Roof Penetration detail through Metal Roof 1

When I built my house with the steel roof, the recommendation that worked was to route the vent to the peak of the roof to minimize custom cuts to the steel roofing. It may imply an additional 20 ft of PVC vent pipe in the attic, but so what.

What is a "B-vent"? What size are the penetrations? What is the profile of the steel roofing? The appropriate flashing details depend a lot on the size of the penetrations.

One problem with penetrations in a metal roof that is almost universal is that there is a backwater lap at the upslope side of the penetration. To prevent this, you have to lap the sheets over the penetration flange. Otherwise, you will be constantly sealing the backwater lap.

Unfortunately, roofers tend to use asphalt mastic to attempt to seal such penetrations. That only works for about a year, depending on your climate. The asphaltic material hardens and shrinks and the leak comes back.

If you can't get away from the backwater lap, then set the penetration flange in a full bed of butyl sealant, fasten it down, then use a high grade silicone sealant around the outside edge. DO NOT USE THE CRAPPY SILICONE SEALANTS THAT YOU CAN BUY AT YOUR LOCAL HOME IMPROVEMENT CENTER!! Use something equivalent to a Dow 790 or 795 material.

There is a company in Canada that makes very high quality penetration accessories. It is Thaler metal industries....


Also, you can check with Roof Curb Systems...another manufacturer of roof penetration accessories...


Finally, check the details given by the National Roofing Contractor's Association (NRCA)...
I have attached a flashing accessories guide from ITW that gives a variety of penetration flashing products....hope it helps...

Ron

hokie66...I'm assuming the OP is referring to a Type B gas vent...nothing special in terms of roofing, except that they typically have no mounting flange and are just passed through a hole in the roof. Because they can be hotter than other vents, the typical plumbing stack seal (EPDM bonded to metal flange), has a shortened life. You can use a lead boot seal, but you have to get the diameter just right and clamp it to the vent to prevent leaks. Can work, but tricky.

B-vents are particularly problematic to seal properly and maintain that way.

Ron

Ron,

I see. The Dektite seals are the standard here on some very hot roofs, but I don't know how hot gas vents are.

Why do you suggest using butyl under a flange and silicone around the edges? Roofers here use silicone for both. I thought butyl was essentially superseded.

hokie66...Butyl tends to remain pliable, as long as it isn't exposed to air or directly to sunlight. Silicone, urethane, and others will harden in time. Silicone can be used under the flange and will remain pliable for a long time when not exposed to sunlight/air. Substituting silicone for the butyl would be acceptable.

Ron

Penetration-Free Attachment
A first rule about any rooftop mounting is to do it without penetrating the roof membrane whenever possible. While this may seem obvious, on metal roofs, it is often violated. The norm for attaching things seems to be: "anchor to structure through the roof." When this is done, it not only threatens weather integrity, but also violates the thermal-cycling behavior of the roof membrane.

Round Penetrations
Round shapes, such as plumbing vents, should be flashed through the roof using EPDM (ethylene propylene diene monomer) rubber pipe flashings. Although these parts are widely available in a variety of colors, black has the greatest UV resistance and, hence, the longest life. Standard installation is to simply stretch-fit the rubber to the pipe. Utilizing a stainless steel draw band at the top of the flashing to further secure it will ensure that the flashing never inverts itself, and will typically add about 5 years of life to the assembly. The part has an integral aluminum compression ring laminated to the rubber base, which should be sealed to the roof panel using butyl copolymer tape. It should then be secured using #14 x 7/8-inch tek screws with #1 drill point at 2-inch centers through the compression ring, rubber, and butyl, and into the metal panel. Finally, excess butyl tape should be trimmed away, and a bead of one-part polyurethane sealant filleted around the joint created (base-to-roof). This bead will hide and protect the butyl from direct exposure to sunlight, ensuring a longer life. After a service life of 25 years or so, this assembly is easily replaced.

When attaching the pipe flashing, it must be anchored to the roof panel only - not into the building structure or deck. To do so would create an inadvertent "pinning" of the roof panel, violating its freedom of thermal movement. Ideally, these flashings should be centrally located on the roof panel so that there is free drainage to both sides with no interruption of the seams. If the location of the pipe interrupts a seam and it cannot be relocated, then a preformed adapter plate can be fabricated to span both panels adjacent to the seam and the pipe flashed per the above to the adapter plate. Companies that pre-manufacture curbs will make such adapter plates upon request.

When installing pipe flashing, remember:

Use unitized EPDM rubber pipe flashings (black preferred).
Locate centrally in panel.
Use stainless steel draw band.
Butyl tape beneath base, then fillet with one-part polyurethane.
Do not pin to structure or deck.


See page 38:

Hi

Thanks Ron,

Do you have known any source of material for Reference to select right type of Sealant materials? Now a day there is a lot of available products. I always found lots of terminology and value they use in Product data sheet, which confused me to correlate with application. Easy to go with experience but if contractor proposed alternative than it’s difficult to approve with out experience with it. It too small detail but can causes significant problems. Any input?

Thanks,

Rakesh

Rakesh...ASTM C920 gives some good information on the specifications for sealants. I have attached a specification section that I use for specifying sealants...

Thanks to all for your great input and time.

Rakesh