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Exploring Octagonal Roof Framing with SketchUp
comments (3) January 30th, 2013 in Blogs
Matt Jackson, member
Octagonal roof framing revealed
Roof Tour deJour:
This video is a virtual tour of a typical stick-built octagonal roof. Using the orbit feature of SketchUp it is simple to explore how all the rafters and other components are assembled:
The model has an octagonal center block to recieve the full length common rafters which then requires the 22.5 degree hip rafters to have sharp top angles which are difficult to cut with a typical circular saw. A sixteen-agonal center block would be more of a challenge to make but would simplify rafter cutting. The hips would require only a simple plumb cut. In the roof modeling tutorial below I'll show the steps I use to create a center block making a 16-sided one in the example.
An octagonal roof framed with trusses is a different animal even though the geometry is the same. Truss roofs are generated almost entirely with engineering software and assembled by following a truss erection plan. Although the math is the same, much less in-depth understanding of the process is required so I didn't get into it in this tutorial.
Beyond the Math:
From a recent viewer comment on my You Tube Channel I learned that blocklayer.com is a great resource for carpenters. The site has an impressive selection of excellent calculators for figuring rafter lengths and angles for any standard roof configuration. There is a special section within the site for figuring math required for gazebo roofs of any shape and pitch imaginable; need to build a pentagonal gazebo? It can be done! A SketchUp model can't match the speed of these calculators for crunching so many numbers but it should help visualize "beyond the math" how the complex measurements and angles are integrated into various framing members and how the members interconnect to form unique roof shapes.
Polyagonal Roof Modeling Tutorial:
While working up the models for this blog post I ended up with a number of techniques that simplify the process. The tutorial example shows how I model an octagonal roof. With a little adaptation you can use the same steps to model any-agonal framing. You'll see the difference between roof framing in "the real world" and framing at The Digital Jobsite where all the intuitive SketchUp tools make quick work of what is challenging carpentry. Well, at least it should be quick work. If you watch the video you'll see me bungle through the sequence of steps to model the hip rafter which I can hardly call quick. At least you'll be able to model more quickly after watching my mistake.
Octagonal Model and Measure:
To get the rafter lengths and angles for an octagon roof from this tutorial just reveiw the "Model and Measure" series at the Digital Jobsite where I walk through the steps of using model components to generate rafter cutting information.
Feel free to visit the Trimble SketchUp Component Warehouse where the models created for this tutorial are uploaded. As an added bonus I also uploaded a basic pentagonal roof framing model just to show it can be done and how such a roof would look.
As always, thanks for visiting The Digital Jobsite blog. Feel free to share comments and questions here...
posted in: Blogs, adasd
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The Digital Job Site
Get the inside scoop on how builders, remodelers, and architects use the free 3-D modeling software Google SketchUp to design projects and present them to clients.
You'll find practical advice on how to use this powerful drawing tool, and get some insight into how some experienced builders and designers have workied it into their set of everyday tools. If you are new to SketchUp, you might want to get warmed up with a few of Google's free tutorials.
Comments (3)
Posted: 2:41 pm on May 14th
The rafter assembly method you describe is probably more common than the way shown in the model for this post. I considered drawing the model without the central "ridge" block but decided it was more straightforward with the block. I see it as a matter of carpenter preference; a choice between making a faceted ridge or the sharp angled plumb cuts. With the shear number of intersecting rafters, thoughtful nail placement is involved either way.
BTW I have a model for circular roof framing in the works and use spaced blocking as you explain; to reduce the mass of wood at the peak.
Your comments add to the value of the discussion here and are appreciated.
Matt
Posted: 4:33 pm on February 5th
Posted: 9:52 am on February 4th
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