# joist spacing



## frodo (Oct 25, 2016)

any one see a problem here

The floor joists are going to be exactly 3"x 8" since the span calculations are done based on the size of the timbers. They will be milled to that dimension.

Spacing is probably going to be 24". I haven't had to move any of the floor joists which is why I have been so diligent in getting the stacks organized (Your feedback has been excellent).

The span is 11' 4" which is more or less split down the middle for the two bathrooms. Not luxurious bathrooms by any stretch but comfortable.
jb9 is offline  Report Post  	

2nd story,  full bath, 11'4'' span at 24 oc   ?   3x8


----------



## nealtw (Oct 26, 2016)

Don't know enough about the timber but for 24 oc the sub floor goes up in thickness 3/4 or 13/16 not sure but more than 5/8 for sure.


----------



## Mastercarpenty (Oct 26, 2016)

The subfloor would be my concern too as well as the fact that most other aspects of building systems (sheetrock, light-boxes etc) are designed to work with 16" OC construction. 

When you change one thing it can affect many others so you always have to keep the big picture in mind- lose sight of that and your problems will always multiply. Construction isn't a simple thing when you really understand it in depth.

Phil


----------



## Sparky617 (Oct 26, 2016)

Why not go with floor trusses or wood I-beams instead of dimensional lumber?  Floor trusses have a great advantage open space in the cross-members to allow for running of electrical, plumbing and HVAC without having to cut into the joist.  They have a bigger span than dimensional lumber too.


----------



## nealtw (Oct 26, 2016)

Sparky617 said:


> Why not go with floor trusses or wood I-beams instead of dimensional lumber?  Floor trusses have a great advantage open space in the cross-members to allow for running of electrical, plumbing and HVAC without having to cut into the joist.  They have a bigger span than dimensional lumber too.



The wider the OC span with I beam  the taller the beam, just guessing but is sounds like he is replacing old construction full 2x8


----------



## Snoonyb (Oct 26, 2016)

What's the price difference between 3X8 milled and 4X8 dimensional, off the shelf?


----------



## Mastercarpenty (Oct 27, 2016)

Sparky617 said:


> Why not go with floor trusses or wood I-beams instead of dimensional lumber?  Floor trusses have a great advantage open space in the cross-members to allow for running of electrical, plumbing and HVAC without having to cut into the joist.  They have a bigger span than dimensional lumber too.



Firefighters are now being trained to not enter a house built with floor trusses or IJT's  once the active involvement in the fire is 5 minutes or more, but with dimensional lumber they have 10-12 minutes to work safely in, and maybe more if heavy timbers are used. With dimensional lumber, the charring affects the core strength slowly and gradually, but with truss floors the stressed members are smaller and once _any_ of them is compromised strength is rapidly lost. Same for IJT's and the center OSB/Plywood panel. 

While this is an unlikely event it shows that for the gains offered by engineered products there also are more tightly limited parameters which they work in. We've all seen bathroom joists chewed to toothpick-size by plumbers running drain pipes, but the floor only sagged eventually and sometimes that took decades or additional damage like water. We've all seen overloaded floors sag badly but not break. The failure mode of engineered floors is far more rapid and catastrophic; when they let go for any reason it's all at once and it takes less outside influence to reach the failure point. 

I'm not a 'Luddite' and there are some engineered building products which I love but given the choice I prefer conventional floors. With one house I built we had the plumber and HVAC guys there when we designed the floor beam and joist layout so that we could avoid compromising the structure by putting things in their way which they'd need to cut through, and where such cuts were unavoidable we could plan for additional strength to compensate. It also had the benefit of ensuring that nobody needed the same space. In another house with floor trusses the planned use of metal HVAC ducts couldn't happen because they weren't placed as the trusses were laid and they didn't have the needed access afterward. In most houses the Architect, lacking practical building experience, does not consider those needs and they design floor systems with specified loading limits for initial strength only, not for the real-world conditions we have to build with. Since these things are engineered and specified in the plans we can't make changes and the end result is often the opposite of the intended result- a weaker less substantial house where future problems cannot be as easily rectified as they are with conventional construction of dimensional lumber. You can't 'sister' an engineered floor joist or beam across them to add strength; you have to call in an architect or engineer to redesign the system.

So there are several very valid reasons somebody might want to stay with the old ways of using dimensional lumber for floor construction. In most homes the only real advantage of engineered floors is lower cost and more convenience for the builder; they are not always truly a better way to build.

Phil


----------

