Question: Pros and Cons of 12' studs vs 8'+4' walls for a 1.5 story?

[HoustonDave]

I purchased the Builders Cottage plans and I am building a variation on the 1.5 story plan, making it 16' wide x 24' long.  We plan on doing 2x6 walls for more insulating capacity. 

We want to have 4' wall height on the half story to increase headroom without the requirement for steep pitch.  The loft will run about half the length of the building.

Would 12' studs be significantly stronger than doing a standard 8' wall, then 4' wall? 

Considering that the floor of the loft will span half the length of the building, this seems like 8' + 4' walls would more strongly brace the walls against outward forces (at least where the loft is because the flooring would fully sit on top of the 8' wall).

Alternatively, running 12' studs seems like it would be stronger in the areas where the loft was not present.  I expect 12' studs would be more expensive.

Can I get some feedback on this?  Pros and Cons?

Thanks!

[HoustonDave]

Just finished reading this discussion.

http://countryplans.com/smf/index.php?topic=9129.0

First, what I'm asking about is a 12' balloon construction vs 8' + 4' platform construction, using proper terminology.

Second, the original design does not support 12' wall height (max 10') because as John pointed out:

Unsupported wall height is limited to 10' in cathedral areas and taller walls require an engineering review if they are longer than 10' (such as a 12' high wall in a 14' wide living room). I understand this is to be a separate issue from how you support the roof load. The concern is lateral loading on tall walls.

Thus if you do a full length loft all the walls are braced by the floor system and the lower floor could be 8' or 9' or even 10' tall, but you couldn't have the full 12' tall wall open in the cathedral living room. This complication was one of the main reasons I didn't design the walls with 12' studs to begin with - I wanted the flexibility to have open cathedral spaces where the owner desired.

What it comes down to is, to do 4' knee walls on a loft (and therefore have 12' walls in the open area) you have to either give up having a full cathedral ceiling in the open area (you could do a coffered style ceiling) OR you have to incorporate an expensive or difficult ridge beam to remove a portion of the load from hitting the sidewalls at an angle (causing spreading).

All this is spelled out in the above link.  I didn't catch that thread the first round of searching.

[bayview]

   Using 12 ft studs would be "balloon framing" construction . . .    8 ft with an additional 4ft on top of the second floor is "platform construction".    Both have advantages and disadvantages.   

   Balloon framing would be more ridged.   The platform framing will have a tendency to "hinge on top of the second floor.   (My opinion)
   
   Balloon framing requires a ledger board.   The ledger board fits into a notch in the exterior studs.  The floor joists extend over the ledger boards and are then nailed or bolted to the exterior studs.   

   The disadvantage is in case of fire.   A fire would easily move from the first to the second floor within the wall.   I feel this danger could be reduced by installing a horizontal "filler" on top of the floor joist within the wall, and between the wall studs..   This 'fire block" would reduce the chance of fire going upward.   And, give a "nailing surface" for drywall and moulding.

   There are special requirements for the thickness of the ledger board.

   ("Extra filler indicated in red on the platform diagram")

/.

[bayview]

What it comes down to is, to do 4' knee walls on a loft (and therefore have 12' walls in the open area) you have to either give up having a full cathedral ceiling in the open area (you could do a coffered style ceiling) OR you have to incorporate an expensive or difficult ridge beam to remove a portion of the load from hitting the sidewalls at an angle (causing spreading).

   Wouldn't . . .    this work?      Other opinions please . . .

   Extend the ledger board into the area with the open cathedral ceiling.   Place a board across the open area every 4 ft and nail or bolt to the exterior studs.   This would help eliminate lateral movement.

   Of course, properly installed collar ties would also aid in the lateral movement . . .

/.     

[MountainDon]

.... collar ties would also aid in the lateral movement . . .
 

Terminology can be confusing and misuse leads to even more confusion.   A collar tie is the connector in the upper third of the rafter, from one rafter to the other. Prevents separation and/or lifting of the roof structure under extreme wind conditions. Wind up the side and over the peak causes lifting forces much like an aircraft wing. Not really designed to prevent lateral movement. A rafter tie, on the other hand, holds walls inline.

[MountainDon]

HoustonDave... so the idea is to raise the sidewall height and build the roof with rafters, not trusses? A truss like a scissors truss can provide headroom and does not create issues with outward forces on the sidewalls.

Just a thought.

[HoustonDave]

Bayview, thanks for the additional information and the great visuals. 

We will absolutely be ensuring there is good fireblocking at the 8' mark throughout the cabin.

Don_P posted some very good analysis (see my link in my first response below) about the impact of increasing wall height and having alternating rafter ties spaced every 4' instead of every 2' causes a doubling of the stress on the connection between each tie and the wall.  That may be acceptable for smaller spans and 8'-10' walls, but probably not a good idea for 12' walls.  Basically, you are really pushing your luck if you have 12' walls with a cathedral ceiling or any rafter tie spacing less than 24" centers.  And having rafter ties every 2 feet kinda defeats the purpose of having a cathedral style ceiling.

MountainDon, thanks for the clarification and the graphic as well.  Speaks a thousand words.  Right now, wife and I are looking at either a coffered ceiling by moving the rafter ties to the upper limit of the bottom 1/3, or as you suggest going with scissor trusses.

[John Raabe]

You have a pretty good analysis of this. Let me summarize:

• If you have engineered trusses for the roof you can platform frame the loft walls. See if the truss company can produce the type of ceiling shape you want in the loft and great room.
• A 12' tall unsupported wall in the great room (with cathedral ceiling above) cannot be longer than 10' without tension ties between the two walls. These ties can be either engineered, or prescriptive spaced @ 2' o/c in most current code areas - (used to be 4'). If this wall height can be kept to 10' or less, ties are not needed. This assumes the roof loads are not thrusting outwards.