“All About Flashing” article by Martin Holladay, March 1st
In the new article “All About Flashing” the picture at the beginning of the article shows a window cap flashing detail. This is the same picture I posted in reply (comment #3) to a question from Tanner Cunningham on Jan 23rd, 2019.
The detail was rejected by Malcolm Taylor, who states in comment #4:
“That was the detail suggested some years ago. The problem with it is it dumps the water behind the siding at the jambs. Current best practice is to bend the ends up not down, to make end-dams.”
Malcolm then provides a link to an article from Hammer and Hand showing a better method.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
There are lots of opinions on this, of course, but I agree with Malcolm on the superiority of window caps with end dams.
As you might imagine, I don't create new illustrations for my blogs. I use what's available.
Below is an illustration of head flashing with end dams.
Sure seems as though end dams on head caps makes as much sense as end dams on sill pan flashings...
Not being snarky, just comparing the logic.
We were halfway through our siding/trim install when I saw Malcolm's quote in the Window cap thread. It made sense to me so we switched and both my sub and I agree that this is a better detail. Thanks Malcolm.
I'm not convinced that end dams are worth the goofy look, having to notch siding, etc.. As long as the WRB and window flashing are done correctly, how much water is really being managed one way or the other?
Perhaps the people who were tearing out rotten sheathing and window frames noticed that the older style of flashing over the window (no end dams) was dumping water behind the siding. If so, maybe that's why sites like Hammer and Hand recommend turned up end dams.
Scott, perhaps; I've torn apart my share of old homes and have seen some damage. But with good WRB details, including either a rain screen gap or sill flashing that kicks out onto the face of siding below, I don't think the end dams are necessary. Additionally, the cap should be sloped more front-to-back than it is end-to-end, so most rain should drain down the face of the trim. The dams aren't tall enough to do much for wind-blown rain; in fact they leave a path for rain to enter between the cap flashing and the head casing, where I have also seen moisture damage.
There a few a lot of layers to this debate and a lot depends on what climate we are building in.
In some areas, like the Canadian prairies, builders rarely use flashing above windows at all, instead taping the head just like the jambs. That approach seems to work where there is no cavity behind.
Once you have a rain-screen gap, a head-flashing makes sense, and because the flashing extends back to the WRB behind the cavity, it is vulnerable to water making it's way directly into the gap with no impediment, the way it can't on a wall without a cavity.
Our code acknowledges the problem of wind blown rain by mandating differing heights of the end dams based on what climate zone you are in. There shouldn't be any larger gap left between the flashing and head-trim with end-dams than you would leave if there were none.
I think it is a mistake to start relying on the presence of a rain-screen cavity to deal with potential water intrusion. Cladding is described in our code as "the First Plane of Protection", and is supposed to be detailed so that it stops water intrusion on its own. The gap shouldn't be used as an excuse for poor detailing on the exterior, and just moving the protection back to the WRB behind.
Malcolm, all good information, thanks. My approach has long been to make the building watertight before any cladding is applied, and to think of the cladding's role as the first line of defense and protecting the WRB (and obviously aesthetics). It's an approach I learned as a carpenter on Nantucket island, 30 miles out in the Atlantic, with nearly non-stop wind and rain that usually comes in from any angle other than straight down.
Malcolm, where is the information on code mandated end dam heights? Are there details showing how the flashings are supposed to be installed because now it seems that climate plays a factor in the window flashing design.
It's at BCBC 220.127.116.11.4)c)
The BC Homeowner Protection Office published very good guidebook to code compliant detailing which is called the Building Envelope Guide for Houses.