How to Cut Furnace Vents in a Roof
Your roof is a valuable investment. It keeps rain, snow and wind outside. Cutting a hole in your roof for whatever purpose is something to do only after careful planning. When installing a furnace that uses any type of combustion process to produce heat, you will need the vent the exhaust gases out of the house as efficiently as possible. Since heat rises naturally, the path of least resistance is often through the roof. Follow the old carpenter's rule of "measure twice, cut once" to avoid a costly mistake while installing your vent. Does this Spark an idea?
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Things You'll Need
- Plumb bob
- Pencil
- Compass
- Vent pipe
- Drill with 3/4-inch bit
- Reciprocating saw with coarse-tooth blade
Instructions
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1
Find the best location for your vent. Ideally, the vent should be placed directly above the furnace exhaust outlet. Hold the plumb bob string against the bottom of the roof decking. Move the end of the string until the bob is centered above the furnace exhaust outlet. Mark the location on the bottom side of the decking.
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2
Adjust your compass to make a circle slightly larger than the outer diameter of your vent pipe. Place the point of the compass on the center point mark you made on the roof decking. Draw a circle to mark the pipe location.
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3
Elongate the top and bottom of the circle. In order to fit your vent pipe vertically through a pitched (sloped) roof, you must make an oval hole. The length of the oval, top to bottom, will depend upon the pitch of your roof. The steeper the pitch, the more elongated your oval will need to be to accommodate vertical installation of the pipe. You can get a good idea of the exact shape by holding a short section of vent pipe vertically centered on your marked circle and tracing the circumference of the pipe on the decking.
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Drill a three-quarter-inch hole through the roof one inch inside the border of your markings.
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Push the blade of your reciprocating saw through the hole. Cut through the roof while carefully following your lines.
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Tips & Warnings
For most furnace installations, your vent must be located at least six feet from the nearest attic vent or intake and at least three feet from the nearest exhaust vent.
The vent flashing will cover any gaps, up to roughly one-half inch, between the vent-pipe walls and the roof. So you do not need to squeeze the pipe through a hole that is too tight. Purchase your vent pipe, flashing and cap before cutting any holes so you get an accurate idea of the margin of error you will have.
A direct vertical run is most efficient for furnace exhaust but this may not be possible in some situations. When installing a vent, cut through decking and shingles only. Do not cut through joists, trusses or rafters. Adjustable elbow joints are available to facilitate routing your vent through the attic.
Always wear eye protection when using power saws.
Wear hearing protection when using any power tools.
References
- Photo Credit roof vent image by hazel proudlove from Fotolia.com
