How High Can You Build an Attic Stem Wall For Stick Built Rafters
In an unvented attic you can order high heel trusses in order to get enough insulation depth over the tops of the exterior walls but what is the best way to build a “high heel” stick framed attic roof?
Now, I plan on building a second storey “floor” on top of the main floor walls (with sheathing on top of the joists to act as the air barrier) so if I want about 20″ of insulation over the tops of the exterior walls is it best to build a 20″ tall stem wall on top of the attic floor at the eaves or is there a better way of elevating the roof rafters?
The one thing I worry about is the “hinge point” that would exist between the bottom plate of the stem wall and the sheathing on top of the second floor deck. Could outward force from the roof rafters cause this little wall to push out at the top and if so how could I brace it?
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Replies
rockies63,
The height of the wall doesn't matter at all. It's the same at 10" as a full storey wall.
The only stick-framed roofs that don't need to deal with the lateral loads from the rafters are those framed directly on (and attached to) the attic floor. For all others you need to either:
- Use rafter ties in the bottom third of the span.
- Use a load-bearing ridge beam.
There are two other options as well.
You can build taller main story wall that includes the height of raised heel and balloon frame the ceiling lower down from where the rafters would rest. This avoid the hinge point as the studs are now continuous. Air barrier continuity is harder though.
The other option is to extend your ceiling joists past the wall to your facia board. Instead of setting your rafters onto the top of the wall, set it onto the end of the ceiling joist. You can then use a piece of 2x on edge ripped to height right above the wall to carry the vertical load from the roof down. This is how commonly older homes here are framed and you can get a lot of insulation into the space if you have reasonable overhangs.
Akos,
Good points. I should have said "the two most common solutions".
sorry for necro, but i think about something and google it to see how insane i am; basically was wondering how to do a double stud wall with 'stick built' framing -- no trusses, but still have a 'raised heel truss', and thought how one room in my house is framed. which has an 8' ceiling, 10' wall overall, top plate is at 10'. joists run perpendicular to this section of the wall, rafters are sitting on the top plate.
so the question is; how would a wall like that work for the entirety of the house. specifically, how would ceiling joists be done for whats running parallel with rafters. for a 1 story home specifically.
https://i.imgur.com/LrzVRZd.png
was something i visualized in my head and managed to find that photo online. but could the ceiling joists be attached to the ledger with hangers, instead of sitting on top? that'd avoid the notching at least..
but as you noted the air sealing details are annoying. for my house i had to cut a bunch of foam board to caulk and spray foam to seal up the wall from the attic. it was completely open from the attic, could reach directly into the exterior wall!
any input is appreciated. i'm not building a house, this is just educational and i need better hobbies.
Air sealing can be done at the outer face of the walls and roof, by taping all the plywood/OSB panel joints. That avoids problems with all the interruptions to surfaces on the inside of a house. But as with any solution, it needs to be understood in the context of an entire assembly design. I won't go into all that here.
Yes, you can certainly balloon frame the walls to avoid the problem of a hinge. However, you are now attempting to use the upper floor as the bottom chord of your truss. So your rafters need to be attached to your top plates for both bearing and shear, in order to transfer the horizontal load component into the wall. And then the studs need to be connected to the upper floor framing to transfer the load as well. If you were building something like this your building official would probably want to see an engineer's stamp on the drawings.
An alternative was mentioned above by Malcolm. Use a ridge beam. Then you're not dealing with trusses at all but rather simply supported rafters. This works especially well when you have an easy way to transfer loads down to the ground under your ridge. That way the ridge beam doesn't have to span very far. I've used that strategy in a few designs.
Here's a short article describing how ridge beam framing works. https://www.jlconline.com/how-to/framing/ridge-beams-vs-ridge-boards_o