Timber Framers Guild

TF with Sketchup, roof valleys

Posted By: Woody_Dvorak

TF with Sketchup, roof valleys - 12/09/20 07:33 AM

I'm modeling a TF house design with Sketchup. I'm not a pro, just a DIY'er trying to design what I want to build. It's not a simple frame, the building is Z-shaped with an attached garage, so four sections perpendicular to one another. I'm working on perhaps the 15th iteration of the design, 'cuz the more I do the more I learn... I'll realize I've drawn something fundamentally wrong, and it's easier to start over than move 300 or more components around.

That said, I've got (many) questions about drawing construction models with Sketchup; some cover basic aspects, such as: if I'm drawing 8x8 timbers, and we all know 8x8 timbers don't really measure 8" x 8", should I be drawing them as 7.5" x 7.5"?

Beyond the basics, I've run into a challenge specific to TF design. I'm confused about how to join two perpendicular roof sections, at the plates where they share a post. Each of the bottom plates should be on top of the post, and the top plates (forming the roofline) intersect there, too. This pic shows what I mean:


I've studied other drawings of L-, T- and cross-shaped TF buildings. In some cases the perpendicular sections of the building are different widths, allowing the top plates of the wider section to attach lower on the shared post, or to a lower beam instead of the bottom plate. In other examples a perpendicular section's wall is offset a bit, joined to the other section by a short beam and another post, providing separate locations to attach the roof plates.

Neither of those options are optimal for the building I'm designing; these two perpendicular sections are the same width, and the walls are on the same plane (as currently designed). If I need to, I could redesign with offset-walls, but I'm hoping there's an easier solution.

And finally, due to ignorance I'm drawing the entire frame using 8x8's; I'm not an engineer, have very little idea how to properly size timbers. We live in Southeast Alaska, so there's some snow to worry about... it's usually wet and heavy. At present I've drawn the entire design with butt joints, no joinery is modeled yet. I do have the Timber Frame Extensions (TF Rubies) installed; I plan to draw the joinery later, after I've completed the overall design.

Thanks for any help...
Posted By: daiku

Re: TF with Sketchup, roof valleys - 12/09/20 08:50 PM

Hi Woody.

If I'm going to square rule to half-inch under, I just draw the timbers at half-inch under, and not try to draw all the reductions, I know what they look like, and I just figure that out in the shop.

In your design you are making a common mistake made by stick framers who are new to TF design: Crowding too much joinery into a single point. And you haven't even added the hardest part yet, which is the valley rafter itself! I strongly advise you to spread things out so that you get saner joinery. For example, would you be willing to take that gable end on the left of your drawing, and pull it toward you by a foot? I realize that it would break up the wall facing the camera, but it would "spread out" the joinery somewhat. You could also drop the tie beam on that gable end. With the two posts in "queen" position, it's really not acting much like a truss.

Regarding beam sizes, you'll need an engineer to do the math for you. There are no tables you can look up in timber framing. Things that will matter: wood species, snow load, roof pitch, and spans. CB.
Posted By: Woody_Dvorak

Re: TF with Sketchup, roof valleys - 12/10/20 04:38 AM

Hi Clark,
Thanks for the response.

I believe the suggestion you made is what I described as "offset walls"; I guess that's just what I'll have to do, then. I found an example in the SU 3D Warehouse, this pic is a screenshot:

Rather than extend the left section (green in my pic above), I'd want to squeeze the walls of the right section (purple) closer together. I guess I didn't realize this type of feature needed to be designed around in this way.

I do plan to submit my drawing to an engineer after I finish, they'll probably have to redraw the entire thing. Prior to that, though, I intend to get it front of somebody with some experience, to get some idea what I did right and wrong. I'll learn a lot more from critique than I will if I just hire it out.

Thanks again,
Posted By: daiku

Re: TF with Sketchup, roof valleys - 12/10/20 02:32 PM

Your engineer may not have to redraw the whole thing, depending on the quality and detail of the sketchup model. If you draw all the joinery (using TF Rubies, for example), that should be good enough, depending on the engineer. I've worked with FireTower many times in the past, and they were able to work from my sketchup model. CB.
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