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SpaceCadet 01-16-2019 02:43 PM

Small Bathroom Rebuild
If I have long plank tiles and a floor where the deflection may be a bit iffy, is it safer to lay the planks perpendicular to the beams vs along?

cx 01-16-2019 03:37 PM

Fix the floor, Paul, it ain't gonna help to try to stiffen it with your tiles.

Not sure what you're calling iffy. It either meets the tile industry standards or it doesn't.

My opinion; worth price charged.

SpaceCadet 01-16-2019 03:54 PM

Floor's fixed, CX. Theoretical question (although now I might rerun my deflection calculations just in case). Another thread (tile install disaster) got me wondering because someone mentioned that installing plywood will fix flex between joists but not overall deflection. Didn't want to hijack that thread.

jadnashua 01-16-2019 04:25 PM

There are two components of deflection...
- along the joists...doesn't really matter what's on top (within reason) that number is critical
- in between the joints. This is where extra plywood may come into play. The minimum depends on the joist spacing and the ply thickness.

Both must meet industry standards. It's easier to fix a between the joist deflection issue than along the joists, since it can all be done from the top.

SpaceCadet 01-16-2019 07:21 PM

1 Attachment(s)
Alright, checked the deflection calculator, I'm selling the house. 2x8 joists 16" OC spacing over 11.5 is not enough for ceramic tile. Seems like pretty standard construction. How does anyone have tile in their houses!?

The tile I tore up was in good shape. The tile underneath it seemed alright but couldn't really tell reliably in the debris. The original was set on a mud bed and the joists along one wall and the one about 1/3 of the way across are doubled up. The attached picture is before I replaced the rotted 1x12 planks and put down 3/4" plywood (1/2" under the tub along the wall). Was it the mudbed that was keeping things together? Too much to count on Ditra to save the day?

Guess I'm tearing out the plaster floor in the basement and sistering some joists.

jadnashua 01-16-2019 08:42 PM

A properly done mudbed over a wooden subfloor is thick enough to float as a unit, so yes, it is what kept the tile intact.

The most reliable way to add tile back would be to do another mudbed if you can't bring the subfloor up to specs.

cx 01-16-2019 08:47 PM

Up to you, Paul. You can tile over whatever you're comfortable with. Yes, in days of old we got away with a lot of tile over marginal framing when the substrate was reinforced mud. Or even mud over metal lath, sometimes.

I can find you lumber span tables that will say (if you know the species and grade of your joists) that your joists met building code deflection of L/360 when it was constructed. Our Deflectolator is more conservative because most of the time it is used for calculating existing structures rather than planned construction. Remodel work is not usually done over new materials in perfect condition and properly installed as one might expect in new construction. The Deflectolator is also sensitive to the additional dead loads associated with a ceramic tile installation. We like it much better than the new construction span tables when it comes to remodel work.

Ceramic tile industry says you must have L/360 or less for a ceramic tile installation. Up to you to determine if you meet that standard.

My opinion; worth price charged.

SpaceCadet 01-17-2019 08:15 AM

I don't like taking chances. Just wish I'd realized my mistake sooner. The tile isn't down yet but the tub is set in a layer of cement and I'm afraid messing with the joists is going to crack that base. I can't remember how I figured that the floor was good enough, might've only used the span of the bathroom floor instead of span between supports or figured that 3/4" plywood and ditra would do it. The user name and pic are appropriate.

Luckily, what's below the bathroom is the basement and I have access to the entire span of the joists from sill to the support beam running down the middle of the house. I'll have to get around a bunch of plumbing and electrical and tare out a plaster and metal lath ceiling but I hate having a finished ceiling in a utility-type space anyway.

So the plan is (and tell me if I'm planning something stupid, please) sister the two single joists in the main span of the floor with 2x8 doug fir lumber. You can see them in the photo above. There's one about under the right wall but it's more out of the way and there's a heater pipe next to it. Use the full 12' of lumber and lay the ends on the sill and support beam like the original joist. Glue and screw the new lumber to the old, 3" construction screws in pairs top and bottom 1.5" from edge of beam every 8" and PL Premium adhesive or similar. This thread is my reference: https://www.johnbridge.com/vbulletin...d.php?t=124259. The floor is planks with plywood glued+screwed over them so I can't screw the planks to the new joists without taring it all up but I'll try to get some subfloor adhesive at the top to keep it stable and fill the void.

The sisters will need to rotate upright and I don't want to disturb the existing floor (and everything on top of it) too much so I might notch them so they're loose between the bottom supports (beam and sill) and the floor then shim them up to hold them in place. They're not supporting the floor, they're reinforcing the old joist. Does that sound reasonable?

If I go out today and buy the 2x8 doug fir lumber today from a place that keeps them indoors, how long do I have to keep it in my basement aclimatizing before I can use it?

Thank you for your help.

ss3964spd 01-17-2019 09:01 AM


You're sistering plan will work. As has been said here by many you need only to have your sister joists span at least 2/3rd's of the existing joists, and more or less centered on the existing joists.

Having sistered many of my own I can definitely agree that rotating them into place with each end supported isn't going to happen. just cut them short of the load bearing points.

The new joists will not be dead straight (crowned), and likely not flat. Don't try to jack them into place, that'll likely result in the floor above being pushed upwards. If you don't care about producing a flat (in plane) for a finished ceiling then it won't matter if they hang a bit below the existing joists here and there.

Have some construction adhesive ready, and some clamps. Screws screws would be good - lots of them, but I used a framing nailer to install most of mine.

SpaceCadet 01-17-2019 10:25 AM

Ok, thank you, sir. Cut the 2x8 short of supporting surfaces. I hear the 2/3s but seems like I might as well use a longer piece of lumber (although getting an 8' board home would be easier for sure). Is pre-drilling for screws a good idea? I'm patient enough to do it and I'm afraid the old lumber will crack.

Out of curiosity, what's the problem with rotating the sisters into place? Is it because there is finished stuff above the floor or just shape irregularities that always crop up in reality? Is cutting a notch out of the bottom then shimming not practical either? Seems beneficial to have it resting on supports at the end. Just want to understand.

wwhitney 01-17-2019 10:45 AM

Use kiln dried (KD) lumber, as green lumber will take at least several months to acclimatize.

Rotating is difficult for two reasons:

(1) The diagonal of the cross section is greater than the height of the cross section. This could be addressed by reducing the cross section just over the bearing points (planing would be better than notching).

(2) A bigger issue is that neither the new joists nor the existing joists will be straight, they will be crowned. Joists are typically installed crown up. So if you try to rotate up a piece of lumber whose crown is greater than the existing joist, it will hit in the middle of the span. The existing joists have likely deflected downward somewhat, so their current average crown is probably less than your new lumber.

For working around (2), I'm not sure if installing the new joists crown down would be acceptable or not. In any event, the point of the sistering is to stiffen the joists, not to strengthen them, so it is unnecessary to achieve bearing at the end. It is also unnecessary to attach the existing subfloor to the sisters.

Cheers, Wayne

SpaceCadet 01-17-2019 01:06 PM

Is there a wood species I should be looking for? Local lumber yards have kiln dried spruce 2x8s but they store them outside. Home depot claims to have douglas fir too but it doesn't say kiln dried so I'm assuming it's not and I don't trust their quality for lumber. Does it not matter that much? If they're stored outside in a northeast winter (which has been rainy more so than snowy this year), how long do I need to let them sit in my basement before I sister them on?

ss3964spd 01-17-2019 01:21 PM

For your purposes, Paul, and because you're using ceramic and not stone, I'd say it doesn't much matter. Use whatever you can find that is dry.

To me, though, what does matter is finding boards that are not cupped if you plan to use adhesive between the two. If some are cupped, then match them with any of the existing joists that might also be cupped.

Have you decided yet what fasteners you're going to use?

SpaceCadet 01-17-2019 01:35 PM

Probably these: https://www.homedepot.com/p/SPAX-10-...0756/202040973, 3" zinc coated, had decent luck with spax brand.

Everyone here just has spruce framing lumber, no select grade and no other wood.

ss3964spd 01-17-2019 01:41 PM

You're gonna need a mess of those per joist, Paul, hope you have an impact driver!

You're not putting a finished floor on the top, nor a finished ceiling on the bottom, of those sisters. Whatever they have that's dry, and as flat/straight/not cupped as you can find. Avoid any with knots on the edges.

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