Ugh - so sorry this happened because no one checked / validated the right product was delivered before the installation... who is your site superintendent?

I don't think you should mess around with joists (sorry- read roof in there somehow!) - I'd want what was spec'd. Period. Unless, as you say, that by perhaps doubling them up you could surpass the rating of the original spec. Otherwise - it's take it down and put up the right stuff. IMO.

Good luck!

Since the lesser joists are still considerably stronger than what the building code minimum requirement (LL = 30, max deal. = L/360), the issue is how much bounce or shake will result from normal use and how sensitive you and your family will be to it.

The best way to determine this is to perform a heel-drop test in the middle of the room. Have a heavy person stand on their tip toes and then suddenly drop to their heels. If the floor shakes for even a split second afterward, the floor is likely to be unacceptable. Another test is to sit in a chair in the middle of the room reading something and have someone walk quickly across the room on their heels. If you find it uncomfortable, the joists need to be reinforced or replaced. You might consider adding 14" LVL's to the side of some of the middle joists and then test it again. The lumber supplier should be happy to pay for this work.

You should not be concerned about exceeding the original spec but additional joists might complicate the mechanical and electrical installation a bit.

This is spec-house builder and he's done this floorplan numerous times. I've been in them and found the flooring to be at my minimum comfort level, which is why lower grade is so alarming to me. After investigating all this and realizing how small the price differential is, I wish I had known enough before hand to upgrade beyond his standard. The optimist side of me hopes this is my second chance.

I was hoping to get an idea of HOW you add a joist at this point. For half of them, it looks like they could pull off the rim joist and remove the blocking, then slide new joists in. For the other half, they've already built the 32x13 roof for my porch and it is attached at that rim joist level.

I read up on the technical spec's for the joists, and they only require 1.75" of bearing surface for the flanges. Does that mean they could use 3.5" shorter joists, slide them up from inside and then use 1.75" of blocking between the joist and the rim joist?

Since the addition of the reinforcing is for the purpose of stiffening the existing joists and each additional member will be sharing the design load, they may not need to have full bearing at the supports but they should be well nailed to the existing members. Dimension lumber would not appropriate against an engineered product and I would use LVL's instead of I-joists to save space. but if the I-Joists are cheaper perhaps you will not have a choice.

You could also put blocking at the inside face of the support walls and use joist hangers to install I-Joists or LVL's.

Since this is not really a structural issue, add a joist and test it. If you ask a structural engineer or the manufacturer you might be able to nest an LVL against the I-Joist web.

I wonder if 3/4" plywood closely nailed to the top and bottom flange, say every 6", to one or both sides of the existing floor joists would provide sufficient stiffening? One could even add a one piece, continuous filler in the flange space...

Has the builder not make a proposal for your approval yet?

My builder installed an additional set of BLI-40s next to the existing joists. In hindsight, I would have preferred having them space 12" OC instead of keeping the 24" OC, but I still came out ahead of what I was originally going to get. Additionally, it looks like the original joists were actually BLI-60s, but mislabeled at the factory (the engineering stamps on the flanges were 1.8E instead of 1.5E as expected on BLI-40s). Thanks for everyone's input.