Floor Joist size/spacing issues

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Old 04-30-13, 07:59 AM
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Floor Joist size/spacing issues

I've mentioned this before, but now I'm looking to actually resolve the issues and have a bit more info.

As previously mentioned, my joists are 24" center to center in the floor of each level of my house (three floors). The main floor is the most noticeable as it's spongy.
What I didn't realize, is the joists are (true) 2"x8" and not 2"x10" as I had thought. They are nailed in place with the original square nails from back in the 1930's.
The joists in the basement at least, sit on top of the concrete walls and on top of the main beams (2 wooden beams consisting of 6 2"x12" nailed together).
The basement is unfinished, so it's fully exposed to work.

So... recap of issues I'm looking at;
- True 2"x8" joists
- 24" center to center
- modern lumber is not exact 2"x8"
- Joist spans are tops 12' (probably 10', need to confirm).

My original plan was to install joists between the existing, resulting in a 12" center to center, but the modern lumber not being 8" tall will add a bit of a complication.

Suggestions?
As always, budget is the driving limits, not time.
 
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Old 04-30-13, 08:12 AM
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As you now know 2 X 8's are undersized for that span.
Not sure why a narrower floor joist would be an issue.
I would sister new joist to the old ones. If the floor is already sagging in the middle you can use what's called old work joist hangers, there wider.
One way I've done it is to lift the new joist up into place and just use one nail on each end to hold it up about 12" back from the ends. Then install the hangers.
Once there all up I use a doubled up beam made out of 2 X 8's in the middle of the room to lift the center of the floor to get it back to level and through bolt the two joist together using 1/2 carrage bolts.
It's easyer to predrill the holes in the new joist before lifting into place.
 
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Old 04-30-13, 08:55 AM
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joecaption1,
You're thinking sister the joists (addressing the 2x8 issue) and disregard the 24" center to center issue?

I would think adding another 2x8 between the existing joists (making it 12" center to center) would compensate for the 2x8. Going this direction however leaves me with the short fall of modern 2x8s not being true 8" tall.
 
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Old 04-30-13, 10:41 AM
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This is going to sound like overkill, but that is unfortunately the case sometimes when dealing with a situation such as this. Anyway, sight unseen, I believe that the first thing that I would do would be to sister the orginal joists, as Joe described; that is the only way to take the sag out of them, aside from jacking and permanently blocking them at midspan. Once they are as they should be, the next step would be to install additional joists, 12" OC, as you initially mentioned. As for the joists being shorter than the space between the sill and the floor above, again, sight unseen, I am picturing laying probably Advantech on the sill in each bay, and notching the new joists the little bit that would be required for a good fit. After sistering the existing joists though, and before installing any new joists in between, I would spend some time on the main floor, seeing if any additional work is warranted. On older homes such as this, I have seen floor boards that I would have no qualms spanning the 24" that you have, so you may find that all of the sponginess is in the joists, and not in the floor boards. Just 2 cents worth.
 
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Old 04-30-13, 11:10 AM
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It's interesting the different approaches to the same issue.
I have not confirmed if any of the joists have measurable sag in them. It would be assumed there is some at 2x8 old lumber (24" OC) but the rooms above have their walls alligned with the main beams or outside walls.

I'm not sure if the following pictures will help. They don't clearly show the joists above, but do show the main beams and support pillers. The two main beams run down the middle of the house, with the hallways on the main and second floor lining up with the beams (hallway walls are support walls inline with the beams).
The pictures are looking North, going from the west side to east.
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Old 04-30-13, 11:49 AM
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Sorry, and no, I went back to look, but you never mentioned sagging, so I only assumed that. That's easily enough determined though by stringing a line across along side the bottoms of them. Hard to say until you do that, but my assumption would be that, regardless of whether or not it is enough to matter, at least structurally speaking, the original joists probably sag some, giving them what I refer to as a negative crown, and any new joists that you would install in between would have what I refer to as a positive crown. So say the old joists are down a little bit in the center, maybe 1/8", and the new ones, only 12" inches away, are up a little bit in the center, maybe 1/16" once loaded, and so on across the floor, it seems to me that you could end up with a wavy floor, albeit presumably stronger, and less spongy. That said, if they do in fact turn out to be relatively straight, I might consider installing a row of solid blocking down the center, or as close as you can get with the fewest obstructions, and see how that affects the feel of the floor.
 
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Old 04-30-13, 04:23 PM
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Well, aka pedro, I defy you to install a positive crowned joist 12" from 2 negatively crowned ones. Its near impossible - friction alone will prevent this. Be it friction from the new lumber dragging on the subfloor or the trying to bend the subfloor up with nails holding it down. It is challenging enough to sister a joist, let alone try to level a floor by beating a new piece of lumber into place. You also have to deal with the fact that the lumber has square edges to it. Thus the hypotenuse of the cross section is longer than the actual width of the board. Kind of like putting 10 lbs into a 5lb sack. Its very difficult to wedge the new into place.

When sistering, I cut the top of the joist buy 5 to 10 degrees to allow for the hypotenuse to fit past the subfloor to get it vertical next to the original. It is changing the original 2x8 to a 4x8 that will add strength to the floor. You can also add additional layers on the topside to shore up a nice stiff floor.

Northern - a table saw will get you an exact 8" wide joists. You will just need a buddy to help feed it through. Another obstacle is all the obstructions (pipes, electrical, cross bracing, etc) that are already in the way.
 
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Old 05-01-13, 05:22 AM
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a table saw will get you an exact 8" wide joists
My house is a mixture of saw mill and store bought lumber - a table saw works well for adjusting size

A few years ago I inspected a house that a friend of one of the kids was looking to buy. I was shocked that the floor joists were true 2x6s on 2' centers. I was equally shocked at how stable the floors seemed to be.
 
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Old 05-01-13, 07:10 AM
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Northern - a table saw will get you an exact 8" wide joists. You will just need a buddy to help feed it through. Another obstacle is all the obstructions (pipes, electrical, cross bracing, etc) that are already in the way.
My table saw and I don't get along very well. Our relationship started off great, but it didn't like something I said, and tossed an 4x8 sheet of 1" MDF at me the last time we talked....

All jokes aside, I didn't think about using rough cut (lumber mill) wood. There are a couple mills near my home. I'm sure I can get 2"x8" exact through them and trim as needed.

As for the other obstacle.... I'll have to remove the electrical as I go but do not have much in the way of pipe work to worry about. Someone did not take the time to run the pipes nice and tucked in/through the joists. You'll notice in the pictures I posted, the copper heating and dommestic water pipes run all over (where ever it was convinenent for the installer I guess). The wiring you see particularly in the last picture has been already addressed. Most of it was poorly run phone cables and coax. I've been cleaning up old wiring for a few weeks now.

Anyway... The general consensis is to sister the existing joists then address the possible new joists (12"OC) if the sponginess doesn't go away?
 
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Old 05-01-13, 02:55 PM
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True 2" x 8" lumber may be strong enough om 24" centers for the support the floors above need. They're certainly much stronger than modern nominal 2 x 8s.

How are the existing joists braced? Using modern 2x8 lumber to add cross-bracing may be all you need to gain the stiffnrss you'd like to have.
 
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Old 05-01-13, 04:10 PM
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1. How far is the span, exactly- between supports?

2. What is the subfloor thickness? 1x8, 1x6 run diagonally, or perpendicular or tongue%groove, or ? Is the sponginess between joists when temp. supporting two mid-span with 1/2" longer than high 2x4's; to verify weather joist/subfloor problem...

Gary
 
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Old 05-02-13, 06:48 AM
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I apologies for the delay on these photos (much over due).

The first one shows how the joists are braced on the main beam. You'll notice they are sitting on top of the main beam, and attached to the next joist for about a foot. The joist it attaches to is only a short run which covers the hallway. The other side is connected in the same manner to the next joist covering the same span.
The longest span is 11ft.
The spans are ~11ft (west side) + 8ft (hallway) + 11ft (east side).

This first pic is the overlap on top the 1 main beam setup (there are 2 main beams).
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This second picture shows the common bracing between the joists. This is common across about 75% of the floor.
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This next picture is a close up of the floor boards. They appear to be 1" thick, by 6" wide. They run diagonal except for the bathroom on the main floor, which is it's own issue.
I believe they are tongue and grove.
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This last is an overview of the bathroom floor which has sag and is a mess. I'm for sure sistering the joists on either side of this room, and will run a joist on either side of the toilet drain pipe. Not sure what else I can/should do with it. The floor in this bathroom is being replaced (not sub-floor) in the up and coming reno work.
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Hope this helps give a better visual idea of what I am working with. I wanted to get a picture of the work done under the dining room which was tiled by a previous owner. What they did is ran a 2x8 between existing joists (now 12"OC) and sistered the new 2x8 with 1/2 plywood. The newer joists where shimmed with 1/4 plywood and cross braced with 2x8 pieces.
 
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Old 05-03-13, 09:34 PM
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Thanks for the clear pictures!

This first pic is the overlap on top the 1 main beam setup (there are 2 main beams).
Standard good practice. The overlap is less than I'm used to seeing, but with the dimensions of the the joists and the shortness of the spans, it's probably sufficient.

This second picture shows the common bracing between the joists. This is common across about 75% of the floor.
Excellent bracing. X-bracing, seldom done anymore, is, IMX, far superior to block bracing for eliminating springiness and providing stiffness. Is the "sponginess" above these areas or elsewhere?

This next picture is a close up of the floor boards. They appear to be 1" thick, by 6" wide. They run diagonal except for the bathroom on the main floor, which is it's own issue.
I believe they are tongue and grove.
If we're still looking up from the basement, and those are the 1x6s laid diagonally across the joists, then that's the subfloor, not the finish floor. What't on top of those; that is, what is the actual floor? If it's hardwood, is it laid perpendicular or parallel to the joists?

Mike, have you watched the framing from the basement while someone walked, skipped, danced, whatever, on those areas of the floor where you get the "spongy" feel, so that you can see whether you can spot anything moving? If you have, and you can't see any noticeable movement, then the problem may be that the finish flooring is not as securely attached to the subfloor and framing as it might be.
 
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Old 05-04-13, 01:14 PM
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"Standard good practice. The overlap is less than I'm used to seeing, but with the dimensions of the the joists and the shortness of the spans, it's probably sufficient." ------------- actually, you are required to have solid blocking between floor joists to prevent over-turning, per code, for (at least) the last 40 years; Chapter 5 - Floors
Perhaps you meant "standard (non-safe) practice before building codes"... lol.

"Excellent bracing. X-bracing, seldom done anymore, is, IMX, far superior to block bracing for eliminating springiness and providing stiffness."----------- it is inferior to solid blocking by 66%. Though you could add a 1x to the bottoms and increase it by 167%, Table 2; http://www.ewpa.com/Archive/2004/jun/Paper_278.pdf

Look for a lumber stamp on the sides of the joists, you may be border-line for maximum span, last table; Chapter 5 - Floors (I doubt you will find one, too old).

Gary
 
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Old 05-04-13, 10:27 PM
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"Standard good practice. The overlap is less than I'm used to seeing, but with the dimensions of the the joists and the shortness of the spans, it's probably sufficient." ------------- actually, you are required to have solid blocking between floor joists to prevent over-turning, per code, for (at least) the last 40 years; Chapter 5 - Floors
Perhaps you meant "standard (non-safe) practice before building codes"... lol.
No. I meant exactly what I said, and I wasn't talking about blocking. I was talking about the lapping of the joists.

"Excellent bracing. X-bracing, seldom done anymore, is, IMX, far superior to block bracing for eliminating springiness and providing stiffness."----------- it is inferior to solid blocking by 66%.
Gotta love book learning. And gotta wonder where they get their impressive-sounding "facts" sometimes.

A friend came over one day to give me a hand re-framing the porch on my old house. When we had the king joists and true joists in place and were ready to brace, he got ready to cut solid blocks for us. I argued that X-bracing was better, and he wouldn't buy it. To settle it, I braced the center section with X-bracing and he braced the west third with blocks, and we got up and danced around on the framing after we were done.

He helped me cut and install the X-bracing for the third section.
 
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Old 05-05-13, 08:31 AM
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Yep, from truss construction to bridge construction...scaffolding-free standing decks-roofing (Oh and look up at the roof of a big box store next time you are there)-cell towers-oil rigs-cranes-coastal homes...can't think of a single use for cross bracing.....(sarcasm off)

When remodeling, its tough to install cross bracing, however, if you have the floor open, then you have a choice as to which to install.
 
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Old 05-06-13, 05:50 AM
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My previous home (built early 1950's) was also X bracing.

Originally Posted by Nashkat1
If we're still looking up from the basement, and those are the 1x6s laid diagonally across the joists, then that's the subfloor, not the finish floor. What't on top of those; that is, what is the actual floor? If it's hardwood, is it laid perpendicular or parallel to the joists?

Mike, have you watched the framing from the basement while someone walked, skipped, danced, whatever, on those areas of the floor where you get the "spongy" feel, so that you can see whether you can spot anything moving? If you have, and you can't see any noticeable movement, then the problem may be that the finish flooring is not as securely attached to the subfloor and framing as it might be.
I believe it's that sub-floor, then the hardwood. The entire house (except attic room) is hardwood. Some areas have other flooring on top, but it's all original hardwood under it.
The hardwood runs horizontal (left to right across all the photos).

I have not had someone walk across the floor to while I watch from the basement. It's a simple test I should have thought about doing. This would easily nail down the issue to the existing joists or the distance between them.
 
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Old 05-06-13, 05:16 PM
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If that isn't the problem (space between subfloor/flooring), try checking the sponginess (between joists) with 2x4 props under two joists mid-span (as said in post #11). Otherwise, the span is questionable. Blocking/bridging helps distribute a concentrated load, doing nothing for over-all strength of the floor in additional maximum load.

Post your data comparing solid blocking to cross bracing and we can compare it... as with mine, otherwise it is simply an opinion... which you are certainly entitled to. Try to keep it in context, not walls, roof, towers, etc. I'm not arguing the structural aspect, just deflection/vibration in a floor system of the choices listed. Dis-prove it in that application, I'm open for discussion, show some facts... not just opinions.

Gary
 
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Old 05-07-13, 05:14 AM
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Being that I work for an engineering firm, I know what the cross bracing can and can't do. We don't do residential engineering, so scaling down the experience and knowledge of industrial work to home applications can be tougher without going overkill and breaking the bank.

I have heard from others that the cross bracing is prefered over blocking to reduce the chance of sqeaking. Block benefits from being easier to cut and install.

One thing that was done on the area that has 12" OC (under the tiled floor) was they blocked, but sistered that with a 1/2" thick piece of chip board.

I'll try to get someone of some weight to walk on the floor while I watch from the basement. It's hard to get the time with the wife to test, and my son who's willing to help is under 40lb. Will post the info.
 
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Old 05-07-13, 10:40 AM
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Yes, cross-bridging WITH a bottom joist tie is 60% stronger than solid blocking and 166% stronger than cross-bridging without the tie. In other words, c.b. with a tie to complete a triangle is way stronger than an open triangle. This is why there are bottom cords on trusses, ties on sides of towers, floor trusses, etc., one has to complete the triangle for maximum strength. Perhaps I didn't explain it simply enough for others as they jumped on me when simply showing you need the ceiling/1x4 for maximum strength. I can see where it was not explained fully and could confuse some, sorry.

Propping up two joists from below and walking down the middle of them from above is an easy test you can do alone. Even measuring the height to the ceiling above while standing between joists could tell you the amount of sag with a load, before and after propping them.
Many times, squeaking of floors is the c.b. rubbing against each other-at the cross joint, when installing, be sure to leave a gap, or run a hand saw between them afterward.

Gary
 
 

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