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Floor joists, etc...

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Floor joists, etc...

Postby » Fri Mar 18, 2005 9:15 pm

Hello,

Could someone please explain to me the difference in structural integrity between a floor system built with 2x6 joists vs. 2x8's. I know that modular/stick built homes use 2x10's.

I am mostly interested in knowing how well they each hold up to weight of furniture. We have a couple heavy antique pieces and a set of free weights (alot of weights!). Is this an issue I should be concerned about?

Thanks.
Marie offline

Re: Floor joists, etc...

Postby » Fri Mar 18, 2005 9:32 pm

Depends entirely on how far apart the supports are, the joist spacing, the type of wood the joists are made from, the load requirements and accepted level of deflection.

For example:
A No2 grade southern pine (most common) 2x8, 16"OC, 40psf live and 10psf dead with a 360/ deflection (suitable for tile) can span a maximum of 12' 10".

A 2x6 could only span 9' 9"

Raise the loading requirements and the span gets shorter, raise the allowable deflection and the span gets longer.

Note that this is the absolute limit of the board, you would never want to build to these limits, as the floor would still be quite bouncy.
Pendragon offline

Re: Floor joists, etc...

Postby » Fri Mar 18, 2005 10:34 pm

In laymans terms a 2 by 6 floor joist system that is common in MH's should be on 16
inch centers,not 24.Many builders use that system (24 oc)because it passes HUD code and is cheaper to build.Avoid that when looking for a home.If they do use 2 by 6's make sure they are on 16 inch centers.2 by 8's on 16's is the way to go .(many economy builders don't offer that though).Anything on 24 inch centers is going to produce a lot of floor shake.Any one of the above systems will hold your furniture and weights proving you don't throw them down.So the key thing to remember is 16 o.c. what ever the width of the joist.
Bill Fry offline

Re: Floor joists, etc...

Postby » Fri Mar 18, 2005 11:54 pm

Thank you, Mr. Fry, for your easy to understand explanation. I appreciate it much!
Marie offline

Re: Floor joists, etc...

Postby » Sat Mar 19, 2005 1:33 pm

These numbers are from a simple span table, they have no relevance in a manufactured home with 2 by 6 or 2 by 8 floors.
A simple span table is based on certain assumptions like, no cantilever, uniform loading, joists being supported at their ends. None of these assumptions is valid in the typical joist over frame construction of a manufactured home.
trmimo offline

Re: Floor joists, etc...

Postby » Sat Mar 19, 2005 1:35 pm

Bill is right. 16" centers are the key, 2 by 8's are better and outriggers 4' on center improve the performance for any size joist.
trmimo offline

Re: Floor joists, etc...

Postby » Sat Mar 19, 2005 3:30 pm

True enough, but in relation to site built homes, which have no steel frame, or off frame modulars, which also have no steel frame, span tables and the wood used become rather important. My home came standard with 2x8's, but was automatically upped to 2x10's to go off frame. This allowed them to run only 3 lines of support (perimeter and about half a dozen centerline piers), and yes, the floors bounce, not a lot, but enough. I'm going to have to install 2 more rows of supports to eliminate the bounce and inevitable sagging that will occur over time. According to the span tables, I am at the limit of a 2x10 for being able to install ceramic tile (but not natural stone), assuming they used at least #2 grade, it's fine for carpeting though and that's what I have. Had I known this beforehand, I would have insisted on the additional supports.

Properly supported by a steel underframe, you could build a floor out of 2x4's, so the difference becomes how much insulation you can fit. A 2x10 will hold a lot more insulation than a 2x6.

So, then it becomes a trade off.. you can up the size of the joist, with it's increase in cost and loose the extra supports, or go with the small joist and have the extra supports. I guarantee you, unless you demand and pay for it, you won't get 2x6 support structure on a home with 2x8's (or 2x10's).

All that being said.. it is MY opinion that ANY span that exceeds the width of the board in feet is going to bounce (ie 2x6 = 6', 2x8 = 8',etc)
Pendragon offline

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