Breaking the Tenison - Creating a Surface Tension Break on Exterior Surfaces

Here I am routing in the tension break on the bottom of the eyebrow cap.

Water is this weird thing that will defy gravity.

Every plumber knows, that you need 1/4” of fall over a foot, for water to shed, but do they also know that you need to break the surface tension, or else that stuff will head uphill, backwards, and cross-eyed?

They probably do.

But for carpenters, it’s an easily forgettable thing.

Take this most recent window repair. I’m sure for the aesthetics, an ‘eyebrow’ was added above the window. Made of crown, and a 1x cap.

The old eyebrow. No tension break was added, which will lead to water trailing backwards, and running over the crown moulding and into any openings.

The cap was laid flat to cover the crown. While no pitch was added for shed, the cap surprisingly was not rotting.

But the crown below was.

If you’ve ever just stared at the bottom side of your window nosing… as most homeowners do… you’ll notice there is a slight groove on the bottom. While nosing is pitched, this is to prevent water from trailing backwards.

The bottom side of window nosing, showing the groove that breaks the surface tension.

Water can bond to the surface, and at slight pitches, head back uphill. But water will not go straight up. That’s why we add these surface tension breaks.

Just imagine reverse rain falls, and the chaos that would do to our minds.

The new eyebrow caps, with the routed in surface tension breaks.

To assist with the life of this piece, I added a new cap with rabbets and grooves, along the three sides that witness water shed.

In the few moments I add this, it’s usually a quick pass through the table saw. But since this can be visible, I opted for the router, with a guide, and a 3/8 straight bit. Giving me those precious little moments of channeling my inner woodworker.

A 10 degree bevel was ripped on the backside, to help pitch the cap… cause we can always do a little better.

And white Lexel slathered all along the seams. Cause white Lexel looks like it was caulked. And Lexel is super water proof. Big emphasis on the ‘super’ part.

The finished eyebrow, recreated with new crown moulding.

Stay peachy y’all!

A cordless Dewalt 1-1/4 hp palm router, on a plunge base mounted with an edge guide, was used to create the tension break. The bit used was a 3/8” straight bit.