Timber Framers Guild

Creating roofing formulas

Posted By: harryl36

Creating roofing formulas - 01/30/21 02:36 PM

Hello everyone, I am relatively new to all of this but am trying to focus on the roofing angles. What I would like to do is be able to work out my own roofing angles, using trig, without having to rely on the pages by Joe Bartok and book by Sim Ayers, speed squares or construction master calculators. For example, I know that tanHip=tanmainpitch*sinplanangle . I only know that because it has been worked out and written down already. As many hours as I have spent looking at the drawings with that formula, and using my pretty basic trig and geometry knowledge I cannot get my head around this for myself. Any help would be much appreciated
Posted By: timberwrestler

Re: Creating roofing formulas - 02/01/21 09:02 PM

I don't quite get what you're looking to do. If you want to figure things out mathematically, that'll work fine, but the formulas are derived from the triangles in Bartok or Sim's work.

To me, that tanHip... formula is gobblygook. And I love trig. But it's not intuitively understandable.

Is there a particular roof problem that you're trying to work out, or just working at understanding the whole thing?

Do you know about the Hawkindales? All of those trig angles have already been worked out. There's an Excel spreadsheet and some graphics showing the angles here: https://www.tfguild.org/publications/software-tools.

A pretty excellent introduction to timber compound joinery is Will Beemer's articles in the the TFG Magazine called "When Roofs Collide." I think the Guild is opening up the vaults of back issues on the website pretty soon. I don't know how recent the archive will go.
Posted By: timberwrestler

Re: Creating roofing formulas - 02/02/21 06:02 PM

Also, feel free to ask more questions here on the compound work.
Posted By: Jay White Cloud

Re: Creating roofing formulas - 02/03/21 03:28 AM

Hi Harry...I'm not certain if you have access to CAD like Sketchup...but what I find useful when teaching or discussing this subject with clients is have a 3D model of a given compound roof scenario and then simply work backwards from their mathematically to see how each formula applies to the model...Does that make since to you...???...of course you can do it with measured drawings as well, but its not as fast...
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