Plumbing Roof Vents

Unless your backyard has a tall, narrow building with a crescent moon cut in the top of the door, there are plumbing vents on the roof of your home. Each sink drain, tub, or shower in the home has a vertical U-trap that stays full of water. The water in the traps and the water in the toilet keep methane and other gases of decomposition in the sewer system from traveling back into your home. However, these gases have to go somewhere, so the home has vertical vent pipes that carry these gases through the roof to the open air.

Vent pipes expand and contract as the temperature changes, so these pipes need to fit loosely through the sheathing and shingles. The pipe also needs to have room to play inside the neoprene collar or all lead flashing, depending on which one you use.

With both types of flashing, make sure the pipe is basically centered in its hole through the sheathing. The neoprene collar will let the pipe shift and rise and fall. The all-lead collar will be loose enough to let it do the same. A word of caution: when you tuck the top of the all-lead flashing into the top of the pipe, don’t try to pound the lead down tight on the top of the pipe. You might tear the lead, and a tight fit won’t allow the vent pipe room to move vertically.

Lay the courses up the roof until you reach a vent pipe. The lead in the all-lead flashing is slightly stronger than butter, so handle the unit carefully as you lift it out of the box. Slide the unit over the vent pipe to make sure it’s the right size. Carefully check the soldered joint between the base plate and the vertical lead pipe portion of the unit. Make sure the solder isn’t cracked or tom anywhere. Chances are, the vertical portion of the flashing is crimped or dented on one side. Place the unit on the roof beside the vent pipe. Turn the “good” side down the roof where it will be seen from below.

Pick the unit back up and lightly hammer beneath the upper half of the base until the base sits flush on the roof and the upright portion is vertical to the roof. Now try to slide the unit back over the vent pipe. The flashing unit may not slide over the pipe, due to your hammering the base. Sight down the vent pipe to see where, the unit is hanging up. Remove the unit and gently tap around the inside of the hole in the base to round the vertical portion back out into shape. Now slide the unit over the vent pipe.

If it goes on freely, take it off and place it to one side until you need it again. Lay the next course of shingles. The vent pipe will stop the shingle from sliding up to the horizontal course line, but slide the shingle up into position as far as it will go. You need to cut a “U” notch in the top of the shingle. Use your knife to make a mark on the shingle. Keep the marks just outside of imaginary lines even with each side of the vent pipe. (Remember to leave the pipe some play. You saw how the old shingles were cut, allowing room all the way around the pipe.) Pull the shingle away and place the top of the shingle on the course line just beyond the pipe. Sight across the shingle and mark the shingle horizontally, between the two vertical marks. This horizontal mark should be just below an imaginary horizontal line touching the lowest point of the vent pipe. The two vertical marks and the horizontal mark are the outline for the sides and bottom of the “U” notch for the plumbing vent.

Use the hook blade to cut the notch into the top of the shingle. (Cut the notch over the felt. It would be a shame to slice the shingles on the new section of roof.) Position the top of the U-notched shingle on the course line with the U-notch around the vent pipe. You will probably have to trim the shingle a couple of times to make the notch loose around the vent pipe. Nail the shingle in place, nailing as usual just above the keys.

You want the lower edge of the base of the flashing to extend down onto the exposed tabs of this shingle below the vent pipe. This means that the bottom of the base plate will show on the finished roof. This overlap onto the top of the exposed tabs forces any water hitting the base plate to continue down and over the shingle roof. Slide the vent flashing down into place, and see if the bottom edge of the base plate is below the top of the keys in the shingle you just notched and laid. If not, you need to lay another course, notching the top of a second shingle. You won’t need to notch the tops of more than two shingles.

Place the flashing down over the vent pipe. Does the vent pipe stick up above the top of the vertical lead pipe of the all-lead flashing? If so, you need to cut the vent pipe so that it is an inch shorter than the vertical lead pipe of the all-lead flashing. You need to be able to tuck the top of the vertical portion of the all-lead flashing down into the inside of the vent pipe itself. If the vent pipe is too long, remove the all-lead flashing and use a hacksaw to cut the vent pipe so that it is an inch shorter than the flashing unit. A PVC (polyvinyl chloride, or plastic) pipe is not too difficult to cut. If the pipe is cast iron, you can cut it, but it takes some effort with a hacksaw.

Make sure you get the measurements right; you don’t want to have to cut a cast iron pipe twice. (Because I cut a lot of cast iron pipes in the course of a year, I used a hand-held electric hacksaw.)

Nail: Some contractors install the all-lead flashing, but they break the tops off any cast iron vent pipes with a hammer. The cast iron is brittle and it shatters. Breaking it with a hammer leaves a jagged top that can puncture the bend where the all-lead flashing tucks back down into the vent pipe. A shoddy contractor will often drop the broken pieces down the vent pipe, partially blocking it. Hammering can also crack the vent pipe down into the house and let noxious gases seep into the attic.

High winds can blow rain back up under the base of the vent flashing and cause a leak around the vent pipe’s hole in the sheathing. Seal under the base of the vent flashing, using the caulk gun to run a heavy ring of roofing cement completely around the vent pipe. Keep the ring well within the area the base of the new vent flashing will cover. (Don’t put down such a heavy bead of roofing cement that it will ooze out from under the base plate when you seat the base of the vent flashing in it.) The ring will go over the felt directly above the vent pipe, and it will also go over the new shingle surface beside and below the vent pipe. Slide the vent flashing down into position over the vent pipe and push the base down firmly, seating it in the ring of roofing cement.

Nail: Too many contractors do not bed the base plate of their vent flashing in roofing cement. The steeper the pitch of the roof, the stronger the wind must be to cause a severe leak. The flashing may never leak, or it may only leak inside the home during extremely severe rains. The damage will most likely appear when you roof the home again many years from now. When the roof is tom off in the far future, you will find that you have to replace sheathing and possibly repair a rafter. In exchange for that, these contractors save a few minutes and 25 cents worth of roofing cement.

Nail each corner of the base of the vent flashing down to the roof. Remember the problem with dissimilar metals. If you use a neoprene collar with a galvanized base, nail the base with a galvanized nail. If you used the all-lead collar, nail the base with an aluminum nail.

Nail: Too many roofers only nail the top comers of the base of the flashing. They will tell you that nailing the lower exposed comers can cause leaks and is unsightly. Of course, this is baloney. They are saving two nails and the time it takes to drive them and caulk over the heads. That unsecured lower edge of the flashing gives a high wind a place to start tearing at the roof.

Come across the roof with the next course of shingles. This time you will have to notch the bottom of the shingle. Mark the top and sides of the inverted “U” similar to the way you did the lower shingle(s).

Mark the sides of the inverted “U” notch at the bottom edge of the exposed tab(s). Mark the top of the inverted “U” up on the shingle. Cut out the inverted “U” notch. Slide the shingle down into position with the notch up and over the lead pipe portion of the vent flashing.

Nail the upper shingle in place. If the top of the key is close to the vent pipe, either nail high at the top of the shingle or nail off to the side of the key. Just don’t drive the nail in its normal position over the key if that means the nail will add another penetration to the base of the flashing.

When you trim the shingle, leave approximately 3/s-inch clearance around the vertical lead pipe of the flashing. Be sure to cut the legs of the notch straight down. These cuts are going to show on the finished roof. (When you trim the notch, be careful not to hook and cut the soft lead base plate under the shingle.)

Once this upper shingle is in position and has been trimmed, take out a little more insurance and run a bead of mastic back under the sides and top of the edges of the inverted “U” you cut in the tab of the upper shingle(s). Bed the shingle into the mastic by pressing the shingle firmly into the mastic.

Use the hammer to tuck the top of the vertical lead pipe gently into the inside of the PVC or cast iron vent pipe. Any rain that hits the top of the pipe will either run down the outside of the flashing and off the roof, or it will run down the inside of the vent pipe to the sewer system.

There is one more thing to do at the end of the job — caulking. Use a tube of aluminum-colored silicone caulk to seal the notched edge of the top shingle down to the base of the all-lead flashing.

Also, caulk around the top half of the all-lead flashing to protect the soldered joint between the base and vertical section of the flashing. In effect, the 3/8-inch clearance around the top half of the vertical pipe of the vent flashing will be caulked-in solid. Use gutter seal (or aluminum caulk) to seal over the lower (exposed) nail heads. I waited until the end of the job to do the final surface caulking so the fresh caulk didn’t get grit in it and I didn’t drag my air lines through the fresh caulk.

By bedding the base of the flashing in roofing cement and tucking the top of the all-lead flashing down inside the vent pipe, you have sealed the unit. Your plumbing vent is not going to leak even if rain catches you the instant you finish installing the flashing and there is nothing on the roof but felt above the vent flashing. If the vent flashing is the all-lead type, it’s not going to leak in the future either.

VENT FLASHING ON AN OVERLAY

The vent flashing on an overlay is done basically the same way a vent flashing is done on a tear-off. The main difference is that instead of using the chalk lines for the horizontal courses to lay the shingles, you are nesting the tops of the new shingles to the lower edges of the tabs of your old roof.

Wait until you get to each individual vent to remove the old vent flashing. You don’t want to remove two or three vent flashings on a large section of roof and then have to worry about sealing them if you get caught in a sudden rainstorm.

Cut the old shingles overlaying the base of the old flashing. Keep the cuts just outside the edge of the buried base plate. Carefully remove the cut pieces of the old shingles to expose the base plate. Save the pieces of shingle. Pry the old flashing loose with the claw hammer; the nails should come loose with it.

Slide the old flashing up and off the vent pipe. Throw the old flashing away. You may be tempted to reuse an old all-lead flashing, but the solder between the base plate and vertical lead pipe may not last the life of the new roof. You’re better off replacing the old all-lead flashing. Fit and nail the cutout sections of old shingle back into their original position around the vent pipe.

You don’t want to leave these old pieces out, or you will cause a dip or swag under the new vent flashing. Once you have the vent flashing removed and the old shingle pieces back in position follow the procedure used earlier to vent flash on a tear-off. Just nest the shingles instead of following the lines. You will find that the method for weaving these into a shingle roof is basically the same as the method for a plumbing vent. The difference is one of scale.

Large obstructions stop the horizontal courses the same way a dormer, chimney, or skylight will.