Dan Morrison | December 23, 2016

Article // Roofing, Siding & Exterior Trim, WRB & Flashing

When Drip Edge Becomes A Double Agent

The mission of water is to turn houses to mush. Some drip edge flashings may actually help water accomplish this mission. 


Drip edge flashing is supposed to push water away from houses. It is the front line of defense for most horizontal planes in a building: top and bottom of a window, roof eave, balconies, base of wall, etc.

Because of water has a unique ability to stick to surfaces—like when you wash your hands, and then lift them from the sink, and the water runs down your arms to your elbows (instead of dropping straight from your hands). Because houses don't have elbows, people use geometry to kick water away from walls.


Drip edge /drip ej/ (noun)

"The leading edge of a flashing, sill, overhang or other linear, horizontal building element designed to shed water."

RDH Building Science Labratories

Without elbows, water flows over the surface of a house—the wall—until it hits the ground. And soaks into the basement. If the surface of the house is brick, then all of this water can have serious freeze/thaw implications. If it is wood, then there could be serious rot implications. If it is something else, then there are other implications. 

So, can you trust drip edge to do its job?

Jonathan Smegal is a senior researcher with RDH Building Science Laboratories and he wondered that too. So he built an apparatus to test it. 

Driop edge profiles tested by Jonathan Smegal

Smegal tested scenarios using different edge profiles, materials, and overhang distances.


Turns out, there are turncoats in the drip edge ranks: some profiles actually do more harm than good.

Hemmed drip edge metal can actually direct water toward a building

Key findings:

  1. Hemming the drip edge has the greatest effect on its water-shedding ability—and not the good way. Because of surface tension, water follows the radius of the hem, so it actually directs the stream toward the house, even if the hem is angled out. This effect was greater for thick drip edge (12 gauge) than for the thinner drip edge (20 gauge) because thick metal makes a wider radius when you bend it.
  2. Larger overhangs (45 mm or 1-3/4 inches if you live in the states) shed more water than smaller overhangs (20 mm or 3/4 inch). This was the case in every test but one. #TheresOneInEveryCrowd.

  3. Adding a 45° kickout increases shedability from the apparatus compared to a 90° vertical drip edge (image at right shows 90 degree bend that is hemmed—worst of two worlds).

  4. In general, thinner metal distributes water better than thick metal, but thin metal may require a hemmed edge for strength and rigidity, so go back to finding 1 and remember that hemming is bad.

  5. Stone distributes water horizontally into a line of many drips, whereas metal concentrates water horizontally into a few fast-moving streams.
    Sloped stone (8°) with a 3/4 inch overhang shed all of the water to the ground, directing none to the wall. The water moved slower and horizontally before driping neatly to the ground.


Jonathan's recommendations:

  • Something is better than nothing. Drip edges that shed less water (i.e. allow more water to land on the cladding) may still perform well, but they indicate a higher risk of elevated moisture content and potentially staining of the cladding.
  • Thicker is better, but you don't hem it. One practical conclusion suggested by this study is that drip edge performance can be improved by increasing the thickness of the metal drip edge so that it does not require a hemmed edge. For example, it would be better to use 12 gauge without a hemmed edge, compared to a 20 gauge with a hemmed edge, especially when the overhang distance is small.


Read the original research paper:





(Images courtesy of RDH Building Science Labs—Thank You!)



Just about the time a guy thinks he's got a pretty good handle on best practices for quality building something like this comes along to make ya wonder... I would never have guessed a drip edge with a hemmed edge wasn't the best way to shed water.  Go figure!

Some comments...seen water go up hill on low slope roofs by way of wicking...good example is to place a rag overhanging a bucket of water and watch it suck up all the water onto the floor ....surface tension of water is worth observing and considering...wood acts much like a sponge especially when end grain is involved

Daniel Morrison's picture


Wicking water can be a good way to make sure your Christmas tree stays watered, but in construction, as you point out, it can be a disaster.

Capillary breaks, gaps, and moisture tolerant materials are tools that can be used. There is more on moisture's sinister side and how to thwart it here: 



If everyone in the trades spent the time to invest in such attention to detail... we would get less done in a day initially, but we would be making homeownership a delight for many, many more.  Great article!!!


what is the best design for a gable step flashing.


Daniel Morrison's picture

In the BOWA's Best Practice Guide to House Wraps, Doug Horgan says to use a minimum 4 inch step flashing. because you need to keep siding 2 inches above the roof, so give yourself some space to tuck behind the WRB and siding.

More on step flashing here: 

Hope that helps,


I am open to current research and changes. I have been taught the Drip Edge at the fascia needs to be UNDER the roofing underlayment attached to the roof deck. I am seeing many times the drip edge applied over the roofing underlayment. Please set me straight on the correct application of drip edge.

Daniel Morrison's picture


Here's a video we made about it a little while ago: http://www.protradecraft.com/two-ways-flash-roof-edges

I hope that helps,



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