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Strengthening 2x8 Joists


joost's Avatar
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03-05-13, 07:00 PM   #1  
Strengthening 2x8 Joists

I have an bathroom and walk in closet off the master bedroom that sits halfway over my garage. Currently I have removed the partition wall between the closet and bathroom to move it over 1.5 feet for a larger bathroom. Total space of closet and bathroom is 13'4" by 5'4".

I have noticed that in the middle of the area, the floor is springy. The old 30x30 one piece shower was here and I would like to replace it with a 48x36 shower (base:acrylic/walls:tiled). I have two concerns 1)added weight of tiled walls & floor to deadload, 2)tiled floor over what is currently springy.

I removed the drywall ceiling in the garage to look at joists. The five 2x8 joists run the length of the rooms making them 13'4" in between supports and they are 12" on center (with the outside joist being 4' from outside wall). I understand this is pushing the limits of 2x8s. Other issues that I have noticed that could lead to bouncy floors: the original cross bridging was never attached to the bottom of the joists and the OSB subfloor has a patchwoork of seams in the room between and along joists. Note: the joists are in good condition with noevidence of rotting or cracks. House built in 1979.

I am trying to figureout how to strengthen the floor and reduce the deflection. Among my options are sistering the existing joists, adding blocking in between, and replacing OSB nailed subfloor with screwed down plywood.

I have a drop ceiling in the garage so there is space to sister larger joists. However, I have been told that there is no need to use a larger 2x10 joists because a 2x8 would be just as strong since the 2x10 would need to be notched at the ends.

Am I on the right track or should I be calling a structural engineer? Thanks in advance.

 
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XSleeper's Avatar
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03-05-13, 09:34 PM   #2  
Why would the 2x10's necessarily need to be notched? You say there is 13'4" between supports, so what are the supports? i.e: What are the ends of the 2x8's sitting on? What I am getting at is that depending on what these supports are, you might be able to sister with 2x10's (or 2x12's if you wish) if they were in hangers placed on the sides of the supports. I'm picturing a double hanger that would accept both members... with a shim in the pocket of the hanger to take up the void beneath the 2x8. The hanger would really only be there to support the 2x10 since the 2x8 is already resting on the support.

Also telling us more about the "supports" will help us learn if they are vastly under sized as well.

At any rate, the new subfloor should be glued (pl400) and screwed... and only AFTER you have beefed it up to take out any deflection and/or sag.

 
joost's Avatar
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03-05-13, 09:55 PM   #3  
Good question. The 2x8s sit on a load bearing wall at the back of the garage. At the other end, the 2x8s sit on three 2x10s that run across the width of the garage, which is 10' wide. So any sister joist that is load bearing has to be notched to 7.5" to fit on the load bearing supports on either end. I have looked for a type of hanger that would allow for this type of support that sits on the load bearing support rather then butting up against it.

 
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03-05-13, 10:07 PM   #4  
You can notch the larger boards to sit on the support and then use a double-wide hanger to support both members, with a shim as X suggested.

The first thing I would try, after removing everything that's coming out, is screwing the bottoms of the cross-bracing to the 2X8s. Good X-bracing makes an amazingly stiff floor.

 
Wayne Mitchell's Avatar
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03-06-13, 07:16 AM   #5  
I'm assuming that the lumber species of your existing joists was adequate for the existing span. Max span for #2 SPF is 13'6".

I agree with the crossbracing as the first step. I think replacing the subfloor with 3/4" T&G plywood would be my next step. If you decide not to replace the subfloor you might try blocking under the OSB seams.

If the above doesn't stiffen the floor enough I would simply double the joists with sistered 2"X8"s but I don't think you will need to do that.

 
Gary in WA's Avatar
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03-07-13, 09:40 PM   #6  
The 2x10 would not be reduced to a 2x8 with up to a 2-3/8" end notch and no hanger is required (check w. local AHJ): http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf

Double 2x8's (fb.1500# 2 Doug-fir) would give you 129# per sq.ft. or 1684# total over a single at 1010#, and 77# per sq.ft.

You could also add a 2x4 to the bottoms of existing joists for increased strength; http://www.doityourself.com/forum/de...eck-built.html

Gary

 
joost's Avatar
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03-08-13, 12:00 AM   #7  
Hi Gary. Does # mean lbs as in lbs per square inch?

I shared by challenge with an inlaw of mine who does contracting and renovations. He suggested adding another double or triple 2x10 beam under the joists to halve the span from 13' to 7.5'. He reccommended sitting the beam on jackposts that would be installed in the existing garage wall. This seems like it would be about the same amount of work but would increase the overall load capacity for installing tile above and reduce the bounce. Thoughts.

 
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03-08-13, 04:11 PM   #8  
2x8 at 12" centers should be enough to give you 50 lb/sf which is plenty for residential. Are you sure they are 12" oc? like was previously suggested, nail the x-bracing tight. How many rows are there? 1 row midspan should be enough but more could only be better. If you add walls above that run parallel to the joists and it doesnt sit on a joist, add blocking at 24" oc. Glueing and screwing new subfloor will probably give you most bang for the buck. You may have some issues replacing plywood under existing walls. Dont forget to glue and screw a good underlaiment before you tile.

 
Gary in WA's Avatar
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03-08-13, 06:15 PM   #9  
Yes, # is for pounds. If the (bearing) wall studs are 2x4, 16" on center and the wall has a double top plate, replacing it with a beam is pointless...

Gary

 
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03-09-13, 11:50 AM   #10  
The original joists are close to being over-spanned, especially if they have less than moderate Fb and E values, but minimal deflection is desired. Since access to the underside is available, why not just stiffen them up with flat 2 x 4s or 2 x 6s (better), glued-and-screwed to the bottom of each joist? The added members wouldn't even have to be full-length, but just 8- or 10-footers, centered. That's where the maximum bending moment and deflection occur, and where the new "flanges" will do the most good in resisting same.

 
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