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Unread 11-01-2010, 10:14 AM   #31
andy911
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Complications with getting the new joist members underneath existing finished space that is not being opened up - I'm working from above in this case. Plus some hvac bulkheads in the way, not directly below the floor to be tiled but further along inside the middle 2/3rd of the joist.
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Unread 11-01-2010, 10:15 AM   #32
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I'm gonna disappoint Brian, but I dunno what might happen if you sister only the area you intend to tile, or just beyond that area. I suspect that the lack of rigidity will cause an early failure (cracking tile or grout), but I can't tell you how bad it will be, how soon it will happen, or even if anything bad will happen at all. Engineers avoid unsolvable (or difficult to solve) problems by making some simplifying assumptions, then designing the structure based on the assumptions made. Joists with differring cross sections are difficult to solve, so we don't use them in residential construction.
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Unread 11-01-2010, 10:36 AM   #33
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Thanks Bob. I'm just curious as to how the unsistered portion could possibly cause the sistered portion to internally deflect. I suppose there is a levering effect there.

I'll try to get the sistering beams under the floor as far back as possible though. I'm calculated to L/1035 deflection so I'm overbuilding to a certain extent. With 1.5" of ply plus 3/8" slc im thinking it'll be okay.
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Unread 11-01-2010, 04:09 PM   #34
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Well a bit of good news - turns out most of my joists are 12" OC, with only 3 cavities being 16" (ie. the ones that support the bathtub, which I had replaced earlier so I had them opened)

So double sistering is way overkill for most of the floor, but I still need to do it to recess the first course of plywood.
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Unread 11-02-2010, 09:32 AM   #35
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Looks like I have a bit more height room than I thought - maybe I don't need to recess the first layer of ply.

If I double sister the 16" OC and single sister the 12" OC, can I run a 3/4" sheet and a 1/2" sheet of ply? (then 1/4" to 3/8" of slc embedding a heating mesh)
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Unread 11-02-2010, 10:14 AM   #36
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Sounds OK to me. (You intend to sister at least the middle 2/3s of the span, right?) Add a decoupling membrane or a antifracture membrane (on top of the SLC), and you should be fine.
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Unread 11-02-2010, 10:58 AM   #37
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I'm going to do my best - I don't think I'll get 2/3rds but I should get the middle half ie at least 8 feet from the ends.

Does the thicker ply sheet go down first or second? (or does it matter)
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Unread 11-02-2010, 11:02 AM   #38
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Thicker sheet first. You screw the second sheet into the first, so you want something thick to engage as many threads as you can.
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Unread 11-02-2010, 12:21 PM   #39
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Sounds good. One last question - can I sister the joists with PL premium and screws (instead of nails which are technically better for shear)? And what is the course/pattern for the screws in this case?

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Unread 11-02-2010, 12:56 PM   #40
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PL and decking screws are exactly what I'd be using here. No nails. Screws every 6 - 8", and I stagger the layout top-third, bottom-third.
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Unread 11-05-2010, 06:30 AM   #41
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Quick question - with 1.25" of total ply, do you think I need to put insulation under the subfloor for a heated floor? Or, at least, can I get away with a foil insulation between the two sheets of ply?
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Unread 11-05-2010, 06:36 AM   #42
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You're always benefitted by adding insulation in the joist bays, unless there's latent heat in the space below that you'd prefer to transmit more freely into your floor surface.

I wouldn't consider a foil layer embedded between the plywood layers to do anything productive, at all.
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Unread 11-10-2010, 01:03 PM   #43
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Hi, I have two questions about anti-crack membranes:

1)are there any membranes rated to go right overtop of the heating wires that would still provide a reasonably smooth surface for the mosaics? (ie in order to skip the slc step)

2)alternatively, any objections to using the laticrete hydroban roll-on membrane overtop of the slc?

Thanks!
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Unread 11-10-2010, 01:26 PM   #44
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1) No. Wires and mats are lumpy and they need embedment in something to spread the heat. SLC or thinset, then a membrane.

2) None that I know of. Contact the manufacturer to be sure.
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Unread 11-10-2010, 01:57 PM   #45
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Thanks. It does appear their products are compatible with a heated floor according to thier directions, so that sounds like the best way to minimize height.

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