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Author Topic: Best way to strengthen sagging floor joists  (Read 3949 times)

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Offline DR_Buck

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Best way to strengthen sagging floor joists
« on: March 12, 2018, 08:07:59 PM »

We have started working on our "new" old house that was built 1875, added onto around 1900 and again in 1921.    It will be a while until I get to it but I am researching ways to eliminate sagging floor joist in the 1875 portion.    Both 1st and 2nd floors sag toward the middle of each room.    The joists are 2x8 and span 16 feet.    I plan on starting with the 1st floor by running a beam down the center of the house in the basement and craw-space and over time raise it up until the floors are level.    I can leave the beam in place for the 1st when that is done.     When I do the 2nd floor  I plan on doing the same thing only the beam will be temporary.    Once the floors are level I'm thinking either sister on new joist next to each one or screw and glue 3/4" plywood on both sides of the existing joists.    For the 16 foot span I think this should keep them straight for the next 100 years or so.

What would be better screw & glue plywood  or sister joists?     Whatever I do I will do the same for the 1st and 2nd floors even though I will most likely leave the beam under the 1st floor.

Any other ideas I should consider?

Additional Info:

Existing joists are a full 2" thick by 7 1/2"  and are on 16" centers.  THe 2nd floor has no sub flooring.  Wide plank pine boards are edge to edge (no T&G) and nailed directly on the joists.  First floor was probably originally the same but have had a second layer put on top running the same direction as the bottom layer.
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Offline WLC

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Re: Best way to strengthen sagging floor joists
« Reply #1 on: March 12, 2018, 08:41:50 PM »
I'd say your idea on the first floor will work.  On the second floor you probably need to look at a span calculator to see what it says you need to span the distance.  I'd be afraid that the glue and screw of the plywood wouldn't be enough and they would either sag immediately upon lowering the jack or they would sag again fairly quickly.   What do I know, I'm just spitballing here.
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Offline Kbeitz

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Re: Best way to strengthen sagging floor joists
« Reply #2 on: March 12, 2018, 09:37:57 PM »
I would use angle iron myself... Jack it up and bolt it fast to each joists.
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Offline jason.weir

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Re: Best way to strengthen sagging floor joists
« Reply #3 on: March 12, 2018, 09:51:32 PM »
16' on a 2x8 is a long way no wonder they sag over time.  Obviously a permanent supporting beam should be plan A...

What's the distance between joists?  If it's more than 16" I'd probably put another centered between the originals..

Construction adhesive, plywood & ring shank nails will certainly help, many a roof truss have been made that way and they don't sag.

Without seeing your setup, I'd suggest 10" wide plywood if you've got the room & don't go cheap on the adhesive & nails..  Then make sure you have diagonal bracing or straight blocking between the joists every 4' or so..

Here is a bad picture of some parallel chord bridge type trusses I built a while back using 2x4s with 1/2" plywood on both sides, they're 2' on center & span 24 feet and don't move at all even with 2+ feet of wet snow on top.  I know it's not the same thing as your issue but illustrates what you can do with plywood, glue & nails..




Offline Chuck White

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Re: Best way to strengthen sagging floor joists
« Reply #4 on: March 12, 2018, 09:51:47 PM »
I would suggest that you use a bottle or house-jack and raise the floor a little bit over level and screw a strip of 5/8" plywood to the sides of the floor joist that are sagging and then when you ease off of the jack, the floor should be really close to level!

KB's idea of angle iron has merit, but I think that the joists strapped with plywood would be stronger!
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Offline newoodguy78

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Re: Best way to strengthen sagging floor joists
« Reply #5 on: March 12, 2018, 10:08:18 PM »
A lot has to do with what the floor will be carrying once completed. You need a lot stronger floor to carry say a tiled bathroom with a big tub in it versus a bedroom.
I'd personally sister a new joist on either side of of the existing and put three rows of solid blocking across the span. Blocking would end up being 64" oc for a sixteen foot span. 
Just keep in mind the framing lumber in a remodeling project is relatively inexpensive and is what keeps all the finished product looking good down the road. 
Just my opinion best of luck with your project

Offline mrcaptainbob

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Re: Best way to strengthen sagging floor joists
« Reply #6 on: March 12, 2018, 10:23:23 PM »
Our place is about ten years older than yours. The 1st floor bath and the room next to it were not only sagging, but some of the joists were missing. Heavy carpenter any damage in the past. I sistered/replaced/added floor joists under both rooms and did not make any adjustments for floor level. On the top side I gutted the rooms to the floor joists. Actually, I did that before I did the joist repairs. To level I set up a room laser. I then made a diagram of the room showing each floor joist. Then marked off every 12" on each joist. Measurements were taken from that laser line to each joist mark. Yep, there were a lot of them. Calculating the variances showed the highest spot and the lowest. So, I rated the highest as zero. Each of the differences relative to that zero gave me a height for a spacer at each of those 12" marks. It was handy to use the Shopsmith, as it has an adjustable spindle that came in handy for cutting the spacers. I glued each numbered spacer to it's location. Verified with the laser that all were at the same distance. I then set two layers of 3/4 ply. The first was n/s, glued to the spacers and screwed to the joists. The second layer of 3/4 ply was set e/w and was glued and screwed to the first. I made a fiduciary mark at the middle of the room floor joist and measured to the floor and marked the side of the joist with that dimension. Did that ten/eleven years ago and it has not moved not even 1/32 inch in that time. And this was after installing hydronic floor heat and all the cabinets and appliances. It is solid as a rock. A marble stays put and does not roll around even when walking beside it.
Hope that was all clear....

Offline florida

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Re: Best way to strengthen sagging floor joists
« Reply #7 on: March 13, 2018, 08:30:14 AM »
Your first floor fix sounds good but it may take you a year or more to jack the sag out.

On the 2nd floor are you trying to get the floor flat or level? When you start trying to jack a 150 year old sag out of a floor there is a real risk of pushing the exterior walls out.

Flat is pretty easy and the trick is to not try to get the sag out at all. Since your joists are too small for the span anyway I'd sister new 2 X 10 joists next to the old ones then nail them together
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Offline bags

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Re: Best way to strengthen sagging floor joists
« Reply #8 on: March 13, 2018, 08:57:03 AM »
The existing 2 X 8's wont have hardly any carrying capacity since they have deflected (bowed) and will be hanging dead weight (in the center) after they are returned to level/straight. If your gonna use plywood in the repair, it would probably be best to build some TGI's and insert them along side the existing 2X's. A groove can be cut in the flange board with a router.

If you go with the TGI's--- make sure you have the room to get the full length beam up in position. If not--- they can be built along side the 2x's. Plywood plate the joints (if any) on the TGI and block out one side if your gonna attach the 2X to the TGI.

Online Hilltop366

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Re: Best way to strengthen sagging floor joists
« Reply #9 on: March 13, 2018, 09:55:25 AM »
If I remember correctly No.2 spruce/pine/fir (SPF) with a 16' span would require a 2x10 @ 12" oc or a 2x12 16" oc.

The last time I tried to fix a sagging floor in a old house by adding beams, post and jacking by the time I was done I was sorry I didn't tear out the old floor and start over with all new. Take a good look at the condition of the existing lumber before starting the work, poke at it with a awl or knife too, especially near the ends of the joist and the sill beams.


Offline newoodguy78

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Re: Best way to strengthen sagging floor joists
« Reply #10 on: March 13, 2018, 10:36:33 AM »
Good advice hilltop, starting from scratch is sometimes the best option 

Offline JJ

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Re: Best way to strengthen sagging floor joists
« Reply #11 on: March 13, 2018, 10:40:42 AM »
Had a similar problem I fixed by studding a load bearing wall in the basement, with framed in doorway, 1/2 way across the floor joist span.   Made the floor solid, and looked like it was meant to be there.   also gave me a place for more shelfs.

       JJ

Offline Bruno of NH

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Re: Best way to strengthen sagging floor joists
« Reply #12 on: March 13, 2018, 12:00:18 PM »
I would add a joist on one side.
Plywood 3/4" fir ,with glue and nails.
Then solid blocking every 64".
Then on the ceiling side strapping 16" oc to attach the ceiling material to.
This would be for the 2nd floor.
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Offline Wudman

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Re: Best way to strengthen sagging floor joists
« Reply #13 on: March 13, 2018, 05:21:20 PM »
I'll throw out one more option....LVLs (Laminated veneer lumber)  It may be a little more costly than you want to go, but you can sister to your existing joists and it will hold its shape.  I have a trio of 16" ones spanning my living room (22 feet) and taking the second floor load.  There is no spring in the second floor.  Very satisfied.

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Offline DR_Buck

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Re: Best way to strengthen sagging floor joists
« Reply #14 on: March 13, 2018, 07:25:48 PM »
Thanks for the ideas.   Keep them coming.  ;D

I added some additional details to the bottom of my original post at the top of this thread.
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