Why Wall Flashing Installation Errors Are So Common

Originally published by: The Journal of Light ConstructionAugust 27, 2020

The following article was produced and published by the source linked to above, who is solely responsible for its content. SBC Magazine is publishing this story to raise awareness of information publicly available online and does not verify the accuracy of the author’s claims. As a consequence, SBC cannot vouch for the validity of any facts, claims or opinions made in the article.

As Christine Williamson (the architect behind Instagram's Buildingsciencefightclub) explains in a recent IG post, lots of builders confuse the cladding of a building with the building's water control layer, and, she surmises, this is the reason for a very common omission: Not installing through-wall flashing, the (usually) L-shaped piece of metal above an opening or at the base of a wall.

In her post (embedded below), Ms. Williamson aptly notes that installing through-wall flashing and allowing water to drain out is a much more reliable approach to protecting a wall from water damage than trying to prevent water from entering in the first place. As she says it: "You will never be perfect at keeping 100% of water out of the wall - in providing drainage, you don’t have to be. This isn’t a compromise position that 'accepts' poor craftsmanship, it’s just smart design."


One of my favorite walking routes takes me past a lot of garages... nearly all of them poorly flashed. This is such a frustrating construction defect because it’s so common and yet so inexpensive to avoid: simply provide an outlet for the water that gets behind the cladding to drain safely to the exterior. Usually this is in the form of “through wall flashing,” which is a more or less L-shaped piece of metal (made of stainless steel, copper, lead coated copper, or even painted aluminum) that directs water from the face of the water control layer (the code term is “WRB” or water resistive barrier) to the exterior. * It’s really important that when through wall flashing is included, it’s not caulked or sealed to the cladding. The whole point is to let water out! This can be tough to accept intuitively, but I promise providing drainage is much, much more important than blocking entry. You will never be perfect at keeping 100% of water out of the wall — in providing drainage, you don’t have to be. This isn’t a compromise position that “accepts” poor craftsmanship, it’s just smart design. * Sketched with #Morpholiotrace * #architecture #architecturestudent #architecturestudio #architecturaldetail #architecturedetail #designdetail #womeninarchitecture #womeninconstruction #sustainablearchitecture #constructiondetail #buildingscience #buildingsciencefightclub

A post shared by Christine Williamson (@buildingsciencefightclub) on

Be sure to click on all the slides in the post above to understand the scope of the problems and Ms. Williamson's guidance on solving them.

It's surprising to us here at JLC that this omission continues to be so prevalent after all the attention we have brought to it over the years, but we agree with Ms. Williamson that it remains incredibly common.

One solution that works on cantilevered areas is to build a drainable reveal. Click to enlarge.

Stucco buckets. One distinct and especially thorny problem Ms. Williamson points out is a problem we earlier dubbed "stucco buckets" - overhangs where stucco wraps from the vertical wall to the horizontal surface without anywhere for liquid water to drain out (see "Avoiding Stucco Buckets"; Nov/18).

One solution is a drainable reveal, with variations that work for stucco (shown left), as well as thin brick. Another option described in "Avoiding Stucco Buckets" is to divert the water with a channel that drains the water away from the bucket (the closed cladding above an opening).

As Ms. Williamson notes, the omission of through-wall flashing is especially common with stucco, but it's not the only cladding that gets short-changed. Providing an escape route for water is vitally important for all cladding.

Here are some good resources for detailing through-wall flashing in a variety of cladding types: