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FatAugie 08-24-2007 06:52 AM

Plumbing vent flashing
OK, we're finally going to get some decent weather this weekend, so I'm hittin it hard starting Sunday. After I tear off the roof, replace a few sheets of plywood and get the underlayment situation in order, it'll be time for some vent flashing.

My choices are lead or the Oakly rubber/aluminum combo. I know lead lasts longer, but how hard is it to work with? Do they size it pretty close to the vent pipe size? In other words, do they sell 2", 2.5", 3", as different sizes? If so, then I need to climb up there before I start to get the correct measurements (the flashing was the one thing I forgot to order).

What about the Oakly? I like that because I've worked with it before, but I don't like the idea of the rubber drying out. If I go that way, I'm going to fashion a sheet metal flashing to slide down and act as a sunguard. Kind of like a Vietnamese cooley hat with the middle cut out slid down to an inch or so above the rubber.

And finally, when I install the flashing, I know I put down a square piece of Ice and Water damn, then mail down the flashing. Do I put any sealant under the flashing? Roof Cement? Geocell? Walrus Jizz? Anything? Or is the I&W enough along with the slope of the roof. I'm familiar with the way to cut the shingles in when I shingle around it....I've seen the pictures over at of the right way and wrong way.

Any thoughts or insights are appreciated.

Ed the Roofer 08-24-2007 10:54 AM

When you order your boot flashings, you must measure the interior diameter of the intended pipe to be flashed.

Most roofing and plumbing supply houses carry the lead in exact inch increments. The Oatey brand that I have seen only comes in 3" or 4", which are the most common ones needed.

Both the rigid rubber and the lead flashings just slide on the pipe.

If it is a PVC pipe and will have a smooth surface, the oatey rigid rubber ones will slide on smoothly. If you choose this type, trace a line aroun the pipe where the top portion of the flashing will be sealed to the pipe. Remove the flashing now and then install a high qualit sealant about a half inch higher than the traced line around the circumferance of the pipe. Do this when you are ready to install the flashing; ie. when you have the shingles installed up to that point. Then slide the flashing over the pipe and it will seal from the caulking slightly above the final resting position. Your rain shield idea is worthwhile, and they sell those at plumbing heating supply centers, but they are not a tight fit and will need to be caulked too, so why bother.

If it is a cast iron plumbing pipe, the beaded seam would slightly pust the rubber flashing away an prevent a completely tight seal.

When installing a lead flashing, they say to where gloves, so as not to have direct contact with the lead. You can tell that most of us roofers have not heeded that advice. Measure about 4" above the top of the pipe and GENTLY score the lead around the circumferance. Repeat lightly a second time if you barely scratched through the lead. Once it is scored sufficiently, you should be able to tear the lead along this line, or else keep using your stanley straight blade to score until it is completey through the lead.

Be careful. A slight slip can cut deeply with that sharp blade.

The Ice and water shield should only go on the left, right and top sides of the flashing. I would not apply caulk under the flashing flanges, but if you do, then DO NOT apply it under the bottom flange of the flashing, as any condensation from the pipe sweating would have no place to exit from and remain trapped under the flashing and eventually under the roofing materials.

This is way too Dang long of a response post for a pipe boot flashing instrruction!!! :eek: :whistling2:


FatAugie 08-24-2007 11:59 AM

Long but appreicated! I think I've apprenticed under you for this roofing project, I thank you for all the input.

When I got this place, the vents were terminated in the attic space. I knew this was wrong and punched them through. Then I put in flashing that had the rubber boot, but was galv. instead of aluminum. Anyway, I put roof cememt all the way around...and since I've been gearing up for this roof and reading everything I could about roofing, I had a feeling that was wrong to do.

They didn't have lead flashings at the biggest supply house around (B&L Wholesale) when I asked today. They actually looked at me like I asked for someting crazy. I guess they don't get much call for it. Instead they had the alum/rubber boot version so I guess that's what I'll be using. They had the Smart Vent, so I loaded up on that good stuff. They were the only ones around with any decent amount in stock.

Thanks again!

FatAugie 08-24-2007 12:04 PM


Originally Posted by Ed the Roofer (Post 59404)
Your rain shield idea is worthwhile, and they sell those at plumbing heating supply centers, but they are not a tight fit and will need to be caulked too, so why bother.


My idea for the rain shield was to use it like a sun screen for the rubber, I don't need it to be water tight. I figured keeping the sun off it will help keep it pliable for years to come. For $2 worth of tin, I figured it can't hurt. At worst, I'm wasting my time and money. I'm getting used to that :thumbsup:

the roofing god 08-24-2007 12:57 PM

if you don`t caulk all 4 sides of the bottom flashing,you will be liable to wind driven rain,you don`t get much /if any condensation between the pipes that I would worry about it more than wind driven rain

FatAugie 08-24-2007 01:50 PM

It's a 4/12 roof and the bottom of the flange would be 8 to 10 inches at least from the pipe, so it would be hurricane winds.

I guess I could do something like this:


+              +
+              +
+              +
+++++    +++++

Where caulk is represented by the "+", leaving a small trougth for any condinsation to exit from.

I don't know...I'm leaning towards very little if anything on the bottom edge.

Ed the Roofer 08-24-2007 01:54 PM


I never caulk any of the sides of the flashings.

But I do take a square piece of Grace Ice and Water Shield and use a hammer and gently tap out the pattern of the interior circumferance of the plumbing pipe. Then I gently slide the Ice and Water Shield down to stick to the deck.

The reason I tap out the interior is so that when I slide it down to the deck, there is a little excess membrane which will stay slightly off of the decking and remain tight to the pipe.


the roofing god 08-24-2007 08:30 PM

water that gets blown in gets over the top of the cut shingle and down behind the rest-more common on a 4 on12 than a steeper pitch roof check the MCGRAW/HILL CONSTRUCTION manual of pipevent flashing details,there is less chance of a problem w/the I&w in place around the pipe,but water that gets in will eventually rust out the nails below the pipe-tyoically w/the I&W we mark the pipe exterior,make several diagonal cuts (like fingers),pull it down over the pipe,and then cut a 2"wide by 1" longer than pipe circumference,and wrap it tight to the base of the pipe at the roof-around the fingers

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