Is the new beam sized properly for it's future use- carrying the new 2x12's for living space, or just sized to carry ceiling joists for an attic? No point in doing the latter if it has to be re-engineered down the road.
I would do the work from below, not above. I'd get some temporary walls in first,about four feet apart; cut out 3 feet of ceiling, and fit the new girder from below. IMO it's much easier to lift it in than to deal with moving wires so it can drop in.
Allow an extra quarter inch when trimming the ceiling joists. Put a nail in every hole of each joist hanger. It's easy.
The point loads at either end of the girder must bear through to foundation. Look at what you have at the stair well- a rim joist is okay, too, but not a double plate by itelf.
Casey

Need more information. The Porch Guy

The beam should be sized for a live load in the attic (they did plans for the bath as well as new stairs/attic), but I'll double check. I never thought about installing from below. That helps with the "how the heck do we get a 25-foot beam up into the attic through an itty bitty hole" question.

I'll go up in the attic again and see what's at the stairwell.

porch guy: what other info do you need?

Fine Homebuilding number 152, January, 2003, page 80, has an article, "Removing a Bearing Wall", in which they install a beam in the ceiling, just as you want to do. Go to finehomebuilding.com, seach for "Removing a Bearing Wall" to locate the article, but to view it you need to become a member, though they offer a free trial.

Bill

What you propose cannot be done.

1st, the lam beam cannot be 'suspended from the joists on either end'. Beams and girders must be be supported on each end by structural columns which transfer their weight directly to the foundations and footings.

2nd, a lam beam needs to be engineered (by an engineer) to carry all loads placed upon it.

3rd, the size of the ceiling joists for the approximately 18ft + spans must be at least (generally speaking) 2x10s or deeper at 16" on center to carry the ceiling loads.

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The bottom line is that no one here can help you and you will need to engage the services of an engineer or other design professional help you.

We did hire a design pro who drew the plans and consulted an engineer. My DH called them back and they confirmed that the beam is sized to support the live load in the future finished attic. And we can make it just 13' long (the length of the wall we're removing), and use headers across the last 7 feet or so (2 doorways).

On more crawling around in said attic, I realize that I misspoke on how said beam will be supported. It will rest on the outside wall of the house and an interior load bearing wall. We'll then hang the joists from the beam.

So we're off to buy a lam beam and get to work. Thanks a million bill v for the article (duly joined/saved/soaked up like a sponge) and sombreuil for your tips on doing it from below.

I'm happy to report that we've got our beam in! DH and DSS did it with a lot of sweat.
1. After putting up temporary support walls, DH cut the LB wall out as well as the two joists, creating the gap for the beam.

Good job. Looks fine. Hot in that attic, eh?
Casey

Whooeeee, hot doesn't begin to describe it. DH was wringing sweat out of his jeans by the end.

While the ceiling was opened up, we fed up a bunch of polyiso insulating panels that we're going to attach to the rafters. Eventually (next year?) we're going to finish the attic space.