Use the shallower fixtures if you can; that's what they are made for.

Not sure if this helps, however,

in our kitchen we had to install a ceiling mount range hood and 2 canister lights, and guess what?

They needed to be installed precisely where we had ceiling joists.

We ended up having to 'box out' the joists in order to make a hole for the required opening. We actually had our structural person do this.

i can get you pics if you like.

A general rule of thumb is that cuts on the underside of joists should be avoided. Taking 2" of depth out of a 2X8 is a lot. If your joists are over-engineered for the loading, then it might not be a problem, but I don't think it is worth the risk of a sagging floor. If you must make cuts, by all means go for the shorter cans. Would it really be that bad if the cans are not perfectly centered? Sometimes less-than-perfect symmetry is good, depending on the layout of the room and the furniture.

Thanks for your follow-ups, gentlemen. I've already cut out one joist to a 2" depth, so should I box it out for support as "funyellow" indicated? Hard to believe the floor would sag, but don't want to take any chances since it's a customer's house. Really want to center the lights, so will go with the 5" cans and notch only about 1/2" or so. Will this be acceptable or should I add support for these as well? Would appreciate further follow-up.
Thanks very much!

I reread your original post and saw that these are ceiling joists in a suspended ceiling so you can cut them all you want.

If the joists had been supporting a floor a 2" deep notch would have reduced the loading bearing capacity of the joist by almost half and, more importantly, reduced its resistance to deflection by more than half. If you did that to enough joists, the floor would bounce like a trampoline. Comfort would be sacrificed before sagging or collapse.

macv-
A clarification, please? I reread the OP's first post, and I'm not convinced that those joists are only for acoustical tile support. What do you see there that suggests they are not load-bearing for the floor above them?

kerschk-
Unfortunately, if those are load-bearing joists, boxing them in around the cuts is not going to do anything much to restore the load-bearing capacity you removed by cutting into them.

Ceiling and floor joist 'notches' are regulated by building codes and the limits established by acceptable engineering practice and tables.

Generally speaking, one is entirely prohibited from notching or prohibited to 'cut into' any joist in its middle third.

For example, if you have a 12 foot long joist, you are prohibited from making any 'notch' cut in the top or bottom of the joist anywhere in the length beyond the 4 foot point to the 8 foot point measured from either direction.

In end 'thirds'... (the 1 to 4 foot length from either end)... you are only allowed to notch the joist along the top or bottom only to a depth of D/3...or 1/3 the depth of the joist.

If you have a nominal 2x8 joist, your actual depth is 7.25 inches.

This means the maximum depth of any top or bottom notch in end 3rds can be 2.42 inches or ~2 7/16ths inches deep.

Maximum depth of notches in joists at their very ends can only be D/4 of 1/4th the depth of any joist.

In this case, the maximum allowable 'end cut' depth of a 2 x8 nominal (7.25" deep joist) will be 1 9/16th inches...

IIRC, you can notch the top edge one-third, but the bottom edge one-quarter. The greater weakening is affected in bottom notches, so they can't be as deep.
Casey

Apparently everyone has a different code book than I do.

Section R502.8.1 of my IRC book limits the depth of a notch to D/6 [not D/3] so the maximum notch depth for a 2x8 is 1.2" [not 2.42"]. I hope no one is taking the deeper notching advice seriously.

There also seems to be confusion about the difference between "acceptable engineering practice" and residential building code prescriptions.

The code allowances for notches on the tension side of a joist is based on long time field observations where the adjacent joists are assumed to help support a weakened joist via the floor sheathing. In this prescription the code doesn't consider the potential for excessive floor bounce nor is that a consideration anywhere else in the code.

"Acceptable engineering practice" is based primarily on the Timber Construction Manual and would require a joist to provide support for the original design loads for it's entire length. That would result in a maximum notch at the 1/3 point of your example of approximately .6" which is half the code allowable depth and one fourth the D/3 depth claim.

Sometimes the building code is not more restrictive than the rules of good design and common sense.

I checked the predecessor of the ICC, CABO, and its 1&2 Family Dwelling Code used the same lumber notching wording verbatim going back to 1986 and probably earlier since they first organized in '72.

Since CABO changed its name to the ICC and changed the name of its residential code to the IRC, it is not surprising that the prescription for notching has been the same for 30 years.

I ran the numbers and found that the 12 ft example joist could have a 7/8" deep notch at the third point and still be strong enough to support the design load but because it is difficult to achieve a perfect cut I would limit the notch to 3/4" and avoid notching two adjacent joists.

My copy of the IRC makes no distinction between notches in the top vs the bottom of a joist. The depth of both is limited to 1/6 of the joist depth, and the notch length is limited to 1/3 the joist depth. Of course, no notch can be located in the middle third of the span.