How to Cut a Circular Hole in a Pitched Roof
Cutting a hole in the roof of your tiny house, skoolie, or other small space to install your wood stove chimney can be intimidating. Getting the hole in the wrong place or ending up with a leaky structure are common fears but with proper planning, you can do this job correctly and painlessly. Read on to learn:
- How to line up your stove and mark the roof exit location
- How to calculate shape and clearance for installation on a pitched roof
- How to determine your roof pitch
Placement for the Chimney Exit
Before you begin to cut, you’ll need to determine exactly where your pipe will exit. Chances are, you decided on the location when you put together your stove pipe order. Now you’ll need to pinpoint the exact spot and mock-up the pipe to ensure everything fits just right. When it comes to a hole in your roof, abide by the old saying: Measure twice, cut once… better yet, measure three or four times.
If your wood stove will sit directly below the chimney exit, here are a couple of tricks to easily find the center of the hole you’ll cut.
- Get your stove into its final location, make sure your space is perfectly level, and use a plumb bob to find the point directly above the center of your top flue flange (or the center of the rear exit tee if using a rear exit). Mark this point on the ceiling.
- Another option is to place a section of single wall pipe in your top flue flange (or rear exit tee if using) then place a flashlight at the bottom of the pipe, shining upwards. This will cast the exact shape of your pipe on the ceiling. Find the center and mark it. This method works even if your space is not perfectly level (mobile space dwellers!)
Shape and Size of the Hole on a Pitched Roof
Most roofs are pitched so with a chimney pipe exiting vertically, this means the hole you cut will need to be elongated. Just how much it needs to be elongated depends on the pitch of your roof and the clearances required. If you don’t care to head back to high school geometry class and revisit the Pythagorean theorem, don’t worry, we’ve got you covered with oval templates you can print. Scroll down to find these.
Double wall insulated Class A chimney pipe requires 2” of clearance to any combustible material. If you have a metal roof you can cut the roof material itself with smaller clearances of only ¼” but ensure that all combustible materials inside the roof and ceiling are cut back to 2” away from the pipe. If your roof material itself is combustible, you’ll need to cut everything back to 2” clearance. Determine what clearance your roof will require based on materials to help you select the right oval template for your hole.
Find Your Roof Pitch
To use the oval templates for your hole you will also need to determine the pitch of your roof. Try an online roof pitch calculator if you aren't feeling up to accessing the part of your brain still holding on to your high school geometry lessons.
“Roof pitch is the steepness of a roof expressed as a ratio of inch(es) rise per horizontal foot (or their metric equivalent)” - Wikipedia
Once you know your roof pitch, roofing material, and the size pipe you'll be using, download the template set and print out the page that matches your roof pitch. Instructions for use are printed on each template.
- 4” Double Wall Insulated Chimney Pipe (6” Outer Diameter) with 2” clearance
- 4” Double Wall Insulated Chimney Pipe (6” Outer Diameter) with 0.25” clearance
- 5” Double Wall Insulated Chimney Pipe (7” Outer Diameter) with 2” clearance
- 5” Double Wall Insulated Chimney Pipe (7” Outer Diameter) with 0.25” clearance
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