Floor Joist Blocking

Blocking is the least effective in reducing joist flex. Get some engineered floor joists and make the joist thicker.

Offset is fine.

However, check also the joist span, depth, and spacing. If span > ~17 x depth, that could be a concern.

Thanks a bunch. Is the 17 x depth figure in inches? I have a feeling the span is fine (room is about 10x14) but I'll check it.

It doesn't matter what units, so long as they are the same for measuring depth and span. Knowing what kind of wood it is would make a difference, but relatively minor. I estimated the 17 from a quick look at a table of allowable joist spans.

I appreciate the thoughtful response. I will try the string idea. I've looked into sistering a bit but wanted to make that my last resort, as each joist has a multitude of wires or gas lines or heating duct attached.

There is a 2x10 that seems to run the whole length of the bouncy floor section. It runs into a concrete wall at the top of the trapdoor used to access the cellar, and someone cut off the bottom 3/4 of it for about 12" of the end of the joist to make headroom for the trapdoor. The CEO recommended bracing around this area to compensate for that.

The joists are different sizes and spans, ranging from 6" depth to 10", but the areas around the 10" ones seem more problematic. One is sistered along most of its length. The wood is undoubtedly old but I took a look at it with the structural inspector and didn't see any rot or cracks aside from some short hairline ones. (Although he's one of these old-house guys who'll say things like "Well, mine looks like that too...you're probably fine.") Pulling laterally on one of them moves it ever so slightly--less evident to the eyes but more from hearing the heating ducts clink when I try and move it.

The other thought I had with all this is that the flooring might've just warped up from the subfloor, because the bouncing is caused even when my 18 lb. dachshund runs across the room. From what I've read blocking would stiffen the floor to improve that problem, but not necessarily strengthen the floor itself.

"and someone cut off the bottom 3/4 of it for about 12" of the end of the joist to make headroom for the trapdoor."

BINGO! There's the problem, the bottom of the joist was cut away. I'm assuming a 3/4" cut into the 2x10 for about 12", not cutting away three quarters of the joist (which would leave about a 2 inch thick section twelve inches long).

"The CEO recommended bracing around this area to compensate for that."

Yup, that's the answer, you found the weak point, you just need to reinforce it. Sister that section if you can, personally, I'd run a second sisterboard out next to the firat if the first sisterboard needs to be notched for the trapdoor too. If you can't sister a new joist in, then I'd recommend adding a new support column on the other side of the notched section, to help take the strain off the weakened section. Put the new column in as close to the weakened section as feasible. It may not look pretty, but this sounds like an unfinished basement in a Colonial Era House, I doubt there's much more done down there than storage and possibly having a small workshop/workbench for home repairs.

That's what I'm thinking. And no, they did hack off the bottom maybe 6-7" of the joist. My cellar is actually a glorified crawlspace with ~5' clearance -- I can't even stand up straight down there.

"That's what I'm thinking. And no, they did hack off the bottom maybe 6-7" of the joist. "

They effectively reduced that 2x10 to a 2x4. I can only hope that the guys who did that are already dead, as slapping enough sense into them after such a bonehead maneuver would likely end up with their head separating from the neck. (not that they were using their heads for much more than hatracks anyway, but the authorities tend to dislike a person after they've slapped someone else's head clean off.)

Well, the Scream settles it. I'm going to look into sistering the whole length this weekend, probably including the hacked portion. I'm thin enough to squeeze down between a 2x10 and a vent pipe.

As a temporary test, support the cut beam with a jack post against the floor. If that stops the bounce you have the problem identified.

Taking a 2x10 joist and cutting the bottom 3/4 of it away is the equivalent to having a 2x3 or 2x4 joist in that area. They are used for walls not floors. Not very strong for that location next to the trap door. Sistering that joist and also likely the adjacent ones and framing out the trap door is a good starting point for the solution. That will most likely be an effective solution. Unless you utilize an engineer specializing in this type of construction you need to start with the most logical correction technique.

Good Luck, Old Salt

Two possibilities come to mind.

1. Fit an extra stump or two and an extra bearer to shorten the joist spans.

2. Don't want to go that route, then consider a Barap strut.

Normally used for strengthening roof members but they work equally well for floors.

They may go by a different name where you live but are basically a heavy steel cable/rod with adjustable ends and brackets which are fixed to alternate ends of the offending beam/joist and with a fulcrum in the centre. Tightening the adjusters shortens the cable and tensions the beam/joist.

You are likely to have more than one floor joist causing your problem, so an extra bearer can be hung across under the centre of, and at right angles to, those joists and then stressed with a Barap strut or two to the desired stiffness, thus fixing multiple joists at once. If you go this route, you need to prevent the bearer from rotating when tightening the strut. Generally a small diagonal brace or block from the bearer to one joist adjacent to the fulcrum on the tightening side will accomplish this.

This may be your easiest and cheapest option as it normally doesn't involve upsetting existing plumbing or electrical work, and is far easier to install than extra joists.

With a house of 130 year age, it is likely that the existance of building codes at the time of construction just wasn't there. My house is 145 years old and the joists are solid wood construction but spliced. I know I will have to "sister" each one as part of my "reconstruction". I also had to install fire blocks in the walls as the house was balloon framed.

Oh the joys of owing an old house☻¿

And, what electricians and plumbers do to install their stuff is usually the easiest way for them, that means a lot of hacked out areas which make the structural integrity of the remaining joists suspect.

Maybe the joists are fine, but the flooring has warped and lifted up in places. In that case, a fairly light load could push the flooring back down. If that is happening, you could glue/screw it in the down position.

I would locate the area that's giving by having someone walk back and forth across the area while watching from below....Once you locate the problem area, take the weight off the floor, wedge a piece of wood up against the floor and screw it to the joist...repeat until all give spots are supported....

....or you could replace the floor....

You can also stiffen the joists by adding a thick wide plywood plate to the bottom edge, say 1x4 - to make it look like an I beam. Totally boxing the bottom side will provide even more stiffening.

But at 130 years, I would be looking hard at the actual floor boards working up and down on the existing joists - shrinkage, warping, etc. Take a good look at what moves when your heaviest friend traverses the floor above.

An alternative to wood blocking is metal 'x' bracing, if the joists are on standard centers.

Why not support the bounciest part of the floor with an adjustable column jack? This will let you know the magnitude of the problem. If it's not in a objectionable spot, you can leave it in place.

Appreciate the responses to this question.

Long story short, I'm now working with a structural engineer, who has identified lots of additional issues and used the word "scary" a few too many times for my liking. Joists and beams randomly sawed off several inches from any wall, insect damage...the whole bag. Quite glad I didn't just sister everything.

Thanks again.

GA!

Follow up pictures and progress reports will be welcome!

Read 'em 'n weep. Here's the beam to nowhere and the joists sawed off / cut two feet before the wall. Plans are done and work's coming soon.

WOW!

Keep the pic's coming!

And the finished work. Very pleased with these guys...I'm going to have them back to address the *other* sagging floor we haven't tackled yet. On the bright side, no more table rocking! The old beam now sitting on the two short/one long posts was supported on one end by a rotting "post" (I think it's a 2x6), nothing on the other end (cut off an inch before the wall), and a small chunk of wood at a 15 degree angle in the middle.

Those are good solid footings! It looks like you could take a foot of water down there without wetting the post.

Watch your head - especially if you have a hard hat on- the head room has come down.

I appreciate the follow up!

Thanks for the heads-up. The clearance to most of the joists was only about 5' anyway, so I'm used to stooping down there. As our (_b*st*rd) home inspector told me: "You've got a true cellar here--don't ever call it a basement."

To be honest we've taken a foot+ of water on more than one occasion--terrible drainage. But yeah, no more rot!

That's some good solid work, don't forget to send those guys a thank you card, and possibly a 6-pack if that's the tradition in your area.