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Old 05-21-2019, 12:49 PM   #1
tl1552
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24 OC floor truss with 3/4" osb and ditra xl?

I'm looking to do a continuous tile installation on my full upstairs. My floor framing consists of:

2x4 floor trusses 16" deep 24" OC
3/4 OSB subfloor t&g glued and ring shank nailed

My upstairs layout consists of a 3' wide hallway, two 5x8 bathrooms and three bedrooms. For each bedroom the joist spans btwn load bearing walls would be 10', 11' and 17.5'

I'm looking to install Ditra XL underlayment then 6x24 non-rectified porcelain tile.

According to Ditra XL specs they support 24" oc with 3/4 osb or plywood subfloor.

ref: ditra-handbook_eng.pdf
pg:6

Looking at this floor truss spec sheet (I don't think this is my floor truss mfg but I've seen the trusses before the house was drywalled and the look like this design) 16" can achieve L/360 at 18.6' at 75 PSF and 16' at 100 PSF

ref: FloorSpan.pdf
pg: 1

Any advice on if this plan will work out or not given my specs would be greatly appreciated.

Thanks
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File Type: pdf ditra-handbook_eng.pdf (2.22 MB, 7 views)
File Type: pdf FloorSpan.pdf (73.8 KB, 7 views)
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Old 05-21-2019, 12:57 PM   #2
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Welcome, Tino.

Schluter does make that recommendation for their DitraXL, but I find it a bit optimistic. What is the width of your top chord? These actual nominal 2x4s or something a bit smaller?

Using an engineered joist span table from a manufacturer other than the one who made your joists is a bit risky, but it's unlikely your joist structure was designed for a deflection greater than the L/360 required for ceramic tile.

My opinion; worth price charged.
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Old 05-21-2019, 01:01 PM   #3
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Top of the floor truss that the subfloor is nailed to is 3.5" wide
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Old 05-21-2019, 03:00 PM   #4
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That makes me a little more comfortable with that single layer of subflooring on 24" joist centers, Tino. But 20.5 inches is still a long way to span with a section of nominal 3/4" plywood to my thinking.
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Old 05-22-2019, 07:53 PM   #5
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The other option for 24" OC in the ditra manual is to use 3/8" OSB over the 3/4" OSB subfloor then 1/8" ditra. Would this produce a more stable base for tile vs ditra XL over 3/4" OSB subfloor?
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Old 05-23-2019, 08:37 AM   #6
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I've done it twice. The first one was about 5 years ago and had a 26' span; trampolines wish they bounced as well. The truss manufacturers engineer and the architect guaranteed it was "suitable for ceramic tile" so I went ahead with Ditra XL and Quartzlock 2 grout, but it made me very, very nervous. The floor felt significantly less bouncy after it was done. Last update was about 6 months ago and they've had no issues.

The second was a month ago, and it still makes me nervous
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Old 05-25-2019, 09:02 AM   #7
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I've done it before too. On one, this gal ordered an 800 lb vanity that sat on 6 legs. I was worried when the movers came to move it upstairs into the bathroom but everything went well. I still get referrals from this gal.
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Old 05-25-2019, 07:23 PM   #8
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Adding 1/2” suitable plywood screwed to the subfloor at 6” both sides the of the joist will ensure a solid floor. Use screw every 4” at the edges of the second layer and 8” in the field. Position the second layer of plywood the same orientation as the subfloor but offset the edge 6” or 18”.

You can then apply standard Ditra and your tile.

Take a look at this post in the liberty if your interested in the engineering “whys” of positioning of the second layer. Subsequent posts in the thread do a great job explaining plywood grades.

Last edited by PC7060; 05-25-2019 at 07:37 PM.
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Old 05-28-2019, 07:27 PM   #9
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From the ditra manual it looks like you can supplement a 3/4 OSB subfloor with 1/2 OSB then use the 1/8" Ditra. Is there any structural upside to B/C plywood at ~$30/sheet vs ~$13 for OSB.

The install will mostly be over bedrooms and hallways. Two small 5x8 bathrooms I plan on using kerdi band to seal against the tub, walls and seams.
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Old 05-28-2019, 07:34 PM   #10
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The primary advantage to the plywood would be the ability to bond to it, Tino. Some manufacturers of thinset mortar aren't thrilled to have their product used in bonding to OSB. I don't like trying to bond to the stuff with anything at all, truth be known.

If you were using a CBU as your tile substrate, I would have no technical objection, but would still favor the plywood in that application.

My opinion; worth price charged.
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