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Thread: Rafter design on Gazebo

  1. #1

    Default Rafter design on Gazebo

    I'm working on a plan for an open framed roof on a 10' x 10' gazebo using 2x6 rafters. I'm leaning toward using a 4' ridge rafter to create a rectangle hip roof. I like the idea of fastening a 2x6 exterior rim band around posts at top, with a 2x4 (maybe double) top plate attached to it and the top of posts.

    1) Will this be sufficient for roof to be self supporting, i.e. no cross ties and adequate to offset any potential for splaying at the walls?

    2) I know there is no one answer to this second question, but what might you suggest as a reasonable rise on the roof for this design?

    3) This is not a pyramidal roof, but will the hip rafters still run across the central diagonal on the top posts?

    Any suggestions most appreciated.

  2. #2
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    Default Re: Rafter design on Gazebo

    Quote Originally Posted by Bayman View Post
    I'm working on a plan for an open framed roof on a 10' x 10' gazebo using 2x6 rafters. I'm leaning toward using a 4' ridge rafter to create a rectangle hip roof. I like the idea of fastening a 2x6 exterior rim band around posts at top, with a 2x4 (maybe double) top plate attached to it and the top of posts.

    1) Will this be sufficient for roof to be self supporting, i.e. no cross ties and adequate to offset any potential for splaying at the walls?

    2) I know there is no one answer to this second question, but what might you suggest as a reasonable rise on the roof for this design?

    3) This is not a pyramidal roof, but will the hip rafters still run across the central diagonal on the top posts?

    Any suggestions most appreciated.
    I would suggest that 2x6 is more rafter than you need on a 10x10 gazebo!! You didn't mention your locale but even the heaviest snow loads would only require 2x4 on that size building. What you need to think of is bending in the horizontal plane on your top plate from the rafter kick forces which will bow the walls outward over time. Thats what rafter ties do!!

    Top plate should be where you use your 2x6 (horizontal) I would consider a fairly steep roof for asthetics unless you live in a very windy location; make a scale sketch and pitch the roof at something that pleases the eye. The steeper it is, the lower the kick force and the less outward bending on your top plate. Good luck.
    Ken

  3. #3
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    Default Re: Rafter design on Gazebo

    Quote Originally Posted by Bayman View Post
    I'm working on a plan for an open framed roof on a 10' x 10' gazebo using 2x6 rafters. I'm leaning toward using a 4' ridge rafter to create a rectangle hip roof. I like the idea of fastening a 2x6 exterior rim band around posts at top, with a 2x4 (maybe double) top plate attached to it and the top of posts.

    1) Will this be sufficient for roof to be self supporting, i.e. no cross ties and adequate to offset any potential for splaying at the walls?

    2) I know there is no one answer to this second question, but what might you suggest as a reasonable rise on the roof for this design?

    3) This is not a pyramidal roof, but will the hip rafters still run across the central diagonal on the top posts?

    Any suggestions most appreciated.

    2X4?? as KenL said...

    For your amusement...

    http://books.google.ca/books?id=QbdM...esult&resnum=3
    ---
    Will

    “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” —- Mark Twain

  4. #4

    Default Re: Rafter design on Gazebo

    Thanks KenL for explanation on horizontal thrust. Use of a 2x6 for ridge beam makes sense and I will do a sketch to come up with a rise that looks 'pleasing to the eye'. Your suggestion for 2x6 for the top plate is excellent as this gives considerable more horizontal strength to offset the tendency of the roof load to push out the walls.

    I agree that 2x4 is adequate for rafters. What I was aiming for is a vaulted ceiling, with no clutter overhead. I did read somewhere that on buildings 10x10 or less that a square pyramidal hip roof would be self supporting; without ties. I don't have the source, but I also read elsewhere that on hip roofs, the roof pressure is more toward the corners than toward the walls ..perhaps that's what increases the self supporting aspect.

    In the text on architectural design referenced by WillR, it is mentioned that using joist hangers to connect the rafter pairs to each other and the ridge beam is required when their is no horizontal ceiling joist. I assume here that the metal hangers at the ridge beam work like crossties to prevent the rafters from separating at the top..which is what they do when the walls are pushed out.

    I was thinking that larger rafters would be required in a vaulted ceiling scenario, but is this correct?

  5. #5
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    Default Re: Rafter design on Gazebo

    Quote Originally Posted by Bayman View Post
    Thanks KenL for explanation on horizontal thrust. Use of a 2x6 for ridge beam makes sense and I will do a sketch to come up with a rise that looks 'pleasing to the eye'. Your suggestion for 2x6 for the top plate is excellent as this gives considerable more horizontal strength to offset the tendency of the roof load to push out the walls.

    I agree that 2x4 is adequate for rafters. What I was aiming for is a vaulted ceiling, with no clutter overhead. I did read somewhere that on buildings 10x10 or less that a square pyramidal hip roof would be self supporting; without ties. I don't have the source, but I also read elsewhere that on hip roofs, the roof pressure is more toward the corners than toward the walls ..perhaps that's what increases the self supporting aspect.

    In the text on architectural design referenced by WillR, it is mentioned that using joist hangers to connect the rafter pairs to each other and the ridge beam is required when their is no horizontal ceiling joist. I assume here that the metal hangers at the ridge beam work like crossties to prevent the rafters from separating at the top..which is what they do when the walls are pushed out.

    I was thinking that larger rafters would be required in a vaulted ceiling scenario, but is this correct?
    No. The joist hangers are there to react the vertical shear load at the ridge beam. The same effect can be achived by proper toe nailing of the rafters to the ridge for the sort of loads that you will encounter. The 2x6 rodge with 2x4 rafters cut at an angle will achieve the uncluttered look of a vaulted roof if you work the angles right. Use your rafter square to figure it out exactly.


    Any type of roof can be self-supporting if the wall are stiff enough to resist the bending load from the rafter kick forces, so you will be good there based on the 2x6 top plate.

    Ken in Ottawa

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