Vents: Two Pieces Are Better Than One

Vents are installed through the roof to relieve vapor pressure (in the form of water or gas) from the interior of the building into the atmosphere.

Because this is required of in all types of buildings, vents and tubular penetrations are typically standard details on all roof systems.

Although these details are standard practice, installing roofing around vents and tubular penetrations could pose challenges for the roofing contractor.

The potential difficulties exist in completing and tying in the sheet metal work with the roofing work. Typically, sheet metal workers would complete the installation of metal components. However, in the case of metal flanges for vents and tubular penetrations, these components are completed by roofing mechanics because the roof flashing and metal flanges have to be completed at the same time.
In this procedure, the existing penetration is waterproofed in accordance with proper roofing procedures. The sheet metal flange is then applied over the waterproofed penetration and the sheet metal flange is flashed or tied into the roof system.

It used to be common practice to cover the penetration pipe with a one-piece flange fabricated from lead. The lead flange would be fabricated to extend approximately two inches above the pipe penetration. After installation over the pipe penetration, the excess top flange would be folded around the circumference of the pipe penetration. This provided a seamless waterproofing at the top of the vent.

The major concern with this type of application is that there exists a possibility of differential movement between the top and the bottom of the flange. In this detail design, there is no accommodation for movement at the top of the flange because it is tightly folded over the pipe penetration. The natural movement that occurs from the adjoining of the metal (flange) and the roof membrane transpires at the bottom. The movement of the metal flange at the bottom produces membrane splits, which allow for moisture entry into the roof system and eventually the building.

At these locations it may be wiser to consider the installation of a two-piece metal flange. In this application, the metal flange is applied at the circumference of the pipe penetration and is flashed in similar to the one-piece pipe flange. The difference is that a metal cover is fabricated to be soldered over the pipe flange. The metal cover is installed over the inside of the pipe penetration and at the outside of the pipe flange.

The cover provides room for movement at both the inside and outside of the pipe penetration. This also allows for differential movement at the top and the bottom of the pipe flange, relieving the pressure at the bottom of the flange. Waterproofing protection is maintained by keeping the inside and outside of the vent dry.