I am looking at various construction methods and advanced framing popped up, 2x6 24 oc, California corners etc. this is where I am stuck, if planning a stick build with stud walls 24" oc, goes without saying you cant do floors 16" oc, to the best of my knowledge the studs need to rest on top of a joist. so what than, joists should be 24 oc too? that calls for much larger dimensional lumber plus thicker subfloor/roof sheathing, is this how they are commonly built? I guess one could opt for 12" oc floors and rafters, for my consideration I am looking at 12' spans rough guess 8x versus 12x, just twice as many, the floor would be much firmer too, I think! it just feels a bit unbalanced having joists twice as often as studs. I am hoping someone can shed some light on common practices and what size lumber I should expect to need for joists/ rafters, assuming 40/10 load ( I think this is common, snow load for my are is 20? call it 30 for safe measure

If you include a band joists over the foundation sill it doesn't matter what the floor joist spacing is, since the studwall's bottom plate will be resting over a band joist, and the joists are resting on the foundation sill. For upper floors the floor joists need to be directly over the studs to deal with the the lower loading issues related to the single top-plate of the studwall below.  Resting additional joists midway between the stud supported joists adds a lot less structure than you might think, since they are supported at the weakest part of that single top-plate.

Rafters too have to be over studs.

<!--[if gte mso 9]> georec, I stick-built a few hundred houses in the mid to late 70’s before I moved into commercial construction so I am not up to date with the latest codes, but in my opinion, stacking all load bearing framing members is the only way to go. I’m designing a home I hope to start construction on in May or June of 2015 and am going with 16” OC. I know engineers can design floors and roofs to adequately carry any loads using 24” OC framing, but I am staying with 16” OC for one reason; fire.

I don’t see much discussion about this, but I’ve read studies on the time difference for structure collapse between 16” OC and 24” OC framing in a fire and the difference between the two is striking. Floor collapse in 16” OC framing takes considerably longer due to the fact there is 33% more material for the fire to consume before the floor weakens to the point of collapse. A fire bad enough to destroy a structure may be a one in a million event, but I am willing to pay for the increased framing cost and heat loss due to thermal bridging to buy a few extra minutes to ensure my family could escape.

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If I was that concerned about fire I would invest in a new combination potable and fire protection systems like the one we are installing now here in Minneapolis, saving material getting a better insulated wall. There is also the thermal barrier and double-drywall would be my answer with the added benefit of sound attenuation. And there is building with stone or concrete since you are in the commercial building business...

Won't the floor joists all be exposed to fire simultaneously? If there is a fire below an unprotected floor assembly, the spacing would not matter.

What is a big concern is the fact that engineered floors (wood I's) collapse much quicker then solid lumber joists. This has killed a number of poorly trained fire fighters. You may be able to entry a home on fire that has a 2x10 floor system for 20 minutes longer then one that has wood I's.

All joists would be exposed to the fire simultaneously, but with 33% more material to consume, it would take longer for a fire to weaken a 16" OC floor to the point of collapse, so the spacing does matter. Think of it this way; what would take longer to burn, a solid block of wood 16" cubed or a block 24" cubed?

I haven't seen any comparisons to flame spread on solid joists versus engineered, but my guess is that solid joists would hold up better in the event of a fire. I'm still going to use engineered joists on my home as I have some lengthy spans and I am also planning on drywall ceilings throughout the basement level. My intent with the drywall is only to finish the ceilings, but it would help inhibit flame spread. I just want a few extra minutes to get my family out in the event of a fire. The house can burn to the ground as long as my family is safe.

Again, a house fire is probably a 1 in a million event. I am not young, have probably met tens of thousands of people and I can only recall meeting 1 person who lost their home to a fire. It is (thankfully) an incredibly rare event.

Badger, fire suppression systems can be of benefit as long as the fire does not start in or adjacent to a floor cavity. I believe most spray patterns are designed to cover areas 4' and below and I'm just not willing to risk the life of my family that a fire would start below 4' and the suppression system would give me a few more minutes to evacuate.

I'm still working on precisely calculating heating costs for the entire house, but right now I have ball-parked the costs to heat the 2000 sq ft 1st floor at $181 a year using 16" OC framing. I won't consider 24" OC but I did look at building a wall truss assembly (16" OC staggered) to reduce thermal bridging, but my ball-park numbers indicated it would take roughly 150 years to recover the additional framing and insulating costs. I haven't completely ruled out the truss assembly but it's certainly not practical. I think I have the energy efficiency of the house designed about as well as I could to fit my needs. Everyone has different preferences, but 24" OC is something I personally would never consider.

Think of it this way. If I have 10 boy scouts around the campfire and they all have marshmallows on their sticks, it doesn't matter if they are standing 24" apart or if they are standing 16" apart, the marshmallows are going to catch flame at approximately the same time and are going to fall off the sticks at approximately the same time. It doesn't matter how many boys are roasting mallows or how far apart they are. All that matters is that they are evenly exposed to the heat source.
If on the other hand, we give each boy two mallows and they hold them tight against each other (double joist) they will take longer to fall into the fire because one side is not exposed to the heat source.

If I give 5 of the boys regular sized mallows (dimensional lumber) and five mini mallows, the minis are going to fall off the sticks first.