try to look any closer
I used a span calculator (awc.org) and used "Spruce-Pine-Fir" as wood type and it states 12.4' span. Why do you feel that I am over spanned?
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Unread 04-17-2011, 02:15 PM   #7
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Don,

You need to make sure your floor's deflection is within the range that's acceptable for the tile you want to use. You need a stiffer floor for tile than for hardwood or carpet for example, since tile can't flex with the subfloor to the same extent as other materials.

Your joist span might be acceptable in general, while not being stiff enough for tile.

There's a link labeled 'Deflecto' at the top of the page - you can use that tool to check your floor's deflection. For 'Unknown wood' with your joists and spacing, you get L/253, while you need L/360 for ceramic or L/720 for natural stone (higher bottom number is stiffer). If you use SYP instead of Unknown Wood, it comes out closer.

This link has some more info on deflection and subfloor issues: http://www.johnbridge.com/vbulletin/...ead.php?t=1845
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Unread 04-17-2011, 02:56 PM   #8
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@CX & Sean12,

I ran the Deflecto calculator, but of course neglected to change the span distance...
I totally understand the "overspan" indication.

There is really no way of changing the joist design, it would be cost prohibitive.

Any ideas on how to get the floor to become within spec?
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Unread 04-17-2011, 03:34 PM   #9
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What's below that bathroom? Since it's on the 2nd floor, I'm guessing you have a finished room with a closed off ceiling, right?
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Unread 04-17-2011, 03:46 PM   #10
dlongan
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Yea, actually first floor full bath.
Walls/ceiling are wet plaster.

I think I will install 1/4" hardi backer board and wish for the best!!
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Unread 04-17-2011, 03:47 PM   #11
dlongan
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Oh btw found a nice article on backer boards

http://www.tilemagonline.com/Article...00000000174314
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Unread 04-17-2011, 04:18 PM   #12
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Hi Don,

If you have SPF #2 or better, then your joists are fine at an 11.5' span, and I'd be very surprised if you had a lower grade. You know they are SPF?
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Unread 04-17-2011, 04:33 PM   #13
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dhagin,
I really have no clue what type of wood, only that the house was built in 1942.

Is there a way to determine wood type?
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Unread 04-17-2011, 04:37 PM   #14
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But that'd depend upon whether it's northern or southern SPF, eh, Dana?

Look for a grade stamp on any of the joists, Don. That'll give you lots of information. Might need a little help translating it, but tell us what you see.
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Unread 04-17-2011, 04:42 PM   #15
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Now yer gonna make me look in that book again, aren't ya?

...ok, so here in my 09 IRC there is only one category for SPF. No north or south. I'm guessing that this value is based on the lesser value of the 2. You want me to look in another book?
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