Framing Question on plans

Started by Rocksteady, March 10, 2012, 11:22:07 AM

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Rocksteady

 ??? Although I've built several interior walls, I've never built a house from the ground up.  In the plans I have (a 14' wide 26' long structure) it shows the sill, Rim joist, and bearing walls a half inch from the edge of the foundation.  So instead of being 14' wide the framing is actually only 13'11" wide.  Why on earth would this detail be in place?  I'd be reluctant to forge onwards without understanding why this is the case.  To further confound me, the wall framing lengthwise appears to terminate at the edge of the foundation (the wall is 26' feet long not 25'11").  What the heck!?  If someone could please explain this to me, I'd be thrilled!  Other details are as follows: The plan calls for 3/8 inch CDX, 1/4 furring strips behind Cedar lap siding, but neither of these really make sense either, especially considering length wise the wall terminates with the foundation.  Please explain!  I've also ruled out this being some sort of mistake because it's such a precise error to make.  Thanks for your help!

PEG688


  I've built a few kit houses that came as wall packages , we provided the floor platform , the walls came sheathed and the connection between the wall bottom plate's and the rim joist was a series of LTP5 plates

  http://www.strongtie.com/products/CFS/LTP5.asp

  This plate nailed to the bottom plate thru the wall sheathing and directly to the rim joist with no sheathing on it.

   As far as your plan if you're stick framing the place the only reasons I can see to stop the framing back 1/2" is to allow the wall sheathing , once applied,  to flush up with the foundation,   the lathe strip would be a cant strip to flare out the first course of siding , which would  normally be held down 1 " below the top of the concrete.

  Could also be some detail to make attaching any wood decks slightly easier  in theory.

Are these plans from John at Country Plans?


  You may be picking the pepper out of the fly poop worrying about this 1/2" set back.         
When in doubt , build it stout with something you know about .


MountainDon

Ditto on the thought/reason for the 1/2" setback.....

I wonder about the
Quote.... 3/8 inch CDX, 1/4 furring strips behind Cedar lap siding,
The 3/8 seems thin for sheathing. 1/2" OSB is usual, and probably less expensive than 3/8 ply.... stronger too.   

Is the 1/4" furring strip that is referred to vertical or horizontal at the bottom piece of lap siding? If vertical it seems thin to me too.  ??? 
Just because something has been done and has not failed, doesn't mean it is good design.

PEG688

Quote from: MountainDon on March 10, 2012, 01:20:44 PM


I wonder about the    The 3/8 seems thin for sheathing. 1/2" OSB is usual, and probably less expensive than 3/8 ply.... stronger too.   

Is the 1/4" furring strip that is referred to vertical or horizontal at the bottom piece of lap siding? If vertical it seems thin to me too.  ???

Good point , maybe a rain screen wall ??  Seems to thin for that, water would just bridge over a 3/8" gap?? Agree on the 3/8" ply for sheathing as well , 7/16" OSB , or 1/2" CDX would be more what I'd expect to see spec'ed. 
When in doubt , build it stout with something you know about .

MountainDon

Yeah, the wall detail sounds like a rain screen. I can't find the info I know I have saved or bookmarked someplace(didn't look real hard). There is a minimum spacing for the air gap. I think the minimum is 3/8", but not sure. With too little spacing it is possible for water drops to bridge from one surface to the other and just sit there. 

No mention of a weather resistive barrier. (building felt or whatever. Usually you'd have the sheathing, then the WRB, then the furring and then the siding. Ripples in the WRB will reduce the air gap. Better to go larger, like 3/4". Also larger will actually allow for air movement, ventilation. Narrow gaps don't move air as well. European rain walls call for one inch air gaps.
Just because something has been done and has not failed, doesn't mean it is good design.


Alan Gage

I think the 3/8" rainscreen comes from Building Science. If you read the footnotes they go into a little more detail:

This 3/8-inch gap is very, very conservative. It can be smaller. Just how much smaller most of us are not sure about. But with a 3/8-inch gap I have never, ever, anywhere, known of anyone to have a problem related to drying if nothing else fundamentally stupid was not also happening. How do I define fundamentally stupid? How about a missing flashing?

There are "draining housewraps" and "draining building papers" that are about 1/16 of an inch and even smaller that work very well. Even two layers of building paper work pretty darn well. So why not just specify 1/16 of an inch or two layers of building paper? Ah, it depends on the "load" – a fancy word that means sometimes we get lots and lots of rain and when things get very ugly 1/16 of an inch or two layers of building paper does not always cut it. Low-rise houses without much of an exposure easily work with 1/16 of an inch gap or two layers of building paper. But taller buildings with big exposures and no overhangs and lots of rain and wind the 3/8-inch makes me sleep at night. Sometimes someone somewhere has to throw some judgment at the problem.


Here's the whole article:

http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh/?searchterm=mind%20the%20gap

Alan

MountainDon

Thanks Alan... that may be what I remember 3/8 from.
Just because something has been done and has not failed, doesn't mean it is good design.

Rocksteady

Building paper was mentioned in the plans; I just didn't write it down because it's thickness is negligible and therefore, not relevant to my question.  As far as the thickness of the sheathing is concerned: I think given the very small size of the house, the designer assumed that the extra 1/8th wouldn't matter?  I'm not sure, and it's an easy "upgrade".  What really interested me is the conversation about the furring strips.  I have no problem going up to 3/8ths thickness especially if that's what people agree is best.  The plans I purchased are from Tumbleweed tiny house company.  They seem to specialize more in portable homes, and aesthetics rather than best building practices and energy efficiency.  That being said, I love the aesthetics of the plans I have, and I can always beef up the specs for efficiency and best practices.  As of right now I think I want to add 1.5" foam sheathing over the CDX.  Someone said CDX isn't as strong as OSB? is that true?  CDX looks stronger to me, but that means nothing.  I love this website as a resource!!  Thanks for sharing your invaluable knowledge!

MountainDon

QuoteTumbleweed tiny house company.

That explains the reduced thicknesses. They are going for light weight. If this is not going to be buiult to be primarily a trailerable/towable cabin go for the 7/16 OSB for sheathing. I believe modern OSB is better than CDX plywood.

If you want to add foam on the exterior for better insulation, that is a good idea, IMO. What is the framing; 2x4?

What type of siding? What climate, rainfall... ?
Just because something has been done and has not failed, doesn't mean it is good design.