Figuring out a loft with what's on hand

Started by Jackson Landers, April 02, 2008, 11:40:44 AM

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Jackson Landers

I've been working on a little 10'x18' building out back as a warm-up before building the 20x30 2 story house (I already bought plans from John last year).  This is going to be used primarily as a workshop but I want to build in some flexibility to do other things with it in the future.

I'm about halfway through with sheathing the walls and in the next 2 weeks am going to have to decide for certain what is going on with the ceiling and roof.  I'd rather like to put a little loft up there. I'm planning on a 10 in 12 roof, so there won't exactly be a lot of head room in a 10' wide space, but it would be nice to have the loft for storage and such anyhow.

Here's my problem: can I build a structurally adequate loft floor, good for the typical residential load of 40 lbs per sq ft, using stud-grade 2x6s that are 12 feet long?  See, I happen to have a couple of dozen of those 2x6 studs on hand and would prefer not to have to spend money on other lumber if possible.  Obviously I'd be cutting the studs down a bit. 

The American Wood Council span tables I looked at suggest that at 16" on center, this lumber would not be adequate for floor joists in a residential application.  However, am I being too conservative by considering a normal residential live load for a tiny loft area in an outbuilding? If I double up the 2x6s under the loft then would I be ok?

Note that since this is such a small building, I'm not dealing with any code or inspection issues. Just trying to do a good job.
Albemarle County, Virginia

MountainDon

#1
Whether or not you can get away with this would depend upon what you intend to store up there. Only you know that. Is it more likely to be boxes of old clothes or lengths of steel? Big difference in weight.

Examples of code-prescribed deflection limits and live load values are:
    * Living space floors L/360 & 40 psf
    * Bedrooms and habitable attic floors, lofts L/360 & 30 psf
    * Attic floors with limited storage L/240 & 10 psf.

The AWC calculator comes up with an allowable span of 9 ft 11 inches for 2x6 SPF, stud grade, L/240, 20 psf live, 16" OC. That's fine for a typical attic with storage.

Remember, joist span is measured between the inside bearing surfaces, not the outside measurement of the walls. Also note the AWC calculator specifies the minimum amount of the joist that must be supported. In the example above it's only 0.31 of an inch. Of course more is fine/better.

AWC Article on Understanding Deflection & Loads
Just because something has been done and has not failed, doesn't mean it is good design.


Jackson Landers

Mountain Dom,

Thank you, this is very useful.  I don't even know exactly what I'll store up there. A bit of lumber? Rarely used specialty tools? I could see making it into a sleeping loft, since I'd like the building to double as back-up housing in case of emergency.   

Good point about the outside versus inside measurement. It will probably be fine with those joists placed 16" OC. I suppose that just to hedge a bit, I could take any extra 2x6s and double up every other joist. With bridging, that should get me into very safe territory.
Albemarle County, Virginia

MountainDon

#3
The 9' 11" is the outermost limit going by the calc. If you were expecting to load the floor/ceiling to the calculated limit it would be a good idea to increase the strength. Doubling or going to 12" OC, depending on how much extra material you have on hand would work.

If this was a floor with living space, loft, and so on you might find it too bouncy with being at or near the limit. It's usually better to avoid pushing those limits, IMO.

Of course for those reading this, it should be remembered those first calcs were on stud grade lumber, because that was what's on hand. Better grades would give better results / greater usable spans.

Just because something has been done and has not failed, doesn't mean it is good design.