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-   -   About to lay Hardibacker on Subfloor w/ mortar (https://www.johnbridge.com/vbulletin/showthread.php?t=76328)

Alan D 08-12-2009 11:25 AM

Sistering Joist Issue
Hi all,

I am currently remodleing my batroom an planning on laying ceramic tile.

I have 2x6 redwood joist, 16 inch on center spacing and 10ft between beams.

with what looks like 1/2 oak subfloor, using 1/2 ply OSB and 1/4 hardibacker.

I know this does not meet the proper delflection for tile, so I sitered up the joist.

Clearly I did not do enough research as I sister up the joist with 2x4s with 12inch spacing.

I now know I should have been using 2x6, but do you think the 2x4 at 12inch spacing will be enough to meet the 360 deflection I need.

Has anyone else done this?? what are the results, or do i need to get under the crawl space again?



cx 08-12-2009 12:11 PM

Welcome, Alan. :)

Not sure just how a fella would sister 16"oc joists with sisters 12"oc. You're gonna hafta help me with that one.

Perhaps you could post us a picher of what you've done? We like pichers. :)

My opinion; worth price charged.

Alan D 08-12-2009 12:25 PM

The 2x4 that is sistered in between 16oc joist fit all the way, approximatley 14.5 inches in length.

and I spaced them 1ft apart along the joist

SO for a section of joist that run 11ft in the bathroom, I put in 11 sistering

Does it make sense yet?

Sorry new to this stuff

cx 08-12-2009 12:36 PM

Well, yeah, kinda, 'cept that's not sistering, Alan.

And, unfortunately, that's done you no good at all. :(

What you've done is install partial blocking between your joists. While that could have some effect upon the between-joist deflection of your subflooring, it has no effect whatever on your woefully overspanned joists.

What you need to do is remove that blocking (I know, I know) and actually sister those joists by adding an additional joist of equal or larger size beside the existing joists. If you use 2x6 material for your sisters, you'll really want to select a more suitable wood specie, also. Southern Yellow Pine and Douglas Fir are a couple common species of framing lumber that are generally available in most areas.

The alternative is to provide a mid-span support beam or wall under the existing joists to reduce the unsupported span. Can't tell if that's an option for you or not.

My opinion; worth price charged.

HammerMill88 08-12-2009 12:53 PM

CX's 2nd option is the one I would go with since u have already installed blocking.

Just double u up a 2x6 beam 1/2 way that spans across all joists. Use some 2x12 double-joist hangers (at least4 to keep beam from rolling) to support beam where it touches the lowest joists. Those it does not contact, cut some "just tight" shims.

Support ends of beam with concrete blocks or floor-jacks (more expensive). Just tighten unless u want to get any sag out of ther floor at this time.

That's what I would do.

Good luck!

Alan D 08-12-2009 01:05 PM

Thanks for the info guys,

I had this all incorrect to start with. I was thinking of deflection of the boards between joist, not the actual deflection of the joist between the beam.

Looking back at it, the 5 or 6 of the joist joist that span the width of the bathroom about 3 of those had already been sistered (the real Sistering)
and I recall seeing 2 cinder blocks underneath one of the sections.

I will need to go look at it all again, but now I understand what is meant to be happening.

So hopefully I just have to sister up the remaining 3 joist.

Thanks again

Alan D 08-12-2009 01:17 PM


Was just thinking about joey's thread and it seems a pretty good option too and may take that course of action.

Just a question about doubling up the 2X6

Do I double up so it ends up being a 2X12 of a 4x6

Possibly a dumb question seeing as you said a 2 X12 joist hanger but it just seems a little odd to me, but what would I know considering my previous efforts. haha


cx 08-12-2009 01:40 PM

You'd customarily "sister" the joists to create a 4x6 using two 2x6s, Alan. If you could find a way to actually attach the members such that you end up with a 2x12, that would be far, far more rigid, but I don't have any eye dee how a fella'd do that.

I also have no eye-dee what Joey had in mind for his 2x12 joist hangers, either. Maybe he'll tell us.

For adding the mid-span support, his suggestion of doubled 2x6s would be OK so long as you have a way to support the new beam every five or six feet. It's usually easier to use deeper beams and fewer supports, but I can't see what's under your floor from here.

My opinion; worth price charged.

Edthedawg 08-12-2009 01:53 PM

the 2x12 joist hangers are a good idea to hang/hold it all together while working by yourself in a situation like this. Hang 'em on one end of the existing 2x6's so that about 5 3/4" hangs down - enough to easily slide in a new 2x6 that will run across below all the existing 2x6 joists. Then get the new piece in, lift the far end up, and attach it tight w/ another 2x12 joist hanger on that end.

I might look for a 4x12 so i could put up a sistered pair of 2x6's running across the span. Less apt to tip/rock, plus better strength as a pair.

Or if this is getting ANY exterior exposure (I think the word crawlspace was used, dunno if that's interior or outside) one might contemplate using a PT 4x6 to do this work.

HammerMill88 08-12-2009 05:28 PM

Yeah Alan, sorry about the informality earlier...Welcome aboard! There are some really good, knowledgeable people here.

Ed kinda hit on the 2x12 joist hangers. The 2x12 gives you enough hanger to nail onto the joists. You have 5 1/2" of doubled beam on the bottom part of hanger, and the top 5" of the hanger nails into the joist.

At the minimum, I would have a hanger every 48" (e.g. 16" o.c. = every third joist & 24" o.c. = every other joist). Use the 10d 1 1/2" approved hanger nails. They have the shear strength necessary for this application.

I like using 2x stock b/c of opposing grains can make stronger and less twisting than 4x stuff. Down here in MS, we use PT pine stuff as much as possible. If using PT, make sure you use PT approved fasteners and nail/screw the piss out of them together (more is better!).

CX is right. I said put supports at ends. He is more correct that you don't want more than say 6' span b/w pillars for the 2x6's. I would figure about 5 1/2' per support. That means you can have about 2 1/2' hang-over on the ends. I wouldn't really want to go a full 3'. So, for 2 pillars, you could max out at 11'. If you want to push 12' do at your own discretion, but you are better widening on the inside than more hang-over (IMO).

Remember, when "shimming" use "opposing wedges," and just tighten b/w any high joists and then beam. Don't over do it. If using PT, you may want to come back every other week for 6 weeks or so b/c most PT is still wet and will shrink when drying out. You may even have to shim where the beam once touched the joist.

Again, good luck. Also, there are no "stupid" questions, only uninformed/misinformed.

Don't hesitate to ask for further assistance. These guys rock!

HammerMill88 08-12-2009 05:34 PM

Edit: Nailing 2x's together.
Remember, crown, if any goes up. Pick through and find two good pieces with a very slight crown.

Also, if you have a good table saw. Once you nail them together (watch out for nails sticking through), run them through the table saw to trim the edges even to again aid in support and prevent rolling.

If not table saw. Lay them on top of one another and scribe the one that is larger, then trim with skil-saw. You may have to trim both pieces. IMO, this is an imperative step. Top and bottom need to be flush and flat, within 1/32 at least.

Alan D 08-14-2009 12:05 PM

About to lay Hardibacker on Subfloor w/ mortar
Hi again,

I am about to lay down my hardibacker onto the subfloor with mortar, and I was wondering how long till I can walk on it and set my bath tub on top of it.

Obviously I will need to screw it in after it is set down so It will have some pressure applied to it, but will it be ok to walk on or set the bath in.

Or do I need to wait 24hrs

and for the guys who helped me out before, Just want to let you know that I installed the extra 2x6 joist halfway along my bath room so the spacing is more like 5 feet insted of 10feet. Used cinder blocks and wood to hold it up, I just need to fill in a few gaps between the new joist and original joist, and keep an eye on it over the next couple of months.

Thanks again

Rd Tile 08-14-2009 12:11 PM

Is this a free standing tub?

Tubs should be installed over the plywood or subfloor first, then CBU up to it, guess it could be done either way.

But you can set the tub any time after if you like.:)

Alan D 08-14-2009 12:17 PM

not too sure what you mean by free standing tub?

Does that that mean it is not held in by screws etc???

It is a old, heavy as hell tub, the side of the tub towards the wall sits/rest on a piece of wood that runs the length of it, and the side that you step in from goes down to the ground, so it has its own wall, if that makes any sense.

Rd Tile 08-14-2009 12:19 PM

OK, I always install the tub you have first, then build the subfloor for the tile up to it.:)

Free standing tub.

Edthedawg 08-14-2009 12:22 PM

Alan - someone's gonna merge this w/ your other thread about subfloor sistering, i'm sure.

I'm a little concerned that you added a 2x6 spanning underneath all this, but there are still gaps to the pre-existing joists in some places?? If that's the case, then you have flex in the floor still. Shimming those gaps is necessary. I wouldn't start doing any thinset/CBU or thinset/tile installation until that's all taken care of.

Trying to shim later on, after installation, is a sure-fire way for tiles to start poppin' loose.

Alan D 08-14-2009 12:37 PM

The reason I decide to lay it on top of the hardibacker, was because the wall of the tub it not a straight line, It has a shape to it.
I figured it would be much easier to have the hardibacker come out from underneath and join it away from the tub wall, so I would not have to try and cut that shape.

But it would be ok to walk on the hardibacker after i have mortared and screw it in. dont have to wait 24hrs

Alan D 08-14-2009 12:40 PM


Why are you concerned about my 2X6 joist span??

Dont worry, I will do the shims before laying the floor, Just trying to get my all my eggs in one basket before the weekend.


Alan D 08-14-2009 12:45 PM


I said 2x6 joist span, I actually was meant to say 4x6

Does that fix your concern

Edthedawg 08-14-2009 01:00 PM

You could park a truck under there. Heck, you could park a truck under each individual joist if you wanted. If the truck ain't touching the bottom of the joist, then it's doing nothing to support the floor and reduce the deflection in the under-engineered system.

If you go down there w/ a stack of shims and a hammer to close up all the gaps, you're gonna wind up invariably moving the floor surface a little. If you've already put any thinset down to hold either your CBU or your tile, then you stand a good chance of inducing cracks and popping and crunching...

Hence the recommendation to get that all firmed up ahead of time.

(4x6 or 2x6 - either way sounds great. Bigger = better, definitely. Just gotta have it in contact w/ every one of the joists down there.)

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