Curious about how to figure out the total center height of a roof’s rafters if you don’t have design software? Maybe you’re working on a do-it-yourself cabana or carriage house. No problem! You can do this!

The pitch of the roof will determine the height of the ridge board that runs the entire length of the structure. (We discuss roof pitch in a previous blog.) Determining the height of a ridge board lets you know how to cut angles on the rafters and how to set them at the right height for the pitch of the roof.

So, first, divide the width of the exterior structure frame in inches by the number two. This lets you know the run for one side of the roof. Then, divide the run by 12 to find the run factor. The 12 is actually for 12 inches. The outside of the width of the wall framing should be measured on the actual structure frame. Don’t add in the siding of the structure or sheathing if there is any.

Then, multiply the the roof pitch by run factor you just calculated. Your answer will equal the total distance of the roof rises. That will be what you use to set the height of the bottom of the ridge board. Of course, that’s specifically if the roof rafters and the ridge board use the same size lumber. That’s the most common way to do things, but if you are using a more creative design, that may not be the case.

After that, you can figure the top height of the ridge board. Make sure to add in the size of the lumber used for the ridge rafter. If you use 2×6 inch lumber, it will be 5.5 inches. If you use 2X4 inches lumber, it will be 3.5 inches. Then, you have the total center height!

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