• Field Notes 17.02.2010

    Some issues keep repeating themselves, either within the industry, or in dealing with consultants, architects and/or working with other trades. One of them is flashings. Somewhere back in time, windows all had flashings. Flashings over a window or curtain wall head detail  – when under a brick or weather cavity – made some sense. They keep water out of the window system, although there will be different opinions on this point within the industry. Ask any 2, 10 or 1,000 people and you’re likely to get as many different opinions. 

    But under a window, that’s a different issue. Flashings should scare anyone who details them. There are too many open issues to address that require a level of sophistication in the preparation of design details, and more importantly during installation. Borrowing a line from Rodney Dangerfield:  Flashings “don’t get no respect.” 

    As a means of capturing water that may come out of a window system, flashings have been – and are still used – to take water out of a window or curtain wall and direct it to the exterior. That’s all well and good, but the road to hell (or to the courthouse:  I know, same difference) is paved with good intentions. 

    If the flashing is directly exposed to wind and water, the height of the flashing may have to be sufficient to overcome water head/pressure issues, and be sealed to the window in more positive means to prevent air leakage between window and flashing, as well. 

    If a window sits entirely inside a flashing, the flashing should have end dams, and have three legs (back and two ends), with the 4th side open or drainable to the exterior. If there are no end dams, water will spill off the end of the flashing, and if behind the exterior weather seal, may flow into adjacent spaces/voids and eventually become a leak. 

    If there’s any question about the water head height and the height of the flashing, the window system should be sealed to the flashing at the top of the upright flashing legs. This may cause the window sill member’s underside to be immersed in water, so the waterproofing of the sill to verticals has to be properly addressed, too. 

    Flashings longer than 10 feet typically require more than one piece of material to make up the whole of the flashing, and the joints between pieces tend to leak. Since the flashing is exposed to exterior temperatures, thermal expansion alone may tear even the simplest of seals. Setting the flashing pieces on top of one another with a “smoosh” seal between them is possibly a leak waiting to happen, even if the sealant is butyl. 

    Any fasteners that go through flashings just negated any good the flashing was meant to accomplish. Sealing around an anchor bolt may prove to be difficult, especially if the seal is around the whole anchor plate or device, not just the bolt-through-flashing location. 

    Flashings should be sealed to the air and water line of the window system for which they are capturing water. For example, extending a flashing under the full width of a pressure wall system requires excess material, and makes it more difficult to anchor the window system to the substrate. 

    Lastly, setting the flashing on the surrounding substrate requires that the underside of the flashing be sealed to that substrate or to the air / water line of the surrounding construction. That’s often overlooked. Setting the sealant in a bead of flashing may or may not work. Any thermal movement between flashing and substrate will tear, and could eventually leak. 

    So, boiling all this down to a key set of recommendations, it’s important to remember:

      1. Flashing should be a single piece, whenever possible.

         a. It should be extruded, not formed. 

         b. If joints are required, they should be dealt with the same way a coping or soffit joint is dealt with, including, but not limited to sizing the joint for expected movement, working within the capacity of the sealant, ease of installation, etc.  Buttering or overlapping the flashing without taking all factors into consideration is a leak waiting to happen.

      2. End caps are mandatory if a window system starts or stops over the flashing. 

      3. Flashings should not be set in a bead of sealant, but should be sealed to surrounding substrates in a similar fashion as windows or any other material that will move, expand, contract, or act in a different manner than the material to which they abut.

      4. NEVER, EVER let a fastener penetrate the flashing. The flashing may need to be just the depth of the glazing pocket, not the full depth of the system. Or turn the back legs and end dams up in front of flashings. 

    The challenge is in finding a different or more effective way of tying the air/water line of a window or curtain wall system to the air/water line of an adjacent substrate. All I know is based on too many instances of failures, flashings are usually something put on a drawing at the last minute or not thought all the way through. And when it’s not thought out, the proverbial chain’s weakest link just became the flashing.

    Posted by Blogger @ 8:42 am

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