Metal chimneys, pot vents, and power ventilators are large roof penetrations. All of them have a vertical component that rises above the roof and a base woven into the shingles. The lower edge of all the base plates comes down over the tops of the exposed tabs of the lower course of shingles. Instead of a simple notch, you cut several courses of shingles to fit along the sides of the vertical component. The bottoms of the tabs of the shingles laid around and over the vertical component are cut in a rounded curve to fit the outline of the top of the vertical component. Lay these courses back toward the obstruction from the new base and offset lines you reestablished beyond the obstruction. Cut the shingles to shape so they fit along the side of the obstruction.


A modern metal chimney consists of an interior pipe and an exterior pipe with an insulated gap between the two pipes. Even though the outer pipe is insulated from the extreme heat of the inner pipe, the outer pipe still gets hot enough to bum you.

There is a fixed flange that ties the metal chimney into the roof. The flange slopes down and away from the outer metal pipe before it flattens to form a base plate that ties into the shingles. The down ward slope and additional distance from the outer pipe keep the base plate area of the flange cool so that it can safely be woven in with the shingles.

On a tear-off, remove all shingles and felt from around the base plate. Pull the nails in the base plate to remove the course(s) from underneath. When you lay the new felt, slide it under the base plate. Keep the felt well away from the wall of the metal chimney beneath the base plate. If you jam the felt into the chimney under the base plate, you create a fire hazard.

Lay the lower course by notching the top of the shingle for the metal chimney and sliding the shingle into position beneath the fixed base plate. Make sure you have notched the shingle so that there is plenty of space between the shingle and the metal chimney. When the base plate of the chimney flange covers the exposed portion of the lower course of shingles, try to run a bead of mastic under the base plate. This is tricky to do and may be impossible due to the strength of the metal and the fact that the flange is fixed. If you can’t get a bead all the way around under the base, try to get a bead under the lower portion of the base plate. This will keep the rain from blowing back in under the lower edge.

Nail the base plate down to the lower course. I always drove the new nails through the old nail holes in the base plate. The nails go into the sheathing in the old holes, too, and won’t hold like a nail in a new spot, but the flange is fixed in position on the chimney and, as you’ve discovered, it isn’t going anywhere.

Cut and lay the shingles around the side and across the top. Pop the verticals for the interrupted courses and cut and lay the shingles back. It’s fine to mastic the edges of the shingles down to the base and caulk the side and top of the cuts to the vertical wall of the flange. The lower part of the flange won’t ever get hot enough to cause any problems.


On an overlay, the strength of the fixed chimney flange will stop you from sliding another course of shingles beneath the base portion of the flange.

Remove the nails from the exposed lower edge of the flange. Cut away the old course of shingle 2 inches out from the sides of the base plate. (You will have to cut part of the second course up along the sides too.) Remove the entire section(s) of shingle between the two cuts. Slide the old shingles out from under the base portion of the flange.

Lay a new course of shingles by sliding the new shingle beneath the base plate. Cut the top of the shingle so that it is well away from the metal chimney beneath the flange. You may have to slide the new shingle out a couple of times to trim it away from the metal chimney until it nests with the course of old shingles on both sides of the base.

Don’t disturb the remaining old courses of shingle to the sides and across the top of the base. Overlay right on top of them. Keep the nails out of the base plate area and let the new shingles overlap the edges of the old shingles by V4 inch. Mastic the new shingles to the old shingles and come back later and caulk the edges and the exposed nail heads at the bottom of the base.


A domed ventilator depends on convection air to pull heat out of the attic. These ventilators consist of a base woven into the shingles and a vertical component, which opens above a hole cut in the sheathing. The weather is kept out by a protective dome over the vertical component. Most units also have insect screens. The smaller versions of domed vents are called “pot vents” because they look like upside down pots on the roof.


Some ventilators keep water out of the home by curving the sheet metal shaft up and over until the vent opening faces back down toward the surface of the shingles. This type is called a “gooseneck” for the obvious reason.


When a ventilator unit includes an electric motor and a fan, it becomes a “power ventilator.” A power ventilator can best be described as a through-the-roof attic fan. Convection air currents carry a large volume of superheated air through a plain ventilator, but a power ventilator removes the heat more quickly.

The thermostat on most power ventilators is preset at a reasonable temperature. You can set the thermostat at the attic temperature you prefer to maintain. Remember, it’s in an attic where temperatures easily soar over 120° F. Don’t set the thermostat at 80° F., or the unit will run constandy during the day and into the night.

The basic steps of installing any ventilator are the same. However, if you are installing a new power ventilator you have a couple of extra steps. Remember the word ASS/U/ME, and don’t weave a new ventilator into the shingles until you have tested it.

If the old power ventilator has been in the roof for ages, you should probably replace it while you are reroofing. If you replace the old power ventilator, make sure you buy a ventilator requiring the same size hole or one slightly larger than the unit you are replacing. Remove the old shingles and felt. Pry the old ventilator loose and unhook the connecting wires (with the power off). If the new power ventilator is larger than the old one, you may have to trim the sheathing back to match the size of the new opening. A saber saw is slow but does a good job of cutting a circular opening in the sheathing. Remember, there are rafters on either side of where you are cutting, so don’t cut into them.

Now fit the framing and motor down into the hole through the sheathing and hook up the electric wires to the new ventilator. I recommend you get into the attic and hook up the electric wires as early as possible in the morning. People have been known to collapse in superheated attics, so don’t wait until the hottest part of the day to hook up the ventilator.

Now let the power ventilator sit in position until you can see if the thermostat and fan motor are going to work properly. If you are the impatient type, you can run a drop light up into the attic and hold the bulb right under the thermostat until the heat makes the fan click on. Pull the light away: the thermostat should click the fan off fairly quickly. Repeat the process. If the fan clicks on and off properly, it’s safe to go ahead and weave the power ventilator into the roof.

If you are installing a power ventilator in a new location, here are some basic ideas before you cut a hole in the roof. First, keep the ventilator(s) on the less visible portion (usually the back) of the roof.

You don’t want the power ventilator to become an outstanding architectural feature of your home. Second, the ventilator should be located approximately a fourth of the way down the roof from the ridge. Heat rises and the highest heat is at the ridge, so why not put the ventilator right at the ridge? Any break in the shingles for any reason gives you a location for a potential leak. If you put the power ventilator right at the ridge of the roof, it is easier for rain whipping across the ridge to be driven up under the dome of the power ventilator and into your home. Also, the wind tends to have the greatest force and speed coming over the ridge. In a severe storm, a power ventilator is subject to the highest force at the ridge. The wind could tear the dome free from the unit and send it sailing.

Whether, you need the power feature or not depend partially on the climate where you live. Power unit costs many times more than a domed ventilator or pot vent, and the motor is just one more stimulant for your electric meter. I sometimes think that knowing the fan is whirring away in the attic just makes people feel cooler. There are many choices of types of ventilators. The power ventilator is the only one that continues to cost money after you install it.

As with other roofing components, plastic power ventilator units are available, and they are cheaper than the metal ones. Stay away from plastic units. I have seen too many plastic domes that cracked or otherwise failed and allowed water into homes.

With the shingles and felt removed by a tear-off, you can locate the joists by spotting them through the cracks in the sheathing or seeing where the plywood is nailed up the rafters. You can cut out the circle of sheathing from on top of the roof.

You need to cut a hole through the shingles and sheathing and miss the rafters. You will need to know if the rafters are on 16-inch centers or 24-inch centers. In other words, what is the distance from the center of one rafter to the center of the next rafter in the attic? Power ventilators are made to fit between one of the two spacing’s. The package for the new ventilator usually includes a cardboard circle you can cut out and use as a pattern for cutting the necessary circle in the sheathing. This circle is the diameter of the segment of the base that will fit down into the sheathing. Cut this circle from its surrounding cardboard and take it and a drill into the attic with you. Select the location you want and hold the circle against the underside of the sheathing (or over the points of all the roofing nails). Drill four holes up and through the shingles on the outside to outline the diameter—top, bottom, and both sides. (Four nails driven through to the outside can accomplish the same task.)

You want the wood around the hole to be as strongly supported as possible. With plywood, try to center the circle near the vertical center of a full sheet of plywood. With planking, try to make sure all boards you cut extend across the adjacent rafter to the next rafter over. (You can normally tell by the color and grain of the wood if the plank is continuous on the other side of the rafter.) If you cut off a plank that ends at the rafter next to the cut, the plank is unsupported and will drop free if you stand on it. It may be necessary to box in around the power ventilator opening with 2 x 4s. Run the top and bottom 2 x 4s from rafter to rafter, and frame new side 2 x 4s between the top and bottom 2 x 4s. That way all of the plank sheathing is supported around the opening.

Take the circle back up on the roof with you and center it between the four holes you just drilled. Cut the old shingles and felt away using the circle pattern. Use a saber saw to cut the sheathing or go gently with a circular saw. Now set the power ventilator in place and go back to the attic and wire it.

Make sure it works before proceeding. Lift the unit back out. Shingle up one and perhaps two courses under the portion of the base that will be flush with the roof.

You will have to trim the top of the shingles to keep the vent hole fully open. (A few blocked inches make a great deal of difference in the efficiency of the unit.) Now run a very heavy bead of mastic around the outside of the circle. Be careful you don’t use so much around the bottom that it will ooze out from under the base. When you have the lower course or courses in place and the power ventilator bedded in a bead of mastic, nail every 6 inches across the base, sides, and top of the base plate (aluminum nails for an aluminum unit).

Usually four bolts (or nuts) hold the dome on a power ventilator. It is easier to remove the dome and trim and caulk the shingles in than it is to try to trim and work under the dome. After you remove the dome, screw the bolts back in place and weight the dome down out of the way on the roof. The wind can take the dome and send it sailing like a giant Frisbee. (Please don’t ask how I know this. Just take my word for it.)

Lay the shingles up the sides. Don’t nail any shingles through the base plate. You are going to mastic the ends of these shingles in place. The lower shingles covering the sides of the base plate are cut straight down from the widest part of the circular vertical

Component of the power, ventilator in other words, cut the lower side shingles straight down toward the gutter from the widest point on the “walls” of the power ventilator. These shingles will still cover the sides of the base plate by a few inches. You want the water running down the roof to run around the top half of the vertical component, then straight down from the sides and down the roof.

You can cut the shingles at the top half of the power ventilator so that they leave a Vs-inch gap between the vertical component of the base and the cut edge of the shingle.

If the verticals fall so that a shingle would just be a weak sliver coming into the side of the power ventilator, cut a full tab off the last full shingle. Fit in a section of shingle that includes a full tab and the narrow piece you need. (This is just a reminder. We already discussed this method of adding strength.)

To pick up the base and offset lines on the other side of the power ventilator, lay the shingles loose right over the top and pop the verticals down.

Once you have the power ventilator shingled in, lift the loose edges up the sides and across the top and run a bead of mastic under all the edges. Press the shingles down into the mastic to get them bedded. Immediately, run another bead of caulk along the cut edges on the sides and across the top of the base. Bolt the dome into its proper position before proceeding to lay shingles up the roof.

The course of shingles that first overlaps the sides will have its bottom edge resting out a few inches up from the bottom edge of the base plate. Don’t mastic these bottom edges. If water does somehow penetrate around the ventilator, you want it to run beneath the side shingles and surface at the bottom of the base plate and from there to run down the roof on top of the new shingles. Mastic on the lower edges of shingles tends to hold the water beneath the shingle, so you don’t want it there.