4x6 beam spans when used as loft joists

[hnash53]

I am going to build a 16' wide addition on to my cabin, sort of based on the 20x30 plans.

I will have a large loft at one end of the addition.  I want to use 4x6 beams as the loft joists instead of 2-by material.  The loft flooring will be 2x6 t&g.  

Can the 4x6s span 15'6"?  And at what spacing?

Thanks.

Hal

[glenn-k]

From the rough sawn span tables in "From The Ground Up" by Charlie Wing,  it looks like 30 psf live load - 7.5 dead load at 16 feet on 24" centers with anything stronger than Northern White Pine should work, Hal.  

Dimensional lumber S4S 30 psf live - 5 dead  - 15'10"span -  24" centers with Doug Fir - others are a bit short.  With 2x6 walls you are spanning about 15' -I think that is how it's figured.

[blackdog]

Here is a useful span calculator tool I found.  Although it doesn't do 4x material.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

[peg_688]

 I don't have a span chart with me but as Glenn's chart shows it barely makes it. If your going to use that loft I'd go to at least a 4x8, in the long run just making it and doing it a little better won't cost much more.

G/L PEG

[John_M]

I used 4 x 12 hemlock beams on mine.  I spaced them at 48" and used 2 x 8 T & G flooring.  One thing I did was use true 4 x 12 beams.  The mill rough cut them at 4 1/2 x 12 1/2 and then planed them down.  I felt this would give them a bit more beef.  The span was just over 17'.  (My original plans called for 4 x 10 but I went bigger).

You might want to do that with your beams and then have them planed.  Rough cut are nice but hard to stain.

I am not sure if a span table gives the rating of an actual measurement or not? (in other words, a 2 x 12 is really 1 1/2 x 11 1/2 or so)

[glenn-k]

The 4x span charts are hard to find.  That one is from a great book recommended by John as one of his favorites.

The first number I posted above is rough full size lumber.  The second is dimensional lumber - nominal size planed 4 sides - usually about 3/8 inch smaller all around. -Say about 3 5/8 x 5 5/8  Some of the new stuff is even 1/8 less each way or so.

[littlegirlgo]

I like the chart that blackdog posted - but what is deflection limit? And how would I figure my snow load and dead load numbers?
Thanks  

[glenn-k]

Your county or other building department can tell you the snow load and other loads for your location.  I don't know exactly how the other loads are figured.  Live load is snow etc.  Dead load is the materials - roofing - wood etc.  Architect and engineer stuff I guess.

[blackdog]

littlegirlgo...

Chapter 2 "Engineering Basics' of Wagner's framing book explains deflection.  I'm no expert, but here is my take on it.  Deflection is the amount of sag (lets say in a floor joist) that a person finds comfortable.  Looks like 1/2" deflection is commonly accepted.  So if your span is 15', selecting L/360 (L= span in inches) would give a 1/2" deflection limit.  L/180 would result in a higher deflection limit (1") and L/720 in a lower deflection limit (1/4").  Notice how the maximum span changes for the same material using different deflection limits.  Anyhow, with all that said, I just let it default to L/360.

[jraabe]

Yes, on long spans deflection determines the size. Most codes require 1/360 for joists but on spans such as the 19' used in the 20' wide universal I use 1/480 to give a stiffer feel to the floor. In either case there is plenty of strength.

[hnash53]

Thanks for all the replies.  

I am building 16 feet wide.  At one end I'll have the loft.  

But at the other end it will be open to the peaked roof.  I want to use the 4x6s to tie the walls together so the roof rafters don't push the walls out.  The 4x6s won't have any load on them.  They should work for this right?

Plus I'll install collar ties on the rafters.

[Amanda_931]

There are certainly tables for ceiling joists--even if they all they hold up is drywall or light-weight paneling or something.  Probably going to vary if the roof pitch is very shallow.

The one here is for 10 psf live load, 5 psf dead load.  (they do have another, for double those)  Not a truly horrible .pdf file.  The index is the first link.

For instance, #2 yellow pine won't go 10 feet if there are two feet between them.

http://www.southernpine.com/spantables.shtml
http://www.southernpine.com/pdf/table17.pdf

[glenn-k]

They are marginal with full load so should be extra heavy for no load.

[jraabe]

If I understand correctly you would be using the 4x6 as a tension member to triangulate the roof rafters (kind of a low collar tie at the plate). A 4x6 would work fine as would a 2x6 (or even a steel rod) - connect this well to every other rafter (4' o/c). Since this reads as a tie beam its size is more of visual concern than structural. Beefier looks better.

See this note to self on the Builders Cottage...


http://www.countryplans.com/vc.html

[Amanda_931]

I was wondering, even after I found ceiling joist tables, if the only force acting on that was tension.

That is certainly the way buildings collapse--walls head out, roofs sag.

[hnash53]

I had planned to connect the 4x6s to the walls via a ledger board.  I hadn't thought that I would have to connect the 4x6s to the rafters themselves.  I didn't do that in my 16x28 cabin.  I used a ledger board to connect my loft joists to my 10' high walls, but the ledger board was installed at 8' high to give me 2' walls in my lofts.

In my addition, I was going to install these 4x6s at the full height of the walls at the open end of my addition, even with the height of the top plate.  Seems like this would be adequate to keep the walls from being pushed out.

I plan to install 2x6 collar beams, probably on every roof rafter to prevent sag and the pushing out effect of the weight of the roof.

Sound OK?

[peg_688]

Could you put the 4x6's right on the top plate and tie / nail them right to the rafters at / near the seat cut??  That IMO would be a more solid tie point as a opposed to a ledger  and hanger situation, cleaner looking as well. Every other set would be fine if they are only in tension , no floor or ceiling load.

The collar ties , IMO, would be good , generally every OTHER set of rafters is fine , the flatter the roof (less pitch) like a 4/12 or less might require the collar tie on every set.  But if you like the LOOK that  a tie on every one would give you it would be fine',
 
G/L PEG

[glenn-k]

Seems that ledger and hanger would still allow some hinging out as hangers are good for down load but not much good for tension load.  Tying into the rafters would make a positive triangle braced truss with no hinging.

I don't know about the collar ties - they may take care of the problem too - just thinking about where the pushing forces are - bottom of the rafters want to push out under load.

[jraabe]

When you use a ledger board to attach the beam/collar tie, be sure to use hangers and the fat nails provided with it. These nails are designed to provide shear strength as needed for the outward wall forces.  

[hnash53]

John,

Fat nails that come with joist hangers?  Never heard of such a thing.

However, the hangers have a hole to toenail through and I have used these before when I hung loft joists in my cabin.  Is this what you are referring to?  If so, they work great!!  My cabin is rock solid.

[glenn-k]

Simpson has some special fat nails for their hangers that re extra heavy - not as long as I remember.