Subfloor thickness, joist spacing, cost, and performance ratio
I did some searching and couldn’t find anything about my question so feel free to point me to another thread if this has been asked which I am sure it has.
I am looking for some discussion about how 24″ OC I-joist floor systems are performing, what drawbacks they have, and what should be avoided, mitigated, etc.
This is a durability, cost, functionality, and energy efficient question.
Primarily I want to know peoples thoughts on the following or more (please specify if you are talking about OSB or Plywood):
24″ O.C. with 23/32″ have people EXPERIENCED problems with tile, wood, or other flooring installations?
Are there other thickness options besides 23/32 and 1-1/8? 7/8? Is 1-1/8 total cost suicide or can it be mitigated by doing 24″ and maybe even upgrading to a wider flange/better/stronger joist?
OSB/Plywood (I hate OSB….but it does seem to be the go to material so it is hard to get builders not to use it that I work with)?
Basically is there a cost effective, tested (is someone using it), functional 24″ o.c. floor system that compares in cost/functionality to a conventional OSB & 16″ o.c. floor or for a reasonable upcharge represents a good value to the customer (keep in mind that most customers, although they should, do not see, for example, the durability of plywood over OSB as being “value”) that I could sell to a conventional builder or homeowner not primarily/zealously concerned with long term durability such as those of us that usually frequent this forum?
I am asking because I often find traditional builders and people in general do what they have always done and rarely pick the rock up to reveal the diamond hiding under it. Is there a better way?
Thanks,
Spencer
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Replies
Spencer,
There is lots of information available on subfloor specifications for 24-inch-on-center joists. The type of subfloor you'll need will depend on your choice of flooring. Here is a link to an article you may want to read: Guidelines for Tile Installation in Engineered Wood Framing Assemblies.
When it comes to making cost comparisons, there is no substitute for good old-fashioned arithmetic. Get prices on the various joist options and subfloor options and do the math to figure out which approach has the lowest cost.
I prefer to use open web 18"-24" trusses with dedicated and aligned chases for my HVAC duct system and webs for easy plumbing and electrical layout. In the long run, you build faster, cut labor and avoid all the cutting, waste and hassle of TJIs. Floor trusses need to have deflection rates of 480 or higher to prevent bounce. We use 1 1/8 Advantech flooring.
I agree with Armando, deflecting bounce is definitely god, and so is saving time.
I used web trusses (24" on center) and Adventech to do the floors in my house. If you will be running HVAC ducts in the floor assembly, make sure the trusses are sized correctly. I wanted to use metal ducts on my project, but the builder did not check my specification before placing the truss order. Because of this oversight, I had to switch to a flex duct that was small enough to fit openings in truss he ordered. If he had checked the spec, the truss company could have built a taller truss that would have worked just fine.
Deeper floor systems have cost implications beyond a simple comparison with conventional lumber framing. Going with 24" deep trusses can add 2'-6" to the height of a two storey house, and necessitates a more space devoted to stairs.
A good online source for information on substrates necessary for tile is: http://www.johnbridge.com
Hey guys I appreciate you taking the time to respond. While I like the idea of floor trusses they are rarely if ever used by contractors in my area (blown in insulation already scares them) and the depth needed to span anywhere are not really something I see being useful. Malcolm points out a lot of my problems with them. Especially in the house that led me to ask the question. I am maxed out on stairs and they have 10' ceilings downstairs. It would be like climbing a mountain.
Anyway, Martin I was hoping to find someone who is using regular I-Joists at larger spacings with thicker if needed subfloor materials all while providing a good end cost/product value. Sure I could pencil it all out but it is a bit like a DC bureaucrat making regulations for the fishing industry in Alaska. I don't have a lot of practical knowledge of bounce, vibration, cracking, etc. That's why I'm asking people that actually work with this stuff if they had any thoughts about how to go about doing it efficiently and effectively...at the end of the day I'm a draftsman not a contractor.
Spencer,
If you have high ceilings, you want to decrease the depth of your floor joists, not increase the depth. You'd want to have more joists then too, to carry the required loads.
There is no doubt that using open web trusses, makes it quick and fast - it also provides a HUGE area for fire spread. In a fire they fail in less than ten minutes. TGI's aren't as good as solid lumber either, but are better than open web trusses, in the sense that they compartmentalize the fire and smoke at least. You can also buy fire rated TGI/TJI too. I used TGI in my own house and garage to get larger wide open spans. Make sure to use at least 480 deflection limits as Armando said. I'd even go more if you want to reduce bounce.
For tile, you definitely want to avoid 24" OC - in my opinion. I go no more than 16".
I use L/720 for my I-joist calcs. I definitely do not want to increase the depth and that isn't what I am proposing by "bigger". That is a bit ambiguous I realize. I was talking about wider flange joists which slightly reduces the actual distance between joists say when they are BCI (Boise Cascade) 90's and are something like 3.5" wide. The cost is quite a bit more...but I can get other series joists that are not as much of a cost increase that still are 2.5" wide.
Spencer,
If the I Joists all have the same defection specs, It might be worth finding out if the wider flanges affect vibration. The studies I've seen show that the perception that a floor is "bouncy" are as affected by the assembly's vibration characteristics as by deflection. My intuition is that the wider flanges reduction of the span won't do much, but that increased surface area sure makes attaching the subfloor so it is less likely to squeak a lot easier.