An idea to mix deflection rated I Joists?

Question

I'm making some hypothetical blueprint ideas and started thinking about the various floor joist options. In our neck of the woods, I-Joists are by far most common with floor truss systems becoming more common on only higher end ($$$) homes. I do like them, but they are typically 2.5x the cost of a traditional I-Joist (after delivery fees). I can typically pour a 9' basement cheaper and use regular I-Joists than I can go with an 8' basement and use floor truss systems to make up for the missing height and keep everything above the bottom chord of the floor truss. So it doesn't seem like an economical option.

I was thinking about how people are now tiling directly over their OSB subfloor, or sometimes they will tile over 1/4" underlayment, or something like 1/4" densshield type material. I can only assume this is possible with an L/600 rated floor joist.

One thing I don't like about tiling is trying to meet the deflection ratings with plywood or thicker tileboard. Would it be possible to simply order two different types of I-Joists and mix them, or would you notice the moment you step onto the lesser quality floor joist part of the home?

Example: L/600 I-Joist under the kitchen and dining area that has tile, and then L/360 I-Joists under the rest of the home that is laminate or carpet.

Answer 1

I doubt you’d notice the difference in deflection walking from L/360 to L/600.

When we design floor systems, we use the “critical span” to size the joists. That is to say, spans (from bearing wall to bearing wall) are seldom identical. So, we size the joists for the critical span and then use the same size joists throughout (so the floor sheathing is at the same height). The shorter spans will have a “stiffer” system, but it’s not noticeable.

If you’ve ever walked across a “stiff” floor system and then a “spongy” floor system, it’s noticeable. But if you walk across a stiff floor system and a “really stiff” floor system, it’s not noticeable.

So, if you mix the grade of the I-Joists, I doubt you’d notice by walking from one to another. However, if the joists are not “stiff” enough, (L/600) the tile will crack, of course.

An alternative to mixing grades, is to mix spacings of the I-Joists. I do not like joists spaced further than 16” o.c. for FLOOR systems. Perhaps you could space the joists at 12” o.c. for tile and 16” o.c. for wood and carpet areas.

BTW, the main reason we use L/360 is to keep the gypsum board ceilings below from cracking. Anything less, (i.e.: L/240, etc.) you’re asking for trouble...it will crack the plaster ceiling when fully loaded....(when you have your birthday party and everyone is in the same room.)

Also, we add 1 or 2 floor joists under the refrigerator and the freezer...making the spacing 8” o.c.

(Also, we try to determine if the client will have waterbeds.)