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LMHmedchem 03-25-2012 02:09 PM

replacing broken floor joists
 
I have two joists that are cracked and two others that also appear to have some splitting on the bottom edge. These four joists are consecutive, so I think that is more of a problem. There is a full basement, so access is not too bad.

I would like to know how much of a problem this is and if I need to do anything about it. The house is 120 years old and the dip in the floor above the damaged joists has not changed in 30 years. The cracks do not appear new, or to be getting worse. The structure appears to be a bit under framed. The joists span a bit more than 13" and are 7.5"x2" on 16" centers. One of the joists just looks like a bad piece of wood. It is full of knots and there are cracks that run through two of them. One of them has a horizontal split that runs a long way through the length of the board.

The worst board is surrounded by gas pipe and heating water pipe, so there really isn't allot of room for a sister. It might be easier to replace that one if necessary. There are allot of things I could do, from sisters, to plywood plates, to replacement, and I don't know which, if any, are necessary or useful. I could always throw on some 6' plates with adhesive and nails/screws over each compromised ares. I don't know if that helps since it adds weight without transferring any of the load to the beams. I could plate one entire side of each joist and add a wide hanger. This area is the first 4 joists from the back of the house, so there is a corner on one side and the main beam on the other. I would guess this is a strong part of the structure, so it may matter less if the joists aren't in perfect shape.

Suggestions would be appreciated, and please let me know if I need to provide additional information.

LMHmedchem

ddawg16 03-25-2012 04:03 PM

Replacing is good....and not as hard as it sometimes seems....

I had to replace a 10x14 section of flooring in my house...2x8's....it was harder getting out the old joists than putting in the new one.

It would sound as if you need to support part of the floor first....

Then cut the joist....remove both ends....and insert new joist. Move on to next joist....

When I did mine....I just put them in at tilted to one side then used a BFH to knock them up into position.

If it was me...I would try to figure out why they broke. Was it just age or is there some stress in this spot?

The condition of the walls in the area might be a clue.

LMHmedchem 03-25-2012 04:17 PM

Quote:

Originally Posted by ddawg16 (Post 885198)
Replacing is good....and not as hard as it sometimes seems....

I had to replace a 10x14 section of flooring in my house...2x8's....it was harder getting out the old joists than putting in the new one.

It would sound as if you need to support part of the floor first....

Then cut the joist....remove both ends....and insert new joist. Move on to next joist....

When I did mine....I just put them in at tilted to one side then used a BFH to knock them up into position.

If it was me...I would try to figure out why they broke. Was it just age or is there some stress in this spot?

The condition of the walls in the area might be a clue.

Thanks for the advice. My impression is that the cracks are at least 50 years old. It's an odd part of the structure for problems to have occurred, being so close to the foundation. I think that part of the problem is that the 4th joist out from the foundation was just bad wood. If that one went, the other joists close by may have weakened over time. I'm also not sure the 7.5x2 is enough for a 14' span, especially for 120 years of use.

These joists are nor hung, but notched and sit on the beams. The bottom of some of then do no touch the beam, so it's hard to see how the entire height of the joist is carrying load. I will try to post some pictures.

To remove an old joist, I would probably slow jack that area first to level the floor above. I think 1/8" per day is the prescribed maximum. When I have the floor level, I would add extra bracing and remove one joist by cutting it with a circular saw and running a sawsall along the top to cut the nails. Then I would put in a new joist. I would probably cut the old joist into 4" sections that would be easier to manage.

Does this sound right? I don't think I can sister in a joist without leveling the floor first, but I could probably do plywood plates. I would still have to notch the joists and sit them on the beam, but I would add hangers as well, or a 2x4 underneath.

What would I look for in the walls?

LMHmedchem

LMHmedchem 03-25-2012 04:45 PM

3 Attachment(s)
Here are some pictures,

Here is the most badly broken joist. There is another split this bad at the other end of the joist where there are more knots.
Attachment 48121

This is a long horizontal split. I don't know if it's deep enough to be a concern.
Attachment 48122

This shows how the joists are anchored. You can see that there is quite a gap at the bottom of the notch.
Attachment 48123

LMHmedchem

Daniel Holzman 03-25-2012 04:58 PM

Based on the photos, the joists cracked due to the knots. As a point of interest, wood exhibits creep, which is a very slow, continuous deformation (sag) under load. This does NOT weaken the joist, it simply means that a very old joists is likely to sag approximately twice as much as the joist did when the load was initially applied. The initial load causes the joist to elastically deflect, meaning if you remove the load, the joist returns to original position. Once the joist has been in place for several years, the creep is non-elastic, meaning the joist will not return to original position if the load is removed.

Wood is unusual in that it exhibits creep under low load conditions, which explains why old houses typically show sag in the middle of the floor, and this sag does not disappear even if the load is removed. Jacking an old joist back to original position is trickier than it sounds, it is possible to crack the joist if you go too fast, and reversing long term creep is difficult. If the rest of the joists are no worse looking than that, I would consider leaving the joists the way they are. You can run an analysis to see if the joists are undersized, but it is often difficult to estimate the strength of older joists, since the wood available 120 years ago was often substantially stronger than currently available structural lumber.

If you are going to remove the joists and install new ones, do the computations first to make sure you can get adequate strength from commercially available dimensional lumber. You may want to consider I joists or other engineered lumber.

joed 03-25-2012 05:19 PM

Looks like you need some wiring repairs as well. I see several cable that look like extension cords stapled up to the joists and one open ended conduit with wires hanging out.

bob22 03-25-2012 05:30 PM

From the last picture, it looks like about 1/2 to 2/3 of the joist is resting on the plate so you don't really have a full 2x8 joist, more like a 2x6 or so.
Might need to figure a better way of supporting the full joist height at the plate; might need to build out where they would attach.
Hard to tell from the pics.

LMHmedchem 03-25-2012 06:18 PM

Quote:

Originally Posted by joed (Post 885246)
Looks like you need some wiring repairs as well. I see several cable that look like extension cords stapled up to the joists and one open ended conduit with wires hanging out.

The wiring is in good shape overall. Allot of what you see in the pictures is coax and phone wire, some of which is dead. There is even some old doorbell and other light gauge dead stuff I can't identify. I need to clean some of that up, but I am having an electrician add a sub-panel, so I will ask him to to that at the same time. I don't like trying to pull stuff out when I don't know what it is. There is a plastic conduit that comes through the foundation with the cable and phone wires. It only sticks out about 8" and then the wires run out of the conduit. I have some lights disconnected because I am going to move them, but the rest is up to code, no extension cords.

Quote:

Originally Posted by bob22 (Post 885254)
From the last picture, it looks like about 1/2 to 2/3 of the joist is resting on the plate so you don't really have a full 2x8 joist, more like a 2x6 or so.

This is one of the concerns I have. For some of the joists, the bottom edge is sitting on the granite, so those are fully supported. There are others like those in the picture where the bottom is not supported. I think I could pretty easily just bang on a hanger for all of those if that would help.

Quote:

Originally Posted by Daniel Holzman (Post 885228)
If the rest of the joists are no worse looking than that, I would consider leaving the joists the way they are.

Thank you for the information, I am leaning in this direction. Part of the issue is that the worst joist is surrounded by plumbing, so it will not be an easy job to sister or replace it. It seems like it would be difficult to sister a sagging joist without jacking it. You would be trying to scab a straight board onto a bent one and I would guess that in order to get it in place, you would have to push it up so hard that you would be jacking one way or another. If I try to replace that joist, I will jack the sagging area (5 joists or so) over a 1 month period to try to minimize damage to the walls and ceiling, etc.

Would there be a structural benefit to putting 3/4" 5 ply (or higher) on one side of a joist with adhesive and 20 penny nails? If I ran the full length and added a wide hanger, would that add meaningful support to the structure? The only thing I am worried about is parties and such when there might be 20 people walking around. I suspect that if it hasn't gone anywhere in 120 years, it isn't going anywhere now, especially since there is no sign of rot, moisture, or insect damage.

Now is the time though, since I will be finishing renovations to that area next.

LMHmedchem

Daniel Holzman 03-25-2012 07:23 PM

The problem you run into with sistering or laminating onto an existing old joist that has a permanent sag is that the new board you want to put on is of course straight, hence does not follow the curve of the floor above. If you want to support the floor, then you need to scribe the new joist to match the floor before you install it. This is tough sledding, I have never seen it done.

What is typically done is one of three techniques:

1. Remove the old joist completely, replace with a properly sized joist to handle the full load. You have to jack the floor back to level to do this right.

2. Sister on a new joist without jacking the floor back to level. You simply nail (sometimes nail and glue) the new joist to the old one. The floor is left with a dip in the middle. You only do this if the old joist is structurally inadequate, which can happen if the load increases on the floor above, or the old joist was always structurally inadequate.

3. Jack the old joist back to level, very slowly and carefully. Once level, release the jack, sister on the new joist. This can work if you have time, patience and a little luck. You typically have to overjack the joist by perhaps 1/4 inch in the middle, since as soon as you release the jack the joist is going to deflect downward.

You can certainly add plywood on one side or both sides, but unless you jack the floor back to level, all you get out of this is a stronger, stiffer, but still curved, joist. Jacking the floor back to level, then adding plywood on the outside, is tricky because you need adequate horizontal shear between the joist and the plywood, else you may end up with a failed connection between the joist and the plywood. This type of failure can ruin your day.

It all comes down to a very careful analysis of what you want to achieve. Do you want a stronger joist? Stiffer joist? Flatter floor? Different solutions depending on the goal. Just as a note, I have inspected many houses with this exact problem, and if the joists are structurally adequate and stiff enough to support the type of flooring the owners want, I generally recommend living with a curved floor. It adds character to the house.

Gary in WA 03-25-2012 07:48 PM

R502.8 Drilling and notching. Structural floor members shall not be cut, bored or notched in excess of the limitations specified in this section. See Figure R502.8.




http://publicecodes.citation.com/ico...7401403358.jpg
From: http://publicecodes.citation.com/ico...002_par027.htm

Gary

LMHmedchem 03-25-2012 09:06 PM

Quote:

Originally Posted by GBR in WA (Post 885340)
R502.8 Drilling and notching. Structural floor members shall not be cut, bored or notched in excess of the limitations specified in this section. See Figure R502.8.

The notches in the joists are ~2", which is slightly over the prescribed 1/4 limit since these are 7.5"-7.75" joists. Do you think the fact that the bottom of the joist is not anchored (I can put my hand between the joist and the beam) is a problem?

Quote:

Originally Posted by Daniel Holzman (Post 885328)
TIt all comes down to a very careful analysis of what you want to achieve. Do you want a stronger joist? Stiffer joist? Flatter floor? Different solutions depending on the goal.

This is what I am churning over. My first priority is to insure the the structure is safe. If it's not safe, then I need to correct that. If it is safe, then I need to decide if I want to correct the sloped floor, leave the floor and try to shore things up some, or just leave it.

I believe that the structure is probably safe. If there was a consensus that it wasn't, I would probably look and correcting the issue with the floor at the same time I reenforced the structure, since I think I would have to do that to replace the joists.

If I want to do anything more than that, I would want the joist structure to be stronger and not worry about the slope. It doesn't creak overly much. I guess plywood plates and joist hangers would be the best way to add some support without having to tear into things too much. Would I have to scribe the plywood plates very much? That kind of thing is not too hard to do with plywood, but I don't see anyway around either scribing or jacking, and scribing is less involved. I don't suspect I will do any harm by adding some plywood and hangers.

LMHmedchem

Gary in WA 03-27-2012 12:27 AM

"The notches in the joists are ~2", which is slightly over the prescribed 1/4 limit since these are 7.5"-7.75" joists. Do you think the fact that the bottom of the joist is not anchored (I can put my hand between the joist and the beam) is a problem?"----- install some shims (plastic, not wood).

Gary

LMHmedchem 03-30-2012 05:58 PM

I have a friend who is an engineer and was a contractor for a long time and I was able to get him to come over and have a look. The worst joist has a crack big enough that you can see through it, so it really isn't doing anything anymore and he agreed that it needed to be repaired. The other joists are not so bad and can be left. He made an interesting suggestion to get rid of the dip in the floor. He said to support the sagging joist and then gently pry the floor boards up off of the joist with a combination of a jack and pry bar. Raise the floor boards back up until the floor is level, shim the old joist, and then add the sister or plating and make that flush with the leveled floor. I'm not really interested in leveling the floor enough to do all of that, but I thought it was an interesting suggestion.

What I am going to do is to reinforce the broken joist with 3/4" plywood plates down one or both sides, depending on what I have room for around the plumbing. I'm going to use PL 400 adhesive, 2" deck screws, and 8D framing nails. I also have some 4x4 hangers to add so that the full height of the beam will be supported at the ends.

Comments or suggestions?

LMHmedchem

joed 03-30-2012 06:41 PM

Quote:

He said to support the sagging joist and then gently pry the floor boards up off of the joist with a combination of a jack and pry bar. Raise the floor boards back up until the floor is level, shim the old joist,
Sounds like a lot of work when you can just jack the whole joist back up and sister to it. Why would you want release the floor boards from the joist.

LMHmedchem 03-30-2012 06:57 PM

Quote:

Originally Posted by joed (Post 888688)
Sounds like a lot of work when you can just jack the whole joist back up and sister to it. Why would you want release the floor boards from the joist.

His oppinion was that a 120 year old dry joist would be permanently displaced and very hard to jack back to level. It would also fight the new member in trying to return to its displaced position. I think that there is also less chance of cracking walls and ceilings when leaving the old joist in position.

I've never done this, so I can't really comment on how hard it is, or effective, I just thought I would post it to see if anyone has experience with this or a different opinion.

LMHmedchem


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