Joists for framing and other questions...

[John_M]

I am having an cabin framed in for me by some Amish Builders.

I have to put in the foundation and flooring.  The cabin will only be 18x32.  I am going to use a post and pier foundation.

Since I will be placing skirting around the foundation and the stapling Tyveck on the underside before insulating, do I need to use PT joists, or can I use regular construction lumber.  Quite a bit of cost difference I would think.

I was also going to put in three parallel manmade beams that would be 9 feet apart (18 foot total width) and then run either 2x8 or 2x10 between them at 16" oc.

Could I just use two beams and then run engineered I-beams the 18 feet?  

I will be putting in the fundation this summer, so I am starting the planning process now.

Thanks for the help eveyone!

[RAB]

John:
If you're using a plywood T&G style subfloor the perm rating is quite high and you shouldn't need a secondary vapor barrier if all the edges are glued (good practice to do anyway).
My idea for the floor is split the 18 in half so your joist span would be about 9'. At 16 OC a 2x6 will work just fine. Actually a 2x6 will also work at 19.2 OC just be sure to use #2 and better- be aware that 19.2 insulation is sometimes hard to find at big box stores.  A 2x8 will work for 24 OC but if you're using ¾ plywood this thickness will seem at bit springy unless your built like my 7 year old daughter.
For a cabin I would rather hand pick the joists than buy TJIs due to cost.

Ryan B

[jraabe]

John:

You should not need PT joists assuming they are at least 12 above the soil. The only other reason to go PT for the joists is if there is a real termite problem (then all the framing should be PT).

If you choose to go with engineered I-joists, an 11 7/8" GPA-40 @ 16" o/c will span over 19' at 1/480 deflection which should give you a solid floor.

Go to this site: http://gp.com/build/literature.aspx#Engineered%20Lumber and download the 3 page floor spans chart (PDF) or the Engineered lumber product guide to learn more.

[John_M]

Thanks for the info!!!  Is there any fear in using the engineered I-beams outdoors?  They would be up at least 12" off of the ground and then skirting will be placed around the foundation.

Also, can the beams be cantilevered?

[John Raabe]

I-joists can be cantilvered. They should not be used exposed to the outdoors as would deck joists. You don't need to worry about a little rain on the framing but unless you can get PT treated I-joists you won't want them exposed to the elements over a long period of time.

Visit here for more info on I-joists and engineered lumber: http://gp.com/build/ProductGroup.aspx?hierarchy=pc&pid=1063&cat=Engineered%20Lumber

[Mike]

Speaking of joists, is it stronger to lay the joists on top of the beam or use joist hangers to support them?  Simpson seems to have a lot of choices out there.

What are the benefits of one over the other??

 Always wondered about this.

[John Raabe]

Having a joist sit on top of a beam substantially increases its stiffness and usually its strength. The backspan on the other side of the beam helps balance the load.

[Ryan B]

Mike:
Also you can have joist set on beam or ribbon wall and still use hangers.
You can adjust elevations by either hanging or setting on top of plate in a concrete perimeter style foundation.
Costs for joist hanger less than $1.00 ft., more time for hangers, lower craw space and a lower entry floor level.
Top of plate rim joist costs $2.70 ft, less labor, taller crawl space, and higher entry floor level.
Have done many of both – it's a wash either way.

[tristan]

sorry to dig this old topic up but I wanted to piggyback on Ryan B's comments regarding the pros and cons of hanging the first floor joists versus sitting them on top of the sill. I understand the influence on height and dropping he first floor joists into the crawlspaces seems like a way to make our two-story a little less "tower-like"

However, I'm concerned that hanging the first floor joists will make it hard to run electrical/plumbing through the external walls.  Basically you'll have to bust through the concrete foundation to access the exterior wall cavity.  Is this a serious concern or not really that big of deal.     

[alex trent]

You really think 8" joists on a 20 foot high house map a visual difference?

Just seems to me that putting them on top is a nice simple way to get a strong base.  No hangers and all that mumbo jumbo (and I do love Simpson stuff myself)