Hi, I'm designing (and hopefully will be building soon) a small 14'x24' cabin with a loft. My question is about how the dormer roof is framed. The floor joists act as rafter ties under the dormer, but I feel like the dormer walls above that may not be tied together well enough. Will there be any spreading of the dormer walls with the design shown below? The dormer section is 10 feet long.
(https://i.imgur.com/raV8aKj.png)
(https://i.imgur.com/I0xulqX.png)
There is thrust there, more so with that shallow pitch. You could tie the roof with level ceiling joists or use a ridge beam, the gable wall is a good support at that end. A post to loft floor and either carrying that post down or designing that edge as a capable point loaded beam for the loft edge end. Whatever the stairs are could be part of that bearing all depending. The last tied common rafter couple can be designed as a supporting truss as well. Follow the load paths all the way to foundation elements and ground. You'll need to check the WFCM for the other tie spacings/heights.
A hint on rafter ties...they're actually attached to the rafters.... :)
Not sure but I really like your drawing program. What are you using?
Quote from: Don_P on February 20, 2020, 07:38:08 AM
There is thrust there, more so with that shallow pitch. You could tie the roof with level ceiling joists or use a ridge beam, the gable wall is a good support at that end. A post to loft floor and either carrying that post down or designing that edge as a capable point loaded beam for the loft edge end. Whatever the stairs are could be part of that bearing all depending. The last tied common rafter couple can be designed as a supporting truss as well. Follow the load paths all the way to foundation elements and ground. You'll need to check the WFCM for the other tie spacings/heights.
Good options, thanks. I'm also considering the loft without dormer now.
Quote from: WISteven on February 20, 2020, 12:54:18 PM
Not sure but I really like your drawing program. What are you using?
I used Blender. https://www.blender.org/
That would basically be pretty useless. I'm working on a 16' wide with a 12/12 pitch and full attic , I can just walk down the center under the ridge without stooping.
Quote from: Don_P on February 20, 2020, 04:40:12 PM
That would basically be pretty useless. I'm working on a 16' wide with a 12/12 pitch and full attic , I can just walk down the center under the ridge without stooping.
That's the same conclusion I made after visualizing what the space would be like in my drawing.
I think I'm going to go tall and use scissor trusses. Hopefully one every 4 ft is enough. It's going to be a pain getting them up there.
Updated drawing:
(https://i.imgur.com/lFFnVT0.png)
(https://i.imgur.com/3K1kx0E.png)
Any feedback on time to learn that software to lay out framing like this? Wondering if I should pursue this or sketchup......Or something else?
Quote from: Nate R on February 21, 2020, 10:26:35 AM
Any feedback on time to learn that software to lay out framing like this? Wondering if I should pursue this or sketchup......Or something else?
I used Sketchup for years, but have recently become very determined to find a replacement because it keeps getting worse and worse. After a long search, I've realized that Blender is The Answer for me, but it took some serious dedication to get passed the initial learning curve (Well worth it IMO). If you are familiar with modeling in another software, expect a week or 2 before you become productive again. If you are not familiar with 3D modeling, it could take much longer..
Visually its very nice. I tried it yesterday but will have to wait till I get a new computer. I agree with the motivation, thanks for searching it out.
Just something for those following to look for. As you drive around notice shed dormers on older houses, you'll see by the sagging ridge the difficulty some have had with that detail.
On this plan you currently have a weak gable at each end. Studs must run unbroken from points of lateral support, generally from floor to ceiling, which in this case is floor to roof. At the loft end, balloon frame, unless it faces high wind 2x6 should work fine. At the tall greatroom end I'd probably use 2x8 balloon framed studs. The bottoms and tops of the studs should be very well anchored at each end as well. I've sandwiched steel plate in those tall walls within the studs in the past to help stabilize those tall walls in the wind. And of course reconsider that foundation as you think about wind, there is more guidance on all that in the WFCM as well. First pass thru I'd study tables 2.1-2.18 to get a sense of the forces and resistance required.
https://awc.org/codes-standards/publications/wfcm-2018
Quote from: Don_P on February 21, 2020, 09:07:06 PM
On this plan you currently have a weak gable at each end. Studs must run unbroken from points of lateral support, generally from floor to ceiling, which in this case is floor to roof. At the loft end, balloon frame, unless it faces high wind 2x6 should work fine. At the tall greatroom end I'd probably use 2x8 balloon framed studs. The bottoms and tops of the studs should be very well anchored at each end as well. I've sandwiched steel plate in those tall walls within the studs in the past to help stabilize those tall walls in the wind. And of course reconsider that foundation as you think about wind, there is more guidance on all that in the WFCM as well. First pass thru I'd study tables 2.1-2.18 to get a sense of the forces and resistance required.
https://awc.org/codes-standards/publications/wfcm-2018
Thanks Don, you've given me a lot to think about. I will be studying the WFCM and redesigning some aspects of the walls.
I don't have a lot of flexibility on the type of foundation because the building will be in a spot that can (very occasionally) receive a lot of water, and it needs to be able to flow under the building. The 15 footings will consist of 16"x24" concrete pads with inset "L" bolts to anchor the beams to. Luckily, is it very easy to excavate down to solid bedrock. I will drill and epoxy rebar into it to avoid any possibility of settling.
Here is my latest design. Changed walls to balloon framing and added stairs and an attic on one side for storage. (I'm sticking with 2x4 walls) Comments welcome.
(https://i.imgur.com/s5LD2l6.png)
(https://i.imgur.com/R9nDZV5.png)
(https://i.imgur.com/DUYbKq3.png)
[imghttps://i.imgur.com/rEehJgc.png]http://[/img]
(https://i.imgur.com/e23BKZN.png)
(https://i.imgur.com/R5iyEBm.png)
(https://i.imgur.com/oafVMSK.png)
I suspect your stud lengths preclude 2x4 walls. Since you have lateral support provided by the loft floor and attic floor those would be natural plate points, and stud lengths to check for what size you need to use.
At 14' wide you don't need the center row of piers, 2x10's or larger will work for joists across 14'. You can lose the girders and build them into the floor rims, what you have drawn is not laterally supported to prevent rolling, if you have a triple rim or so (check the girder tables) flush with the floor it provides that support. Settling was not my main concern with the piers, overturning, or lateral is. Generally you can build that foundation as a crawlspace with blowout sections. At a minimum I'd build corner walls ~4' long each way at each corner to provide that foundation lateral.
That latest design is getting pretty tall there. Assuming you are in a windy area, those walls will receive a tremendous force and without a perimeter concrete footing???? Honestly, I really like your first design with the dormers. Have you considered just using a short section of ridge beam over the loft area with the beam carried by the loft to support the ridge? Seems simple and will take out the thrust with minimal design changes. You could leave the cathedral portion the same using the ridge board as you have drawn.