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colosseo 06-04-2008 10:31 PM

Hello all,
I have two questions in regards to a project I am working on:
we are sistering existing joists (together with doubling those at the stair opening) of a 100+ years old 19' wide 2-floors row-house before removing the main wall at the first floor. Joists are of a variable size 2x8, 3x8 and 4x8.
Does sistering mean also pocketing the new joists into the party walls or not? In other words: would pocketing be required only at the "doubled up" joists at the stair opening.
Would it be preferable (in terms of cost of material and labor and structural stability) to sister every second joist with 3X10 or using 3x8 for all the joists?

Advice is extremely appreciated, thanks in advance.

gaspar sanz - canarios from suite espaņola

Maintenance 6 06-05-2008 06:24 AM

To truly sister a joist you should extend the new piece over top of the load bearing surface, same as the original. If your plan is to sister the joists and then remove a bearing wall underneath, you should rethink this. What is the total span on the 8" joists after the wall is removed?

colosseo 06-05-2008 06:59 AM

18'-4". So. sistering implies always pocketing? joists are 16" o.c. Apparently there is a problem trying to use new joists with a length over 18'-4", it seems difficult to put them in place.


Termite 06-05-2008 08:05 AM

The advice so far is correct. You should use FULL LENGTH members, and they must also rest on the bearing point. Otherwise they're subject to excessive shear loads.

As for removal of the wall, I hope you know what you're doing. You really need to consult an engineer regarding serious structural modifications to a home. I don't care how thick it is, an 8" deep member clear-spanning 18'4" is going to bounce like a trampoline.

Maintenance 6 06-05-2008 12:40 PM

18'6" is too long a span for an 8" high joist. If there is any load bearing on this span, it will not only be springy, it will be dangerous. I would have to assume, (you need to know and are the only one who can tell us) that there is also a bearing wall on the second floor that is carrying your second floor ceiling joists, and that this wall is located overtop of the first floor wall, or at least in close proximity. Removing the first floor bearing wall will transfer all of that load onto the joists that you want to "sister". Like Termite said, you need an engineer to evaluate your situation. You may be able to place a beam under the joists that will carry all of the load, but again, you need to have a pro look at this.

colosseo 06-05-2008 06:30 PM

We hired an engineer who called out the joist sistering (quoted here: sister every joist with same size lumber <3X8> and double up at stair opening) for the proposed removal of the main wall at the first floor (wood framing with 4x2.5 and lath/plaster. There is a main wall also at the second floor, as well as at the basement level spanning for the length of the stairs. There is also a main wood girder (not continuous, divided in two parts) supported by the foundation walls and three brick columns in the cellar. The 2nd floor main wall does not go all the way up through the roof attic space.
The house has been inspected by a licensed engineer and reputed in structural sound conditions.
The question for us is: does sistering imply pocketing therefore using same length lumber? is there a distinction between sistering and doubling up?
Also is it preferable to use nails or bolts?
Thanks for the precious advice.

Termite 06-05-2008 08:05 PM

Sistering is the act of attaching one member to another, and it is common practice that the sistered member is the same length as the old member.

At your bearing point, you definately need to pocket the new sister joist in all the way and provide adequate bearing for it.

You'll have to bolt them together. 3" nominal members are too thick to effectively nail them together.

colosseo 06-05-2008 09:22 PM

Thanks for the advice thekctermite. Your point is clear.
Only, this makes the main wall removal not economically feasible for us and will require revisiting the entire project.

Thanks again, best.

Termite 06-06-2008 07:59 AM

Consult your engineer about this. He can run calculations of the shear forces that would act against the existing joist at its bearing points, to see if it could handle the additional loads imposed by not providing direct bearing for the sister joists.

colosseo 06-06-2008 10:17 AM

So I guess the answer is that sistering can be done either ways depending on an accurate load analysis and calculation results of the shear at the existing joists load bearing point.

Thank you very much for your precious advice.

BigJimmy 06-06-2008 11:59 AM


Originally Posted by thekctermite (Post 128211)
You'll have to bolt them together. 3" nominal members are too thick to effectively nail them together.

Your SE should also prescribe the correct nailing schedule. I had to sister existing floor joists before removing part of a L/B wall above the floor and I was surprised to see what the schedule was, esp. at the ends.

Also (and I'm no SE), I believe that there is a difference in shear strength between framing nails and comparable-sized lag screws (I think that nails offer greater resistance to shear). Again, check with the SE. And if you DO use lags, make sure to pilot-drill the holes with a bit that is equal to the root diameter.

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