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Unread 09-22-2019, 09:20 AM   #1
BrendonTW
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(Picture) Blocking vs adding extra joists prior to tile install

Hello all, newbie here. Homeowner, not contractor.

I am in the processing of adding a new bathroom to the second floor of our house.

I am attaching a picture. I am needing to know if I need to add blocking, or if I should add joists between or nailed directly to the existing joists. (Not sure what the term for that is).

Would prefer to just add blocking, but want to do it right, so I'll do whatever is necessary. Here are some details:

2x8 joists
11.5' free span (yellow line in picture is where beam is underneath in the garage)
wood is unknown but good condition
Defletion number using john bridge calculator scores me at 275. I would prefer to be well above 360 because I like going conservative)

All areas where floor is removed will be tiled (15x8 approx)

Area A is shower (approx 55x55 inches) - schluter shower system going here and tiled floor and 8' tall walls (probably 750 pounds or so ? when all is said and done)
Area B is water closet - toilet room
Area C is main bathroom area, double vanity and open floor space, no tub
(C also points to some supports to keep joists from twisting, not sure if these are still necessary when blocking or additional joists are added?)

Notice the shower is installed directly above the beam, and is also sitting immediately adjacent to the load bearing wall that the beam sits on. (not in the middle of the beam span)

I am planning on installing 3/4 ply subfloor as well as 3/4 ply underlayment prior to installing DITRA.

Looking for advice on what to do to beef up the floor.

Do I need to beef up the area under the shower since it's sitting right on the beam? If so, what would be a sufficient beefing mechanism?

Do I need to beef up under the main bathroom area since it's in the middle of the joist span? If so, what would be a sufficient beefing mechanism be?

Is 3/4 + 3/4 sufficient for subfloor/underlayment? Is it overkill? Will going that thick alleviate the need for joist reinforcement?

If I reinforce joists, do I need a full 1.5" of plywood under the ditra?

Thanks in advance from this newb.
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Unread 09-22-2019, 10:18 AM   #2
Carbidetooth
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Welcome Brendon. Lots to cover.

I think what you're calling blocking is actually bridging. While bridging may add some small about of coupling between joists, its purpose is to keep joists from rolling. Structurally speaking, it will add nothing to deflection numbers in and of itself.

The way to where you want to go is sistering joists along existing or adding additional in between. One other possibility is adding mid-span support, but I'm guessing that's a non starter.

If you get the structure where it needs to be, there's no need to be concerned about loading of subfloor in the shower or toilet area.

Nothing added to top of joists (subflooring) will mitigate need for stiffening structure to achieve L360. Ceramic tile can be successfully set on 3/4 sheet subfloor provided it has adequate framing underneath.

I add 1/2" ply second layer to most of what I do for a belt and suspenders approach.

One other consideration is drain plumbing for all that you plan. It has to fit within framing for the most part, run downhill and be vented. Not so easy if starting with nothing.
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Last edited by Carbidetooth; 09-22-2019 at 10:24 AM.
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Unread 09-22-2019, 10:35 AM   #3
BrendonTW
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Thanks for your reply.

So I need to either double the number of joists by adding them in the middle, or sister the new joints to the existing?

That shouldn’t be a problem. Is it imperative that the sistering joists run all the way from the load bearing wall to the beam and sit on top of each? Or will running them to where the edge of the load bearing wall is be ok? Reason I ask is I’m not sure if I will be able to slide the new hoist between sill plate of second floor wall and top plate of first floor.

And did I understand correctly, that I do not need to beef up the floor under the shower and toilet since they sit directly above the beam? Only beef up the open bathroom area between the load wall and the beam?

For adding sister joists, how many nails per foot should I do? Any particular mailing pattern?
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Unread 09-22-2019, 10:52 AM   #4
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What is the span on the side of the beam that is not shown? Looks like the bathroom footprint extends 3' beyond the beam?

Cheers, Wayne
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Unread 09-22-2019, 11:00 AM   #5
Carbidetooth
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How about we do it with numbers for clarity.

1. Yes, either

2. Sistering doesn't require that there be bearing on ends. Additional joists do.

3. Yes. The beam really isn't a concern(assuming it's not compromised and also adequate for support) but everything above it is. As I look more closely at your photo, the joists to the left of the bearing beam (wall?) will need to meet deflection numbers as well. Are those joists similar in span to those on right?

4. More common is screwing. Assuming correct screws, I'd be confident with 6" staggered spacing. Add PL Premium if you're quick and want the additional strength and squeak resistance. 3" nail spacing, but if using framing nailer, you may be limited in access between joists.
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Last edited by Carbidetooth; 09-22-2019 at 01:34 PM. Reason: Added detail for clarity
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Unread 09-22-2019, 11:29 AM   #6
BrendonTW
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Beam span is about 12’ and is made of 2 2x8 or 2x10 (need to measure). Yes, bathroom (toilet and shower area) extend about 3’ to the opposite side of the beam. As mentioned, the shower portion is immediately adjacent to the load bearing wall that the beam rests on.
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Unread 09-22-2019, 01:25 PM   #7
wwhitney
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Yes, the details on the beam construction are important and need checking.

But the question was, the floor joists lap at the beam, so joists continue outside of the picture, off to the lower left, how long is the span of those joists? Since part of the tiled area is over those joists, their deflection also needs to be considered, not just the 11.5' span in picture.

Cheers, Wayne
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Unread 09-22-2019, 02:09 PM   #8
BrendonTW
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Those are also 11.5 to the next beam.
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Unread 09-22-2019, 02:33 PM   #9
wwhitney
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Quote:
Originally Posted by Brendon
Beam span is about 12’ and is made of 2 2x8 or 2x10
Please check this info, as (2) 2x8s spanning 12' is way less than what is required.

If you think about it, you have each single 2x8 joist carrying 11.5' x 16" of floor load. The beam is carrying 12' (the span) x 11.5' (half the joist span on either side) of floor load, or 9 times the area as a single 2x8 joist. So it better be a lot bigger than just 2 floor joists.

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Unread 09-22-2019, 03:23 PM   #10
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Yes, I should have been more alarmed as I posted that. I was incorrect. It is made of 3 2x12’s.
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Unread 09-22-2019, 04:38 PM   #11
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You want to sister at least the middle 2/3'rds of the joist's span. Running each end to the support does increase the strength, but getting the middle 2/3rds gets most of it.

If your floor isn't flat and/or level, the sistered joists can be installed slightly above the existing ones to give your subfloor a nice flat/level surface.
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Unread 09-22-2019, 05:37 PM   #12
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Making some progress. Two more to go. Two of them went between two
Old joists. This throws the o.c. spacing off a bit but I think this should still be ok based off of the calculator. Used 3” screws with 6” staggered pattern and included PL premium fastgrab adhesive between as well. The sistering joist runs about 95% of the length of the original joists. Still need to add bridging between them.
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Unread 09-22-2019, 05:38 PM   #13
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Might throw nails in every 3-4” as well for safety
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Unread 09-22-2019, 06:11 PM   #14
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The blocking isn't a big deal once the top and bottom are anchored especially if you use adhesive when installing the subflooring. It's more important during initial construction when a long joist could move around prior to the ceiling and flooring being installed. Done correctly, it does help to share point loads, but so does a properly installed subfloor.
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Unread 09-22-2019, 06:58 PM   #15
BrendonTW
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Makes sense. Thanks for the help all. I’m assuming my beam is strong enough. I can’t really find any information as to how I can check that.
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