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StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 7072
Experience:  Structural Engineer
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Gambrel house. Expand 2nd floor bedroom space into adjacent

Customer Question

Gambrel house. Expand 2nd floor bedroom space into adjacent attic. Attic floor structure is inadequate. Area is 22' wide with 2x6 floor joists 16" OC that are unsupported below. The structure is a truss -type arrangement which includes members running
from upper rafter down to joists, which would need to be removed. Upper rafter pitch is 5/12 (22.5 deg) and lower rafter pitch is 29/12 (67.5 deg). Knee walls are located approx 3' in from exterior walls and run from joists up to where the two rafters join.
Wall height is approx 7'-10". 2x6 collar ties run across the top of the two knee walls. I am looking for a solution to remodel this structure (truss?) to make livable space with a floor to match the level of the adjacent space which has a floor built with
2x10 joists. This would be done with no beam or wall below. Jack
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  StructuralEng replied 1 year ago.
Hello, how are you doing? I'll be happy to assist you.
Expert:  StructuralEng replied 1 year ago.
Can you post a sketch?Even 2x10 (or 2x12 for that matter) are not adequate to span 22' for occupied space.
Customer: replied 1 year ago.
Let me know if you need details or have initial questions
Expert:  StructuralEng replied 1 year ago.
Can you post a sketch?
Customer: replied 1 year ago.
I understand the 22' span. The idea is to use a truss arrangement, sistered joists, lvl, I joist or whatever for a creative solution
Expert:  StructuralEng replied 1 year ago.
Are you able to post a sketch showing the arrangement you have?
Customer: replied 1 year ago.
Yes I can do that. I will take a little while.
Expert:  StructuralEng replied 1 year ago.
No problem. I will check back periodically. It will help me provide you the best answer.
Customer: replied 1 year ago.
Here is a basic sketch
Customer: replied 1 year ago.
PLs let me know if the sketch came through ok.Jack
Expert:  StructuralEng replied 1 year ago.
Yes. I see the sketch. Thank you
Expert:  StructuralEng replied 1 year ago.
The upper 2x6 can stay, but the lower 1x8s need to go away, correct?
Customer: replied 1 year ago.
Correct
Expert:  StructuralEng replied 1 year ago.
The vertical 2x6s (wall) at the change in roof slope and also stay, correct?
Customer: replied 1 year ago.
That's correct but they are 2x4 walls.
Expert:  StructuralEng replied 1 year ago.
You need 1.75" x 11.875" 2.0E LVL at 12" on center to just barely work.
Customer: replied 1 year ago.
Is it correct that the 11-7/8 would stand on its own without a truss structure? If so, what I'm looking for is a way to restructure a truss arrangement so the bottom cord can be 9-1/4 to 9-1/2.
Expert:  StructuralEng replied 1 year ago.
That is not a truss structure and the lower joists are helping to support the roof. There is no truss behavior with the lower joists
Customer: replied 1 year ago.
Ok then I would need to remodel it in to truss structure
Customer: replied 1 year ago.
By the way, what loading are you assuming on the floor and what would be the deflection?
Expert:  StructuralEng replied 1 year ago.
There is not a way to make a truss out of that and have the open space you want. You need the joists to be flexural members. Trusses are composed or members loaded primarily with axial load only.
Expert:  StructuralEng replied 1 year ago.
I use 40 psf LL and 15 psf DL for floors and the roof.
Expert:  StructuralEng replied 1 year ago.
That's right at span/240 deflection
Customer: replied 1 year ago.
That would be too much bounce I think. What about the idea of adding members with gussets as necessary to actually make a truss out of it, even if iit means losing some interior space?
Expert:  StructuralEng replied 1 year ago.
Trusses require diagonal members at every joist to resist all forces as axial members. When loads are transverse to the members, they resist load in flexure.
Expert:  StructuralEng replied 1 year ago.
You can't have a truss AND an open space. trusses require diagonals that eat up the space.
Expert:  StructuralEng replied 1 year ago.
1.75" x 14" LVL gets deflection down to span/400.
Expert:  StructuralEng replied 1 year ago.
About 0.65"
Customer: replied 1 year ago.
There are "gambrel attic" trusses that do just that and that is what I'm talking about. I'm afraid I ended up in the wrong place here so I need to sign off and say thanks for the effort.
Customer: replied 1 year ago.
I am going to follow tthe just answer procedure to find another expert.
Customer: replied 1 year ago.
Please make my question available to other experts.Thanks again
Expert:  StructuralEng replied 1 year ago.
Ok. I'll be happy to opt out. I'm sorry you don't like my answer, but a truss is a truss and that is not a truss. Good luck with your project.
Customer: replied 1 year ago.
Thanks and I am very sorry for the inconvenience.

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