Every sink or appliance in your home that exits water into your sewer system needs to have a vent pipe that extends up through the roof of your home. The reason vents are required is to allow fresh air into the waste line behind the water when you flush a toilet or drain a sink and to allow sewer gasses to escape outside the home.
The vent works like a game you may have played with a straw as a kid while waiting for food at a restaurant. If you put a straw into a glass of water and keep your finger over the end the water will not drain out. When you remove your finger and air can enter the straw from the top the liquid can run free. The same thing happens in your home’s plumbing. If you did not have vents or if a vent got blocked the air would not be able to enter the system allowing your waste to exit the home.
On the roof the pipe vents will exit in various locations. Most homes have between 3 and 5 vents that can be individual vents or shared vents.
A shared vent will need to be larger then a single vent. Your kitchen sink vent is probably a 2 inch pipe however if it was a shared vent with your clothes washer or a down stairs bathroom you would need a larger vent up to 4 inches in size. The average size home will have vents from 2″ to 3″ inches in diameter and normally wont have 4 inches or larger unless the home is older and many fixtures are shared on the same pipe.
Where the pipe exits the roof a hole will be drilled or cut with a jig or reciprocating saw. Hopefully your plumber has taken care when making the hole for the pipe to exit as small as possible.
How the Vent Pipe Flashing Works
Most newer homes will have a flexible vent pipe flashing that is made of aluminum and a rubber seal that the pipe passes through. This type of flashing is common and inexpensive for the contractor to install because one flashing unit can fit pipes from 1-1/2 inches wide to 3 inches wide and there is no custom work needed to size the flashing.
When the shingles were originally installed the vent pipe was already in place protruding from the roofing plywood decking. The contractor that installs the shingles should have laid down a piece of roofing tar paper and then shingled up to the bottom of the vent pipe.
When you reach the vent pipe you then insert a vent pipe flashing and continue adding shingles around and above it as normal.
Replacing A Dry Rotted Vent Pipe Flashing
Since the rubber material that fits around the pipe is susceptible to breaking down from UV Sun rays it is only a matter of years until you will need to replace the flashing. The rubber will become gummy and then start cracking. Once the rubber part breaks down your are likely to first notice a problem when your roof leaks during a rain storm.
Measure your pipes
You should first get up on the roof and measure the inside diameter of the pipes that need new flashing. Waste pipes are always measured from the inside diameter.
Once one pipe flashing has started to crack it is probably a good idea to replace all of them. This will mean less overall time on the roof and peace of mind later.
Make a list of the flashing sizes you will need and the following materials for the project.
- 1/2 tube of 100% pure silicon exterior flashing caulk per pipe
- roofing nails you can reuse the old nails but you need extras just in case.
- 1 can of white enamel exterior spray paint
- A 1 x 1 foot square piece of roofing membrane / ice and water dam per vent pipe
Tools
Utility knife and heavy scissors
- Caulk gun
- hammer
- 2 foot long flat thin prybar
- metal 4 inch putty knife
Removing the Old Flashing
It is important to take your time removing the old plumbing vent flashing. 
First you do not want to damage the shingles that you will need to detach and second you should carefully watch how the flashing fits between the shingles as you remove it so you can insert the new one in the opposite way.
Where are the nails?
Vent flashings usually have a minimum of six nails holding them in place. It is attached at the bottom below the pipe by two nails that are exposed. There are two in the middle of the flashing that should be right under the first row of shingles that over lap the flashing. And there are two or three nails at the top of the flashing that are most likely holding both a row of shingles in place and the top of the flashing.
Prying Back The Shingles To Get To The Nails
So now that you know there are 6 nails used to hold the flashing in place you will need to pry back the center row and the top row of shingles to get to them.
Shingles are held in place by a bead of asphalt that holds the front lip of the shingle to the one it is covering. You need to pry that lip up about an inch at first to loosen the shingles.
Using your 4 inch putty knife start on the middle row where it overlaps onto the aluminum flashing and insert it as flat as you can. Take care not to cut through the shingle by going on an angle. Now work your way out away from the pipe and loosen the first full shingle or about 10 inches on either side of the aluminum flashing. It is important to give the shingles enough room to flex freely when you pull the flashing out.
Repeat the process for the next row up to expose the top nails under the shingles.
Removing The Nails
Now that your shingles are free you need to remove the nails.
Start at the bottom and insert your 4 inch putty knife between the flashing and the shingle and pry up slightly so you can insert your flat prybar. By putting your putty knife under your prybar you will not damage the shingles under it when you pry the nails out.
Now that you know how it is done you proceed up the sides and the top with your prybar until all of the nails are removed. You can reuse the nails if they are not bent or damaged.
Removing the Flashing
You will note that the flashing is under the middle row of shingles you loosened but you can not pull the flashing back far enough to get it out.
To remove the flashing you will need to turn it 90 degrees and then slip it out under the shingles. If you find the shingles need to be loosened more on either side do that with your putty knife. Do not try to pull the shingles up with the flashing or you will brake a shingle and you will be in deep #### because it will be almost impossible to find an exact new shingle to match at the store.
Take your time and remove the flashing gently.
Sealing The Roof With Membrane
Now that you have the old vent flashing out you should brush away any loose dirt and then cut a piece of roofing membrane / ice and water dam material to fit over the opening that was cut when the vent pipe was installed through the roof.
Membrane material is similar to roofing shingles but the back of it is completely covered with the same tar used to seal down the front edges of your shingles and it is also rubberized rather then a fiberglass material used for shingles so it will mold well once the sun hits it.
To cut the right size position the piece of membrane on the top of the vent pipe centered in both directions then mark the pipe on the back of the membrane with a magic marker.
Cut a hole by using the inside of your line that you marked and it will fit snugly around the pipe as you position it on the roof.
Now cut away the excess material. You want the metal vent flashing to overlap about an inch in all directions.
Apply a bead of silicon around the opening for the vent pipe then remove the backing plastic from the membrane and position it in place over the hole. Follow that up with a bead of silicon around the vent pipe where it passes through the membrane on the top surface.
This way if your vent flashing ever fails you have the membrane to back you up and you should not have a leak.
Installing The New Vent Pipe Flashing
Now it gets a little tricky to install the new vent pipe flashing you first need to work the opening of the rubber where the pipe will pass through by stretching it. You just want it flexible so the pipe will pass through easily.
Now put a small amount of silicon on the inside of the rubber flashing and spread it around so it will slip over the pipe without problem.
Lower the flashing onto the pipe rotated 90 degrees … or the opposite way that it came off.
At the same time you want to lift the top two rows so that the metal flashing can be rotated under the middle row of shingles.
BEFORE the flashing is pressed down to the surface of the roof you want to apply a bead of silicon on the shingles about an inch in from the edges.
Now press the aluminum flashing flush to the shingle surface.
Nail the bottom two corners about an inch in from each side the same position as when you took the flashing off.
Nail the two middle edges of the flashing.
Nail the top shingles in place through the flashing on the outside corners.
Gluing down the Shingles
Now that everything is nailed down you want to apply a bead of silicon about a half inch above the asphalt adheasive that you broke through to loosen the shingle. By placing the Silicon above the asphalt bead it will still be able to remelt and bond the shingle in place.
Over the aluminum flashing you want to use a large quarter sized bead of silicon to glue the shingle to the new flashing.
Once all the shingles are secure all you need to do is clean the vent pipe with some acetone or other strong cleaner such as brake cleaner or carburetor cleaner. This will clean the asphalt off the pipe quickly.
Painting the top of the Rubber Flashing
In my install I took an extra step and painted the black rubber surface white to protect it from the UV Rays of the sun. You can do this too and it should provide a few extra years of life but don’t expect it to last forever.
You may also want to repaint the flashing when you check your gutters for leaves in the fall.
To do so you can use a piece of news paper with a hole cut in it.
Dont try to just paint the flashing without it or you will get paint on the shingles.
If you do get paint on the shingles use an acetone type product immediately to remove the paint. Applying too much cleaner could damage the shingles so use a paper towel.
Notes about this job
It is amazing that only a few cups or a gallon or so of water can cause serious damage to your drywall ceilings.
If this happens in the middle of winter this can be a serious problem because getting up on the roof to make the repair is dangerous and if you hire a contractor who is qualified there is still a possibility that they could crack and destroy your shingles which freeze hard in the winter.
In the Fall when you are cleaning your gutters you should take care to inspect roofing flashing around all of the openings of your roof and spray them down with the hose if you have any doubts.
If you do have a problem in the winter it is probably best to just cover the flashing with a few layers of Aluminum Duct tape until spring. You can completely cover the rubber part by wrapping a cone shape with the aluminum duct tape and it will last for months.