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Old 01-20-2010, 11:41 AM   #1
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Need to add floor support


Seems like when your family finds out you are pretty good with remodeling and have a bunch of tools they all need something done!

My aunt's house is about 70 years old and there was an addition about 25 years ago. It has a full basement with poured wall and the house is a 2 story. The walls seem to be holding up good, the problem is the when you walk on the first floor there is a "bounce" that you can feel and things rattle. The addition is fine, this is just in the older part. I looked in the basement and there is a beam supported by poles in the middle of the older parts. The floor joists are 2x8's and span about 15 feet and I think this is where the problem is. I took a 8 ft level and there is a dip in the middle of the joists.

So I believe I have a couple of choices but need some advice. One would be to sister in some more 2x8 or 2x10 joists to the existing joist. Would I try to raise the old joists to level before I sister in new joist? My other option I see would be to add another beam in the middle of the span so the existing 2x8s only have about 8 ft to span. And use the metal support poles to slowly raise the new beam to level. This would cut some head room out but think this would be the easiest option right now.

I will attach a rough sketch showing her basement layout. Looking for some advice on these options or if anyone else has another option. Thanks

Need to add floor support-basement-layout.jpg

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Old 01-20-2010, 05:27 PM   #2
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Need to add floor support


2x8, 16"o.c. = 12'8" span 24"o.c. = 10'3". I would add a beam and posts, with new concrete footings under each. Post cap and bottom brackets for safety:http://www.strongtie.com/products/ca...ost_bases.html http://www.strongtie.com/products/ca...post_caps.html Elevate them above the slab with the brackets. Add solid blocking above the beam. What is the beam span, one story or two? http://ftp.resource.org/bsc.ca.gov/t...2_page0376.pdf

Be safe, Gary

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Old 01-20-2010, 06:17 PM   #3
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Need to add floor support


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Originally Posted by GBR in WA View Post
2x8, 16"o.c. = 12'8" span 24"o.c. = 10'3". I would add a beam and posts, with new concrete footings under each. Post cap and bottom brackets for safety:http://www.strongtie.com/products/ca...ost_bases.html http://www.strongtie.com/products/ca...post_caps.html Elevate them above the slab with the brackets. Add solid blocking above the beam. What is the beam span, one story or two? http://ftp.resource.org/bsc.ca.gov/t...2_page0376.pdf

Be safe, Gary
Thanks for the info and links Gary. It is a two story house. The post cap and bottom brackets you linked are for wood beam supports right? I was thinking of using the metal posts that are adjustable so I can set everything in place and slowly adjust them and bring everything up to the right height.

The span for the planned beam is roughly 22 ft., I looked at the link you posted about spans and it shows that for 4-2x10's I would need a post every 7ft2in. so would need a post on each end and two in the middle.

What would I need for footings for the posts? I have a big milwaukee rotary demo hammer so I can chip out the floor no problem. Just don't know what I would need for footings. Do I just dig out the dirt and then pour concrete in? Or is there more to it?

Thanks for your help.
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Old 01-20-2010, 06:36 PM   #4
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Need to add floor support


string line the existing support beam to see if is has sagged as well.
it might need to be jacked up a bit.

Any idea if there is a span chart for double 2 x 8's? you might be able to sister them an avoid adding the new beams/posts.
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Old 01-20-2010, 06:58 PM   #5
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Need to add floor support


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Originally Posted by jlhaslip View Post
string line the existing support beam to see if is has sagged as well.
it might need to be jacked up a bit.

Any idea if there is a span chart for double 2 x 8's? you might be able to sister them an avoid adding the new beams/posts.
I thought about sistering another 2x8 or even a 2x10 to the existing but there is a lot wires and other stuff in the way so thought the extra beam would be the best way to go, probably cheaper also. Also I would think that the current joist would have to be leveled out before sistering the new joists in? Or just jack each one up and add the sister joist and then lower the jack? If I can just jack an individual joist up and add the sister and then move on to the next one I might do that with 2x10s. If a single 2x8 is good for a 12'8" span I would think a 2x10 sistered to a 2x8 would be good for 15' span but not sure. It would be a lot of work to rip out all the electrical and other obstructions like plumbing and do this but then I would only lose 2 inches of headroom which wouldn't be horrible and wouldn't have a 10" beam hanging down with posts in the middle.

Lots of things to think about!!

Thanks for the help
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Old 01-20-2010, 08:23 PM   #6
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Need to add floor support


Based on loading of 40 pounds per square foot, and an allowable d/L of approximately 400, doubled 2x8 joists are good for about 16.5 foot span. Deflection controls in this case, unless you have unusually high floor load requirements. There is no need to jack up the existing 2x8 joists unless the floor is so far out of level that you find it unacceptable. Jacking up the existing joists to level will not add any strength to the system, will simply allow you to level the floor if that is necessary.

It is important that the sistered joist be adequately connected to the existing joist, or else you will not get the full benefit of the added structural member. Typically you nail the joists together using 16p nails every 8 inches or so, offset top to bottom. Personally I like to use structural screws, however it is a bit more work, especially if you don't have a heavy duty cordless drill. If you do use screws, make sure they are structurally rated, and predrill the joists using the proper sized drill (should be the diameter of the root of the screw).

My preference would be to sister the joists rather than add beams, since my basement is already tight quarters without adding additional posts and beams. Your choice of course.
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Old 01-20-2010, 08:53 PM   #7
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Need to add floor support


Well the floor sags close to an inch in the middle of the 15 foot span. So I would think that I would need to half way level the existing joists in order for the new joist to line up with them? I would like to make this floor as level as possible and don't think it would be to hard to jack the existing joists up before sistering the new ones and then hope they hold pretty level when the jack is let down.
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Old 01-20-2010, 11:21 PM   #8
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Need to add floor support


Apparently this house and mine was built with better than "standard" joists, because I have 2x8s on 14'6" span (12"o.c.). One thing I can say is that the wood in those joists is HARD... much harder than the soft pine I can pickup at one of the big box stores.

I had the same problem (except my house is only 10 years old, and when I checked the sagging of the joists, there was none).

What I did was to add 2x10 sisters to every third joist, added two sets of blocking (each about 3' way from the center of the joists, and used lots of shims to make sure the blocking was tight), and then add 2x4s laid flat to the underside of the rest of the beams to sort of create an I beam. The idea of the 2x4s is sort of like the online suggestion I've seen to add strapping to the underside of the joists. The idea is that when the joist bends, the under side of the joist streaches. By attaching something to the underside of the joist, that something must streach as well. However using strapping seems to be cost prohibitive. The only straping I could find was the 25' rolls for $25/roll. Given that you want a solid piece of strapping under each joist, that's a $25/joist fix... a 16' long 2x4 is MUCH cheaper.

Now with a 14'6" span, I still have some bounce, but it is much reduced.

While I think I've heard the suggestion that sagging joists should be made strait before sistering, it would seem like leaving them with the sag would lessen the amount of bounce after sistering. The idea is that these joist now want to sag. If you jack them up and sister them, the natural sag will tent to pull the floor back down. But then again, perhaps that "strain" against the new joist actually prestresses the new joist helping to make the floor less bouncy.

Of course if you don't correct the sag before sistering the joists, the joists will sag forever.

The other thing I've heard is that you have to be careful jacking up the joists to a new level because the house has now settled with this sag. Straiten the joists back up too quick and you can develope cracks in drywall above.

I learned about a great tool for working in tight spaces like between joists... pnumatic palm nailer. A decent one will run you about $80 from what I could find online. But I recently learned about a place called Harbor Frieght Tools. They basically have cheap import tools. Most of these tools are not going to be up to the high standard a contractor would need for using the tools every day. But a plam nailer for only $40? I've already gotten my money's worth working on my bouncy floor even if the tool never works again.

Having said all that, I'll admit that Gary's answer is going to be the BEST from the stand point truly getting rid of the bounce. But in my case, I wanted to preserve the floor space below to finish in the basement room, and I didn't want a bunch of new posts in the way. So I instead went with "less bounce" rather than the near removal of the bounce using Gary's idea (after all, you effecively turn a 15' span into <8' spans).
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Old 01-21-2010, 12:45 AM   #9
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Need to add floor support


Here is the site HooKoo is talking about: http://www.finehomebuilding.com/PDF/Free/021184090.pdf



And the other site I may have used, bridging will distribute a given load, not make it stronger, it will feel stiffer: PAGE 749: http://books.google.com/books?id=DWs...um=4#PPA747,M1


Be safe, Gary
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Old 01-21-2010, 01:22 AM   #10
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Need to add floor support


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Originally Posted by GBR in WA View Post
Here is the site HooKoo is talking about: http://www.finehomebuilding.com/PDF/Free/021184090.pdf
That's one of the web pages I found when I researched the subject. One of the other pages simply showed the strap mounted to the bottom of the joist for it's full length (but I think what they show on the aboved reference page migh be better).

I tried doing a Google search to see if I could find the other page I found... didn't find it, but did find where Google has already indexed this page.
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Old 01-21-2010, 05:43 PM   #11
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Need to add floor support


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Originally Posted by GBR in WA View Post
Here is the site HooKoo is talking about: http://www.finehomebuilding.com/PDF/Free/021184090.pdf
Thanks for the link, that is a lot of really good info. After reading that article I believe my best coarse of action is to get a couple of support jacks and a beam and set them up under the joists in the middle of the span. After it is jacked up to level I will then glue and nail a 2x10 to the existing joists. And also add 2 rows of blocking in between the joists. Then let the temp support down and hope everything holds! I would only loss 2 inches of head room by using a 2x10 and I wouldn't have any added beams or posts so I think this would be the best bet. It is going to be a lot of work to disconnect all the wires running threw the joist in order to sister the new joists, and there is also a bunch of heating and plumbing that will need to be moved or worked around. Sounds like it's going to be a fun job.

Should I slowly raise the floor joists to minimize cracks in walls or doesn't that matter? I read to raise things slowly when doing this.

I'm going to try and find some decent support columns to use to temporarly raise the joists. The only ones I could find at the box stores are the standard round metal ones that have an adjusting screw on one end but they look pretty cheap. In the article from fine home building they the have some nice adjustable ones but not sure where to get those from. I looked on the website of the ones they showed in that article http://www.ableexport.com/FSeries1.htm but they don't show prices or where to buy them from. I sent them an email so hopefully there is an place locally to get them.

Does all this seem like a good plan of attack? Thanks for all your help.
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Old 01-21-2010, 10:44 PM   #12
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Need to add floor support


Maybe I'm missing something here...

How do you plan to sister a 2x8 joist with a 2x10 joist if the 2x8 joist is being held up by a temporary support?

As for dealing with the existing wires, would it be possible to just notch the joist such that the new joist would fit around the existing wire? You could even retain most of the strenght of the joist by either back-filling the notch with a block of wood and shims.

The other thing that would work (just not be as strong) would be to attach the sister joist below the wire. For example, if the wire is 2" from the top of a 2x8 joist, then get another 2x8 joist, but attach it 2" from the top. The new joist would still protrude 2" below, and would not be AS stiff as a full 2x8... but sometimes the easier way is "good enough".
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Old 01-21-2010, 11:05 PM   #13
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Need to add floor support


In the article that was linked from fine home building the said you can sister a bigger joist and it will give more support, even though you have to notch the ends. It tells about it in the fine home building article GBR and you linked to. As for the wires I don't want to go threw all this trouble only to have it still sag because I chose to notch pretty much every joist. It might work but I think I would rather do the extra work and make sure it is going to be solid after wards. I'm fairly good with electrical so that shouldn't be to much of a problem. Should be able to work around most of the plumbing. Probably just take down all the sheet metal dusts that are nailed to the joists and replace them when done.

This is the plan so far unless anyone sees anything wrong or has a better idea.

Found some pretty nice jack posts online made by ellis manufacturing, you just put a 4x6 cut to the length you need in and screw it up to where you need it. They look a lot sturdier and better made than the cheap looking ones at home depot and menards.

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