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WillK 03-02-2011 09:58 AM

Replace or sister floor joists
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So my floor joists in my 1917 house are 2x6, the first 10 floor joists are rotted at the end similar to what is shown in this picture. They have been sistered, but the sister does not extend the full length to the triple 2x6 beam in the center. At the triple 2x6 beam, the joists butt against eachother, they do not overlap.

The structure is supported as seen by stacks of things, in this case some 2x6 wood pieces which sit on top of some cement blocks, no proper footings. I'm implementing plans from an engineer to replace this with 6x6 support posts holding up triple 2x10 beams near the exterior end of the joists, these will be on specified footings.

The engineer's plans specified using the existing triple 2x6 beam in the center, but it's on stacks of cement blocks and is sagging between 2 colummns where the boards are spliced away from supports - all 3 boards splice between 2 columns, and this is where the floor sags, and it's part of where the stairway is supported. I plan to replace this center beam with triple 2x10 with joints properly supported.

Here's my question: the span between beams will be 11'. I want to either sister 2x8's onto the damaged joists OR replace them. I'd rather replace them, but I don't think I want to go as far as replacing all floor joists since there are floor joists that are not rotted. The floor joists have clearly been replaced because they are not true 2" x 6" boards like what would have been in use at the time of original construction and which I've seen in use elsewhere in the house.

If I replace, it makes it easier to put in joist hangers where I notch the joists for the beams, which need to be placed for supporting the 2x6's which aren't being replaced.

If I sister, it makes it easier because I don't have to remove old boards and devise temporary support while this work is in progress... But I don't know how I get a joist hanger in, or is there another way? Or do I fur in 2x2 under the 2x6's and use a triple 2x joist hanger?

Which way should I go?

Gary in WA 03-03-2011 10:08 PM

Can you sister on the good board side and add a double hanger cutting the original joist end back making room for the hanger, using through bolts?


WillK 03-04-2011 08:31 AM

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As much as I'd like to map out the state of my structure so I can have a complete picture of what I'm dealing with, every time I try to do that I find something that holds my attention because I study it until I'm too tired and/or overwhelmed to keep up a consistent effort to get through the whole crawlspace.. I'm kind of taking it as one section at a time, but I'm also going to be forced to deal with all of these joists since I'm putting in a new beam that supports all of them for the full length of the house.

So with that said, I made a diagram illustrating as much as I know about what's what at this point.

The problem is that I assume I would ideally want to be able to nail a new sister to some board that is good along its full length. I don't have such an entity, the older joist that has the rot on one end will not provide attachment at the exterior end, and the newer sister doesn't provide anything to connect to at the center beam - or anywhere near the center beam.

loftezy 03-04-2011 11:04 AM

Do you need joist hangers because you are going to remove the built-up wood supports (are those stacked 2x6's really the extent of your foundation?!) at the end of the joists? You may not need joist hangers because you should be allowed to have a small length of your floor joist act as a cantilever. But this all depends on where you live and how far your new 2x10 beam will be from the end of your floor joists. How far from the end of the joists is your engineer telling you to install the new 2x10 beam? Also, your engineer should be able to tell you if you need hangers.

If I were doing the work at my house, I would leave the existing floor joists in place as long as there was not a insect issue. I'd leave them for two reasons: 1) reducing the amount of work by not having to remove them, and 2) there may be seams in the subfloor that are supported at those existing joists.

I would also run new joists from the exterior wall to the center support if possible and not worry so much about only sistering the rotted portion of the old joists.

WillK 03-04-2011 11:33 AM

The diagram shows what is currently existing, what I plan on doing is sistering 2x8 onto the existing 2x6 joists. My planned use of hangers was based on the presumption that I would need to have the existing 2x6 joists bearing on the new beams, therefore the 2x8's would need to be notched and the joists are to support the 2x8's at the notch.

I'm leaning towards setting new beams so that the 2x8's bear on the beams without notching. To make up the difference to support the 2x6's I would add 2x4 lumber between the 2x6 and the beam (rip the 4" dimension down to 1 3/4")

luckattack 03-08-2011 12:42 AM

Looks like someone had discovered the rot and sistered the joist to the existing thus preventing a failure from occuring. If I were you, I would sister on another 2x6 to the side without the sistered joist. This is the method with the least amount of work. I wouldn't notch a 2x8, too much trouble when a 2x6 would work.

WillK 03-09-2011 08:55 PM

I forgot I had meant to respond...

That would work if the rotted joists were the end of the story, but really it's just the beginning of a big remodel. I'm starting by establishing a sound platform under the first story onto which I can re-establish solid level floors. On top of that I will be building new bathroom and kitchens in a rearranged first floor designed to provide load bearing walls to support a full second floor with 3 bedrooms and a bathroom where currently I have a half second floor with 2 bedrooms only whose floors are sagging from overspanned joists held up by an overspanned ceiling beam (triple 2x6 which has been sistered into a five-tuple 2x6, one end of which is supported by a "load bearing" wall which runs parallel to floor joists and is between joists, the other end of which is going to a walk-through openning which I have doubts that it has a properly sized header, although it doesn't have sagging so this may actually be one of few things done right)

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