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Attic remodel: Floor Joist question

jonrossen's picture

I'd like to throw this out to the great 'brain-trust' here on the forum!

I have an old house that has a new 2-story foundation. I have a set of stairs (non-compliant-too steep and narrow) that goes up to the rather funky attic. Right now I'm analyzing whether it would be worthwhile to put a large bedroom with bath up there. I have a good design solution to make the stairs legal. The next main issue I'm grappling with is the 'floor joists' which also are the ceiling joists for the plaster ceilings below.

The joists are 2x6 and according to the basic span table I looked at are undersized for the span. I could run 2x8s but in order to keep the floor at the same level I'd rather not. I've been thinking of perhaps 'sistering' 2x6s with the existing joists....doing a nail pattern with 16d nails to essentially create 4x6s. I realize that they might not calc out as strongly as a 4x6 but certainly would be beefier than a single 2x6. Does anyone have any ideas of taking this approach rather than putting in 2x8s? (Obviously I need to know what my floor height will be in order to lay out the stairs correctly.)

(Btw, the work in this area will not be a full 2nd story addition but will be built within the envelope of the existing roof for the most part with the exception that there will be two sizeable gable shed dormers, one on each side of the ridge.)

Thanks guys for any feedback!

It's going to be difficult to (post #183581, reply #1 of 9)

It's going to be difficult to practically impossible using 5-1/2" wood. I think you would have to look at steel, and you would have to have an engineer draw it up.

What is the span? and (post #183581, reply #2 of 9)

What is the span? and spacing?
2x6 sistered might be all you need if 2x8 is adiquate.

This site may help they make many load bearing joist/beams.

Here is another site that has tons of information and calculators. You can do span and load calculations with this site.

"The joists are 2x6 and (post #183581, reply #3 of 9)

"The joists are 2x6 and according to the basic span table I looked at are undersized for the span."

How do 2x6 joists look on your span table at 12" OC instead of 24"?

If that's not good enough, how about 4x6s dropped in between the existing 2x6's?


Thanks for all the feedback. (post #183581, reply #4 of 9)

Thanks for all the feedback. I don't have all the 'micro-data' of this issue, I've just looked at it froma 'macro' view so to speak so I don't know the exact span down to inches and the existing spacing of the existing joists. I do know that from a general look at my as-built plans and a simple span table that 2x6's won't work even at 12" O.C. and I was under the assumption that I'm at 16" O.C. now. However, I won't know for sure until I pull up the flooring or cut into the weird wall covering on the existing knee walls and look into the space on the other side.

I was just attempting to get opinions on sistering 2x6s to existing vs putting in 2x8s or perhaps 4x's or metal. Someone earlier stated that dealing with 5-1/2" lumber would be tricky...I guess the assumption is that it wouldn't be the same as the existing joists? If so, couldn't I simply put shim material on the plates to make the top plane equal to the existing joists? Also, that would tend to keep the bottom of the joists off of the plaster and lathe (particular the plaster 'keys'---where the plaster goes in between the lathe to the other side, in this case on the attic side)

Just a thought, but will you (post #183581, reply #5 of 9)

Just a thought, but will you have adequate room for the bath drain plumbing with 2x6 joists? Seems like 2x6 joists might cause you some plumbing headaches with your drain lines.

Disclaimer: I am not a plumber. Just trying to think about possible issues for you.

I made the comment about (post #183581, reply #6 of 9)

I made the comment about 5-1/2" lumber. It's not out of concern that other joists might be a slightly different size, it's that 2x6 is rarely used as joists, it's not big enough to span much of anything, there's not much space in a 2x6 floor to run plumbing, etc. Adding a bunch more 2x6 or going to 4x6 is only going to make the floor framing that much more dense and harder for the plumber to do anything. In certain situations it could work, but I would plan all of the drains carefully before committing to it, and if you want a tile floor I would calculate deflection.

David, Ryan, Thanks very much (post #183581, reply #7 of 9)

David, Ryan,

Thanks very much for your input; I hadn't given plumbing enough attention...I'll look into it and I appreciate you guys bringing it up.
David..thanks for clarifying your original comment, I appreciate that.



I've been putting thinking (post #183581, reply #8 of 9)

I've been putting thinking cap on again about this issue as I start my preliminary floor plan design for this attic remodel.  I don't think using 2x6s would pose much of a problem for the plumbing.  I'm 'stacking' this new proposed bathroom in the attic space over the existing bathroom and I may even have some flexibility downstairs for building a soffit to create more space if need be.  I think my big plumbing lines will more than likely run parallel to the floor joists.

A contractor friend told me the other day that on a attic remodel that he awhile back, the existing floor joists were 2x6.  In that job the architect and engineer spec's out some metal hardware to go alongside each joist to beef them and make them deflect less.  I believe he told me that what was spec'd were some sort of metal studs, not really designed for this application but this engineer had done this before and found it to be a good solution.  Has anyone here ever tried this method of beefing up floor joists by attaching some type of metal along the sides of the joist in order to eliminate having to add more wood?

There've been several (post #183581, reply #9 of 9)

There've been several discussions of metal "filch plates" between sistered members for strengthening headers.  I suppose if you got heavy duty steel studs they'd spec stronger (and definitely stiffer) than wood.  (Would also help in assuring a flat floor.)  But you have to make sure that they're attached adequately so that they can't twist, as twisting robs them of strength.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville