Strengthening an Old Floor

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Old 12-19-11, 09:26 AM
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Strengthening an Old Floor

Hi Everyone,

I have an old 1.5 story home (1910-1920) that is balloon construction. The first floor is supported by a 8x6 solid beam with 2x8" joists 16" on center. The beam is supported in the middle with 2 - 6" diameter tree trunk posts about 7' apart from each other and each is about 7' from the foundation. The 2x8" beams span 11 feet.

Our first floor is pretty bouncy so I plan on stiffening it up by sistering new 2x8" boards to the existing using construction adhesive in a zigzag pattern and 10p framing nails. I know that using larger lumber (2x10 or 2x12) would really stiffen it up more but our unfinished basement only has 5'11" ceilings already so I don't want to loose any more head room.

I am currently in the middle of a vinyl siding/insulating project. I am tarring off to expose the wall studs and insulating from the outside (we previously had NO insulation in many of our old walls).

Since I have the walls open I figure it will be the perfect time to slide new beams in from the outside having them sit on the sill plate.

(sorry for all the background info)

I have 2 questions:

1) Will it benefit us at all to glue 1/2 or 3/4 plywood onto the joists first before sistering the new beam giving us a lumber-plywood-lumber sandwich?

2) I plan on putting 2x8" blocking perpendicular to the joists attaching each joist to the next (all in a straight row of course not staggered). How many should I add per joist bay? Only 1 or 2 or so many per total span?

Thank you all very much for your help!
 
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Old 12-19-11, 03:16 PM
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Something tells me that it's not the joists that make the floor 'bouncy'. You maybe doing more work than is needed. If the first floor is oak, you may just have to secure it to the existing joists.
 
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Old 12-19-11, 04:04 PM
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I think a span on those floor joist will clear up if they are the source of the bounce. I personally think there is a good chance they are. Generally speaking, older homes have very bouncy floors compared to new construction.

A bit off topic from your questions, but when you do your insulation work, make sure you put blocking into those joist bays if there isn't any already. I would put a block down at floor level above the sill and at ceiling level. Repeat for each floor you have. I'm not sure on code, but I think fire blocking is required every 10'. The reason for my suggestion is to stop air flow through your wall cavities from the basement to the attic. Batt insulation does not stop air flow, so it is important to install blocking. Caulking any gaps around the blocking will only help.
 
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Old 12-19-11, 08:16 PM
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Thanks for the responses!

The cause of the problem is definitely the joists; I can see them flex when I'm in the basement and people walk above. The hardwood IS an issue but unrelated... The floor will be redone down the road (I am a flooring installer by trade); structure first.

drooplug: what do you mean that the "span on those floor joist will clear up if they are the source of the bounce"??

As for the insulation, I'm filling the stud bays with polyiso board (got 700 sq. ft. of 4'x4' 3" polyiso for FREE off of craigslist) cut slightly short and spray foaming it in. Our 2x4 walls now have R20 between the studs and another R9 on the exterior. The bays should now be pretty much airtight. Too late for firestops now.

Any opinions on the 2 issues i'm pondering?
 
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Old 12-20-11, 08:05 AM
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If you are sure that the joists are the problem, then continue with your plan to sister them.
 
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Old 12-20-11, 08:13 AM
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Originally Posted by Pulpo View Post
If you are sure that the joists are the problem, then continue with your plan to sister them.
Cool...

Can you comment on this:

1) Will it benefit us at all to glue 1/2 or 3/4 plywood onto the joists first before sistering the new beam giving us a lumber-plywood-lumber sandwich?

2) I plan on putting 2x8" blocking perpendicular to the joists attaching each joist to the next (all in a straight row of course not staggered). How many should I add per joist bay? Only 1 or 2 or so many per total span?
 
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Old 12-20-11, 12:56 PM
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No, make yourself a steak sandwich on garlic bread instead. It will taste much better. I wouldn't bother with the blocks either. Sistering the joists should do the trick.
 
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Old 12-20-11, 04:38 PM
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The plywood won't help any. The blocks are a must. Doing them in line is the proper and best way to do them. Blocking/bridging helps tranfers load from one joist to the one next to it. I don't have a number for you. In my house, the longest span is around 14'. The bridging is centered on those. This is 1928 construction. This is exactly what you are looking for: The Importance Of Floor Joist Bridging - Part 1

What I meant by my comment about span was to determine if the bounce was from undersized joists or what Pulpo mentioned. Knowing the span (distance between the two points of support for the joist) and the size of the joist will tell us how inadequate the joists are.
 
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Old 12-20-11, 05:16 PM
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