View Full Version : floor deflection

02-24-2007, 05:34 PM
since I have moved into my new home (25yrs old) 6months ago, I have noticed a lot of floor deflection in my family room upstairs. the floors were constructed from 2x8 at a 12-13 foot span on 16" centres. I have the joists sitting in my front yard waiting for the day I have enough guts to tackle this. My real problem with getting this done is that the main circuit breaker panel is located on the far side of the room and all the wires that run to the rest of the house have been run through the joists that need sistering! What can be done to this to make it easier? Can I cut the new joists at the level where the wires run and notch, then install the rest of the joist? Who has had this problem - there sure is a lot of wires routed through these joists and I can't have the house run without electric for more than a weekend as I have smaller children and a full freezer of meat. I know I can just wire up a receptical at the breaker panel and move the freezer but that still leaves the rest of the house without power. What do I do? Also the wood has been sitting outside for about a month because I had it delivered and the next day winter came! Snow and ice had done some warpage on the wood but no more than about 1" from tip to tip on the 16 foot pieces. They have been sitting on top of 2x4s perpendicular to the pile.


02-24-2007, 06:13 PM
Notching would destroy the strength of the joist.

Have you considered the possibility of supporting the floor with a central beam below, instead of sistering up all the joists?

You are getting into a project that is possibly a little beyond DIY, and you are getting carpenter advice from a bunch of old plumbers! Now, a lot of the folks on this forum have been around the block, and are pretty savvy! but you might need some on scene help on this one.

02-24-2007, 07:58 PM
You made no mention of bridging................

02-24-2007, 07:59 PM
can't put in a beam with a support underneath as that will destroy the overall view in the soon to be theatre in that location.

I will have some help - but my brother and friends are just as clueless as I am about this. Maybe I'll pull all the wiring out from one side and go from there! :eek:


02-24-2007, 08:01 PM
TedL... what do you mean by bridging? Is that running wood perpendicular to the joists? I want to keep as much headroom as possible as the joists are only 7'6" from the floor already.


02-24-2007, 08:06 PM
My dilemma was about the same as yours, and I solved it by pulling the wires and making some temporary reconnections during the sistering. However, that is not something one should do without knowing precisely what one is doing, and one should never have loose wires merely hanging from breakers.

02-24-2007, 08:11 PM
Bridging is bracing between joists. It helps prevent the joists from twisting and spreads point loads to the adjacent joists, but does nothing to improve the joists overall deflection ratings. If you had 1" of deflection before blocking, you'd have the same deflection after. To improve deflection, you need either thicker, taller, or stronger joists or to shorten their span. You could use steel plate, but that is a pain to get in place and bolt down. Make sure to have them punch holes if you go that route. drilling them is a major pain and time consuming. If you can't get the joists full length, you need them at least 2/3'rds of the middle to do anything useful.

02-24-2007, 09:20 PM
It does have bridging from when the house was built (I think) They are 2x2s at an X between the joists. I was going to replace them all with the extra 2x8s I have.

It looks like there really is no easy way to do this then... removing the wires seems to be about the only way :mad:

Any more suggestions out there?


02-25-2007, 07:08 AM
Removing the wires seems like a huge job depending on how many wires, and where they run. Wires running up in the wall to switches and outlets cannot be pulled down....they are stapled to the studs inside the wall.

You cannot cut the wires and put in concealed junction box splices. Code does not allow that. You could do it if you left the boxes accessible.

The floor has not fallen down, right? Is the deflection such an issue that it must be fixed, or could you just live with it?

02-25-2007, 10:14 AM
when my cat walks on the floor the whole thing moves! - and it ain't a 30 pound cat either! It pretty much is such an annoyance that it needs to be done. Someone walking by the tv and it rocks back and forth on its stand. I just can't let it go anymore. I took a little more of a look at all the wiring and think I could do it with some help from my brother and friends to get it done in a weekend. The wires going up to the second floor are run in an area that doesn't need the sisters. So I think i'm ok. To do it right I need to pull the wires... so i'm gonna just nail the boards with the wires on it and hope it goes all right! Just kidding, i'll pull the wires.


02-25-2007, 04:48 PM

You didn't say where the wiring goes through the joists. Close to an end? Not right in the center of the joist?

I'm no expert on wood, but I think the wood you left outdoors is now wet and will take a long time to dry and then it will warp after that for quite a while. Months and months of fun, or anguish, depending on how you view it.

I would use 8' long plywood (3/4" x 8" x 8') cut along the long grain (if there are an uneven number of ply's) and screw these to the joists, centered in the middle of the joist. As Jim (Jadnashua) pointed out, rigidifying the center 2/3rds of a joist is what prevents almost all of the deflection. This is true.

Based on what you described about a cat making the floor deflect, and a TV (probably backed up against a wall) moving too, then I think I would also buy a few long angle pieces of steel (1/8"thick, 10' or as long as will fit, and angled 8" x 2" or 3") and screw them into one side of each of the joists that are in the center of the room. I have found an easy way to screw through steel without even drilling a pilot hole first. I buy self-tapping screws. I have no problem drilling into soft steel, to make a hole, either. I would use a bit of both methods. I like diversifying the treatment.


02-25-2007, 05:18 PM
... 2x8 at a 12-13 foot span on 16" centres.

I had 2x6s on 16" centers spanning 11 feet, and I did not have anywhere near the amount of deflection you have described. And, sistering has produced a revived frame floor as solid as any new ones I have ever had or built for myself or anyone else. If you do this job correctly and avoid any and all mere patching, you should get nearly as pleasing results.

Be cautious and wise while dealing with your wires, be sure to put the crowns up and definitely use a jack to push your existing joists up to match the new ones before you nail them together well.

02-25-2007, 05:37 PM
the wiring is closer to the end of the joist than the centre - say 10"-15" from the end. 2/3rds of a new joist would be ok, but hows the shear strength of these joists (I don't know the species, but it is 25yrs old) I would be concerned with the old joists failing under the load of the extra weight, considering they move so much as it is anyway.

I'm also thinking that the subfloor is a little responsible for the movement of the floor. Between the joists it moves as well, but the bridging with the 2x8 will definately help out.

Genie... I realize you said to use plywood on the sides of the joists. It's just that I purchased the 2x8s already and would like to use them, do you think if I bring them inside and aclimatize them for a month it will help?


02-25-2007, 05:59 PM
shear strength is not a problem. If it were a problem, everyone would have said so decades ago, instead of developing the rule of thumb that says that sistering the large part in the middle is the essential thing to do.

how long to dry wood out? Maybe someone can tell you.

You could screw the new joists on one side and plywood on the other side. Don't you believe in overkill and doing it right? I am not the right person to tell you when your materials are OK to install. I believe in doing things right, buying better than the minimum acceptable materials, and not saying "ought to be good enough" at the beginning when I have big doubts. In the course of building, I can cut corners if need be, but I don't put that into my plan at the outset.


02-25-2007, 08:51 PM
since I started on the path of renovating my house I have only thought of how I could make the same outcome with less money. At the very least, doing it right. :D If it could be done better than that, then great - will do (if cash permits). So... I think some plywood on the other side of the joists would definately be the "better than that... (if cash permits)."

Does anyone know how long it takes for wood to dry?


02-25-2007, 10:12 PM
.... Does anyone know how long it takes for wood to dry?Carry the wood inside and get the drying started.

Later, someone will advise you on how to measure the humidity in wood.

The actual answer to your question is meaningless to your situation. Here it is: depending on how much humidity is now in the wood, and then depending on how much drying takes place in your house, which depends on your air R.H. and on your house ventilation and on how you stack the wood, the rate of drying will vary by many orders of magnitude.

Bring the wood in, and let air move around it.


Phil H2
02-25-2007, 11:43 PM
Plywood wouldn't be the best choice of material. The plies running perpendicular to the long axis of the joist will do little to prevent bending. A 3/4" thick plywood section will help about the same as a 3/8" (maybe 1/2") thick piece of solid lumber (assuming equivalent species).

02-26-2007, 06:02 AM
I recently moved from a house with the same floor structure as you describe...2x8s spanning 12-13 ft, and had nowhere near the movement you're describing. A 260 lb man walking didn't do what your cat does.

Before you go to all the effort, I would make sure that your solution addresses the real problem (strength of joists). I don't know what the other potential causes are (subfloor not attached properly?) Either get a pro to evaluate, or at least try putting a temp beam under the center of the span and see if that cures it.

As far as using the wet wood, my experience with treated lumber is that it warps and stiffens as it dries out. Extrapolating from that, you may be better able to make the sisters conform to the needed shape if they are still wet when you install them than if you let them dry out.

Good luck...I hope you have a nail gun, or a friend with one!

02-26-2007, 01:36 PM
As far as using the wet wood, my experience with treated lumber is that it warps and stiffens as it dries out. Extrapolating from that, you may be better able to make the sisters conform to the needed shape if they are still wet when you install them than if you let them dry out.

Your 2x8s being a bit damp is not going to be a source of any great trouble. And like Ted essentially said, that might even help make sure they can be fitted tightly against your existing joists.

Since they are going to come inside anyway, bring them inside now and stack them tightly together so they will not be so likely to warp all out of shape while drying and before you can get them up where they need to go.

As already said, plywood would not be a comparable substitute for the strengthening of your joists, and full-length steel plates would be heavy, difficult to handle and install as well as cost-prohibitive.

Your initial plan is what would be best, and while it can be both enjoyable and a learning experience to toss other ideas around, well, your best overall outcome is still going to come as a result of simply doubling your joists.

02-26-2007, 05:58 PM
thanks again guys... I think i'll just bring in the wood and stack them tight like you said. I'll also try a pole in the centre of the floor for now and see if it does what I want.

For a bunch of "old plumbers" you sure know what you're talking about. :D


02-26-2007, 08:34 PM
If the wires are near the top or bottom of the old joists, you might be able to jack the joists level and while the "support" is under them notch the old joists so the wires can be removed, (and possibly relocated to a point above the support wall or beam, so the new ones could be notched at the same point without sacrificing any strength), and not having to cut the wires.

02-27-2007, 05:53 PM
I was told not to notch any of the old joist for strength. And, besides they run about 2"-3" from the bottom of the joist. So I don't think it would be good to take out that much material. I'm pretty much just waiting for my wife to make some room in this one so we can "git 'r' done".

I appreciate all of your assistance... It helped me with the final decision.
It will be done with the 2x8s and lots of PL400, nailed with a pneumatic nail gun and 3" screws. The floor will be lifted with a floor jack to take out any slack in the joists (1"?) before any attaching of the new joists. If it will make them any stronger then I will add carriage bolts as well.


02-28-2007, 05:40 PM
... with the 2x8s and lots of PL400 ... pneumatic nail gun and 3" screws ... [maybe] carriage bolts as well.

I did my 2x6s by hand with 16d cc sinkers near the tops and bottoms about every 15"-18" or so, and I drove them through the softer (newer) wood into the harder. I would tend to think the screws are quite optional and the carriage bolts definitely unnecessary. But of course, neither would do any actual harm.