Exterior wall height without lateral support

[old_guy]

We are kicking around design ideas for cabins on our property in Tennessee.  We will be building on or very near hillsides, and want to limit building width to avoid having the building hanging over the abyss.  One issue with the narrow width is how it limits headroom in the loft.  A simple way to improve this headroom is making the walls taller, giving more clearance over the loft floor.  This leaves 12 foot walls in the open area, however, so seems unacceptable.

Code limits wall height to 10 feet without lateral support.  We would like to go to 12 feet, maybe a little more.  Many here seem to just ignore the 10' limit and, frankly, I agree that 12' should not be a problem.  They put the limit at 10' for a reason, however, and I would prefer to do this "by the book".

Would an exterior porch roof tied into the 12' wall at the 10' mark be considered lateral support?  Would it also need porch ceiling joists tied into the wall and the porch rafters, or some other method of stiffening, to meet the intent of the code?

If not an exterior porch roof, what options are there for having taller walls and still meeting code?  I have seen suggested a shelf or walkway running along the wall, but would like to avoid that if possible.

I noticed that taller walls are allowed in Canada, not that it matters here.

Thank you.  I learn here every day.

John

[Rys]

Following. I too am in TN.  We're in the planning stage of a 20x40 1 1/2.
Very much want to do the 12' walls.

[ChugiakTinkerer]

If you're subject to a permitting authority they will likely want to see an engineer's stamp.  Absent that, you can do a lot of things to stiffen the wall including 2x8 studs, decreased stud spacing, thicker sheathing, interior plywood sheathing, etc.  I would also look at ways to make it a non-load-bearing wall if possible.  Then code allows you to go up to 20' for a 2x6 wall.

Edit to add: Here is the 2015 IRC - http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%206.html
Table R602.3(5) shows max wall height.

Edit #2: Reading the text in the IRC, there is an Exception 2 in section R602.3.1 that may apply.  What's your snow and wind like?

[Don_P]

A porch roof diaphragm does create lateral support although not as good as a roof on perimeter walls, blocking the seams creates a more rigid diaphragm. A ceiling will improve things considerably, hipping it will improve that more by folding the plates. An inspector would not be out of line asking for engineering, you would not be out of line pointing to the porch roof as lateral support. Going from memory check the code footnotes, parts of those tables revolve around a 12' wide room, which is just about useless most of the time.

An exterior wall stud is what the old engineers called a beam-column or younger ones call combined loading. There is a vertical, or axial load trying to buckle the column and the wind is at the same time pushing on it from the side, which makes it want to buckle all the more. This is also why a horizontal break in a cathedral gable end is such a bad idea.

Whenever things are getting outside of normal also bump the grade up. A "stud" gradestamp means the stick can be as poor as a #3 with #1 edges. A #2 is considerably stronger, a #1 better yet and SS, select structural is the strongest.

[old_guy]

Good info, thanks.

Chugiak Tinkerer - Snow is light.  They get an inch or 2 a handful of times each winter, and it is generally gone in a day.  Last year they had a few inches stay for 4-5 days, and that was considered a lot.

Wind - I wasn't sure (I had no idea) how to quantify, so ran the following report.  FWIW, buildings will be about 1/2 way up a continuous slope which covers a 500' elevation change, and we are in very hilly terrain as opposed to the flat valley a mile or 2 to the north:

ASCE 7-10 Windspeeds
(3-sec peak gust in mph*):

Risk Category I: 105
Risk Category II: 115
Risk Category III-IV: 120
MRI** 10-Year: 76
MRI** 25-Year: 84
MRI** 50-Year: 90
MRI** 100-Year: 96

ASCE 7-05 Windspeed:
   90 (3-sec peak gust in mph)
ASCE 7-93 Windspeed:
   70 (fastest mile in mph)

*Miles per hour
**Mean Recurrence Interval

Don_P - I appreciate your perspective on discussions with an inspector.  I am too prone to accept a statement from "authority" as gospel, especially as I grew up in the 60s, and need a boost to offer resistance.  Our county has opted out of Tennessee's decision to adopt the building code, and I still need to learn if I will need to get acquainted with any inspectors.

Building width is planned to be 16', and length somewhere between 24' and 32'.  We will use #2 or better 2x6 for studs, with wall height around 12', and will sheath walls (blocked) and roof in ~1/2" OSB.  Interior wall covering is as yet unplanned, but we could sheath inside of long walls with OSB (or T1-11) if needed.  Loft is planned to be over 50% to 60% of the cabin interior, with the remainder open to ceiling.  We are planning a center beam (probably double LVL) supported at gables, with an additional support near edge of loft if needed.  There will be a full-length porch roof, 6' or 8' wide, on both long walls and probably one end wall.  Porch roof will be sheathed in the same ~1/2" OSB.  Porch roof framing will connect to wall at around the 10' height. 

So, assuming the inspector is satisfied or we have no inspections, should we be concerned about the taller walls?

- John

[Don_P]

I don't think of it as resistance as much as understanding. Most of the provisions in the code, especially structurally, revolve around understanding the forces on the building and providing adequate resistance. If the builder has designed the building but cannot show through prescriptive methods or some pretty simple engineering that he is providing adequate structure. Then the inspector is supposed to ask for engineering. Generally even when I feel like I've got it figured correctly but the inspector asks for that stamp, I can see and respect where he is coming from.

County health dept at a minimum for permits, ask what else when you're there or at the county admin's office.

You've taken half the load off those tall walls with the structural ridge, I'm comfortable FWIW. There will be at least one interior ridge support post. In the greatroom the gable end ridge support post can be tall and slender enough to have buckling problems out of plane. A beam from loft to gable post would help laterally reinforce that very tall wall. You can also make that column a bump out feature in the room. I've wrapped them in oak or fir, or drywall, or run a catwalk out to the gable.  There is a column calc here; timbertoolbox.com , those are just my calcs so they are worth what you pay but it is another way to check your studs and post.  Morning is light too  you'll need a snow load weight in psf.

Chapter 3 has some windspeed info for slopes, there is also a section there on foundations adjacent to slopes and some basic snow load numbers.

[ChugiakTinkerer]

...

Going from memory check the code footnotes, parts of those tables revolve around a 12' wide room, which is just about useless most of the time.

...

Your memory is spot on Don_P!  The exception I cited says

Code Select Expand


2.  Where snow loads are less than or equal to 25 pounds per square foot (1.2 kPa), and the ultimate design wind
speed is less than or equal to 130 mph (58.1 m/s), 2-inch by 6-inch (38 mm by 14 mm) studs supporting a roof
load with not more than 6 feet (1829 mm) of tributary length shall have a maximum height of 18 feet (5486 mm)
where spaced at 16 inches (406 mm) on center, or 20 feet (6096 mm) where spaced at 12 inches (304.8 mm) on
center.  Studs shall be minimum No. 2 grade lumber.


With that 6' tributary length, if you are looking at trusses or a non-structural ridge, then a 12' room is what you get.  But with a structural ridge, isn't old_guy looking at a roof tributary length for the wall of 4', plus whatever overhang?  The ridge beam carries half the width, and each side wall carries a 1/4.  So if the ultimate design wind speed does not exceed 130 mph then doesn't the exception apply?

Granted, if the county hasn't adopted the IRC then there won't be an automatic pass, but it seems like a good starting point for a conversation with an inspector.

[old_guy]

Wow.  A link to the IRC, references to pertinent sections, and examples to help me understand what I am reading.  Thank you both for taking the time to help me reduce my ignorance.  I will take this opportunity to study, and to hopefully learn what is good and why.

The wall-height issue is fairly common here.  I hope that this discussion is valuable for more than just me.

Thanks again.   I have reading to do.

Oh.  Our snow load is 10psf. 

- John

[Don_P]

Oh.  Our snow load is 10psf. 

Cool, your snow is really light. I think you'll read somewhere in chapter 3 that wind will control in that instance at a minimum of 20 psf for live load. Confirm that and add to it 10psf for dead load and the combined roof load would be 30psf. That should get you on the road for figuring ridgebeam, post loads etc.

It is a bunch of dense reading, holler if you hit muddy spots. I've had the inspector reference pertinent sections, tap his foot and shake his head on a number of them... all at the same time