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Old 10-16-2013, 11:39 AM   #1
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Floor Joist Dry Rot Repair


I have a 2"x10"x15' floor joist that has some dry rot about 5' from one end. To repair this, I would like to sister on another 2"x10". Must I sister the total length of the joist (15 feet), or is 3, 4, or 5 feet past the damaged area sufficient?

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Old 10-16-2013, 12:04 PM   #2
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Floor Joist Dry Rot Repair


The new (sister) joist should go the full length; its ends should bear where the rotted joist bears, or held up by a hanger if the original was installed that way.

Here's why:
The rotting joist's strength has been compromised. You need to restore that strength. The bare minimum way to do that is to replace it with an equivalent joist. Full-length sistering is a one-up on that. You end up with something that's even stronger than what was originally there (the bad joist, unless it's rotted all the way through, still contributes strength).

If the new joist is shorter and simply fastened to the existing joist, then the only thing holding up that new joist is a bunch of fasteners. Thus, the net strength of the combination is actually less than that of the original joist (in original condition). This would not meet the objective of restoring the floor's strength.

Code requires joists to either bear on supports or be held up by hangers. Short-length sistering makes joists stiffer, but does not contribute to the floor's strength.

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Old 10-16-2013, 01:51 PM   #3
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Floor Joist Dry Rot Repair


As per Cortell, and also make sure you eradicate the cause of the dry rot first (ie check for penetrating damp or condensation, and ensure adequate ventilation of the affected area).
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Old 10-17-2013, 12:47 PM   #4
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Floor Joist Dry Rot Repair


Looking at the sub-floor above the joists, I see no evidence of water as the cause. Possible the 2x10s were not dry (exterior and interior) when nailed together. Dry rot is where the boards meet in the center. No rot on the sides.

The other point is the 2x10s are above a non-load bearing framed partition wall. Also, the sub-flooring is 2x6 pine tongue and groove board topped with plywood. I suspect that since the partition wall is not bearing load and the sub-floor is so thick, there is little chance of 2x10s failing (right now).

I'm sure I would need an engineering sign-off to validate.
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Old 11-07-2013, 07:00 AM   #5
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Floor Joist Dry Rot Repair


I am continuing to explore methods to correct this problem. Another issue has developed. Water lines, waste lines, and electrical.

On either side of the double 2x10x20 there is electrical and plumbing that go up through the floor (crawlspace) into the partition wall. So a full length sister is not possible. I could sister short sections of 2x10 to bridge around the electrical and plumbing, then install a full length 2x10 next to the short sections. Would that work?

To another question. When constructing a floor joist system over a partition wall, must the joist/s be directly under the wall above? I'm trying to understand how this is done in new construction when plumbing and electrical go into the wall. Double 2x10 is 3" wide. 2x4 framed wall is 3-1/2" wide. If the wall must go directly over the joists, how do you get the plumbing and electrical up into the wall?
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Old 11-07-2013, 08:25 AM   #6
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Floor Joist Dry Rot Repair


If it's a real "wet" wall (drain, supply) It's best to build a 2 X 6 wall not 2 X 4.
The wall can be built along side of the joist not directly on top of it.
That joist did not rot out because it was not dry enough when it went on. Many years ago all homes where built with green lumber and there's plenty of 100 plus year old house with no rot.
FYI, there is no such thing as "dry rot" Dry wood does not rot.
If it's the end of the joist that's rotting some of the causes can be the house built to close to the ground.
High moisture content in the crawl space or basement.
Uninsulated rim joist.
And the 2, biggest most common ones I see is someone built a stoop, deck, porch right up against the side of the house without taking the proper precautions in flashing the wall first.
And built them right up even with the doors threshold. When that's done there's about a 99% chance there's going to be moisture damage.
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Old 11-07-2013, 09:36 AM   #7
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Floor Joist Dry Rot Repair


You are correct Joe. I incorrectly wrote that the wall was 2x4. It is 2x6.

House is on a crawlspace. When I moved in, there was no moisture barrier on the ground. That was corrected as well as having all the joists, plumbing, ducting and whatever else is under there cleaned of any growth and treated. The only joist with an issue was the double 2x10.

The majority of the damage is not at the rim joist but near the center beam that runs down the middle of the crawlspace. Maybe the ground moisture was higher there.

Thanks for confirming my guess that the partition wall does not have to be directly above the joist when pipes and electrical is involved.

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Old 11-07-2013, 01:27 PM   #8
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Quote:
Originally Posted by Earnie View Post
On either side of the double 2x10x20 there is electrical and plumbing that go up through the floor (crawlspace) into the partition wall. So a full length sister is not possible. I could sister short sections of 2x10 to bridge around the electrical and plumbing, then install a full length 2x10 next to the short sections. Would that work?
I strongly advise against it. If there is failure in the utility lines, where those lines are tightly sandwiched in between the two joists, you'll have a hell of a time getting to it. Also, you're sure none of those utilities require more than 1.5"? All it takes is one 2" DWV line to render the idea undoable.

Also, why not just forget about sistering and run a full length joist on each side of the rotting one, unattached? You could leave as much distance as you want between the new and rotting joist, as long as the two new joists are spaced no more than 16"/24" OC (whatever your current span is). That would be a suitable fix, structurally speaking. If there's a point load above the rotting joist (e.g., a very heavy piece of furniture), cut the rotting joist out and install blocking as necessary.

Quote:
Originally Posted by Earnie View Post
To another question. When constructing a floor joist system over a partition wall, must the joist/s be directly under the wall above? I'm trying to understand how this is done in new construction when plumbing and electrical go into the wall. Double 2x10 is 3" wide. 2x4 framed wall is 3-1/2" wide. If the wall must go directly over the joists, how do you get the plumbing and electrical up into the wall?
You've confused me here. Your first sentence says "joists over wall", then ends with "joists under wall". Then you refer to a (2)2x10 as a joist. Care to clarify?
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Old 11-07-2013, 03:41 PM   #9
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Floor Joist Dry Rot Repair


Ok, you caught my mistake. I meant the joists are under the 2x6 partition wall in the crawlspace.

I see your point Cortell about installing new 2x10 joists to either side of the damaged ones leaving a space (not to exceed current on-center spacing) for electrical and plumbing.

I'll post an update with a picture of the area with plumbing.
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Old 11-07-2013, 04:05 PM   #10
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Floor Joist Dry Rot Repair


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Ok, you caught my mistake. I meant the joists are under the 2x6 partition wall in the crawlspace.
OK. So I believe you're asking the following: if you have a non-load bearing wall running parallel to the joists underneath it, does the wall have to rest squarely on a joist? The answer to that is no, but the joist bay below the wall needs blocking. If for no other reason, you can't just fasten the wall's bottom plate to the subfloor.
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Old 11-07-2013, 04:26 PM   #11
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Floor Joist Dry Rot Repair






I have no idea why there are two sets of the same pictures. As you can see, its very busy with wiring and pipes. In the picture with the dehumidifier, you can see a small section of wood damage below the heavy gauge wire.
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Floor Joist Dry Rot Repair-002.jpg   Floor Joist Dry Rot Repair-005.jpg  

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Old 11-08-2013, 09:48 PM   #12
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Floor Joist Dry Rot Repair


A moderator should be able to delete one picture for you. The rot appears to be a very small area and in the 1/3 span close to bearing, not in the middle 1/3 span where I would be more concerned. The other fact is that it is a double joist under a running- non-bearing wall rather than a single joist. There should be plenty of support as pictured, as a bearing wall above requires the double joist arrangement with exception/substitution of a 2x spacer installed every 4' along the whole span for utility pipes, etc. passage, as cortell said; bottom of diagram; http://publicecodes.cyberregs.com/ic...9_5_par011.htm

The rot may be from a leak in your 2" drain line (pictured with clean-out) that runs along the joist between both before exiting. Was there signs of water on the plastic ground cover?

Your insulation is a mess, hardly helping restrict heat loss from above, vapor retarder paper should face living space in heating climate; http://www.energyvanguard.com/blog-b...tion-in-Floors

http://www.advancedinsulationinc.com...Insulation.pdf

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Old 11-10-2013, 04:38 PM   #13
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Floor Joist Dry Rot Repair


Actually, the area with the most damage is on the other end of the 15' double joist several feet from any water drain or supply. I mentioned earlier after removing the insulation no water staining was visible on the sub-floor.

I suspect the boards were already compromised when installed and the high humidity in the crawlspace promoted further damage.

All of the insulation batts are being removed during the repair. Will be re-installed correctly.
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Old 11-10-2013, 09:35 PM   #14
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Unless you push a screwdriver or awl through the top/bottom 2", I wouldn't lose sleep over it. You need that material in a joist for strength. Just so it's not an insect.... keep an eye on it with occasional visits down there, o-boy! Lol, I get under my house every few years..... too soon.

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Old 11-11-2013, 02:19 PM   #15
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Floor Joist Dry Rot Repair


Thanks Gary. After many hours under there, I've learned to accept my "basement for trolls".

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