Yes, you can do it and keep the ceiling intact. We did this just last year. You "sister" the correct sized joists along side of the 2x6. In our case the plaster was sticking up betweed the lathe and joists so the builder put in a small spacer offsetting them when they sistered. All in all the ceilings survived fairly well, we do have a couple of cracks from the new load. Fixable, if we ever get around to it. Hope this helps.

I don't know your span, but I have a similar problem and interested in what is determined here. Is there anyway to get 2x6 to make the span you have? Maybe adding 1 2x6 in each bay (then you would have 12" centers? I read here in the past that doubling dimensional lumber adds 1/2 to the span table is can support, or was that 1/4?

I had an addition added and the new foundation was too high to allow the floors heights to match the existing floors. It was a 16' span. The builder dropped one joist size and placed them at 10" centers. Inspector did not flag it.

This Old House on TV added new joists above the existing ceiling joists on one of their projects within the last year. Tearing out the old ones on your house? Get a second on-site opinion. You might want to replace that engineer completely if others say removal is unnecessary. His plan is expensive and time consuming.

What is the square footage of the 1st floor? What is the square footage of the 2nd floor addition? What is the span of the new I-joists?

The reason I ask is that if you are totally removing the roof then busdriver's suggestion would work. If you are only removing a portion of the roof then you would end up with different rooflines by adding the new joists above the old ones.

There are a lot of considerations to take into account of which there is not enough detail provided. If your structural engineer had these details it is possible he is providing you the best information for this design.

Things such as will these I-joists span all the way from front-to-back (in which case it will affect the roofline) or will they be supported by internal load bearing walls thus you might get by without affecting the rooflines but maybe the engineer determined these internal load bearing walls could not support the additional second story.

I should have added that if the pitch of the roof is not 45 degrees or greater & assuming a soffit overhang of 12" then the I-joists would not fit onto the perimeter top plates without affecting the roofline.

The first floor is about 1100 sq ft. The addition is about 500 sq ft. The addition will run from the front of the house to the back of the house. The roof will be taken off only where the addition will be. We will be adding more posts to the basement to reinforce the the steel I beam. We will also be adding more 2x4 studs to a wall on the first floor that runs the length of the house (side to side and perpendicular to the floor/ceiling joists). THe front to back span of the house is about 30 feet. The current joists run from the front to the back of the house. I hope this answered most of the questions.

Since it sounds like the engineering work has been done to ensure that the addition will have the necessary support on the first level & in the basement I guess there are a couple of ways to think about this.

500 sq ft of sheetrock would be about 20 sheets (factoring in waste). Sheetrock would cost about $10/sheet & then assume $25/sheet to hang & finish. That amounts to about $700 not including the new ceiling texture or replacing any crown moulding, etc. So maybe the engineer is thinking in the scheme of things what's $700.

However, from the sound of it busdriver's suggestion seems totally feasible. Since the roof is being taken out I don't see any issue with running the I-joists from front of house top-plate to the back of house top-plate (again assuming the I-joists are sized for that & the support underneath is sufficient). Since you have that middle wall running perpendicular that means the I-beams are only spanning 15'. I'm not sure why I-beams are required as 2X12's (SYP #2) on 16" centers easily meet the 15' span) so you may want to inquire about that. Personally, I like the I-beams but if this is being done on a strict budget you might save a few bucks that way.

I would not sister (make a connection) the I-beams to the 1st floors 2X6 ceiling joists. I would keep them isolated so as to keep the 2nd floor addition forces (live & dead loads) away from the 1st floor ceiling. Sounds like TOH ran another plate around the perimeter & where internal load bearing walls were & then placed the 2nd floor joists on top of that.

I would think you could install (lifter plates for lack of a better word) which would be 2x4s 22.5" long installed on top of the top plates between the 1st floor ceiling joists and the I-joists would sit 1.5" above the sheetrock thus isolating the forces on those from the 1st floor ceiling sheetrock. You would need to make sure that would pass code.

We are doing what mike13 suggested. We are putting on a 2nd floor and leaving the existing 2x6 ceiling joists and putting lifter ( top sill) plates along the outer walls. We are then running 24" TJIs ( engineered I joists) across. Existing ceilings will be left in tack and we will have really good isolation between the two floors.

An additional benefit is that any second floor plumbing drains, if secured to the new joists only, will transmit very little noise downstairs.

Busdriver is correct but if you want to ensure the quietest drains use cast iron on any vertical segments.

Added benefit to the aforementioned technique is that duct runs can be made effortlessly as well, if you are adding/converting to forced air/central a/c.
Casey