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Greater Watershed vs. Lesser Watershed Slope


hausdok
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We've talked about this here before but I'd never really asked the question before.

When you all see a Closed cut/California/Half lace/Alternative closed valley on a roof, do you all consider the "greater" watershed slope the larger roof expanse, such as a main roof that meets the small roof of a dormer, or do you consider the greater water shed slope the slope with the steepest pitch, such as when a steeply sloped tudor dormer meets the lesser-sloped main roof of a house?

I was taught by my father to always run the shingles from the smaller roof field up the slope of the larger roof slope field about a foot. If the shingle from the smaller field was to end just short of the valley, in the valley or just uphill of the valley, we'd insert a shingle section in the course a couple of shingles before the valley so that the end of the shingle lapping the valley would be just long enough to ensure the shingles ended at least a foot uphill of the valley. Then we'd overlap the shingles of the larger roof field onto those of the lower, trim them off 2 to 3-inches short of the valley on the uphill side, nip off the sharp point at the top of the shingle where it meets the valley and then place a bead of mastic just under the edge of the overlapping shingles to make sure they stayed stuck.

In W.E. Johnson's Roofer's Handbook (Originally published in 1938), Johnson calls it a half-laced valley and says that the shingles of the less-steeply-pitched slope should run across the valley and lie on the steeper slope with the shingles of the steeper slope overlapping them (Like the tudor dormer I talked about above).

Certainteed's Master Shingle Applicator manual says on page 63 to "lay shingles on smaller foof area, across valley and onto the adjoiniting roof area at lea 12:, Embed each shinlge in a 2: wide strip of asphalt roofing cement. and Cut 2" diagonally off the upper corner of trimmed shingle."

Certainteed's instructions pretty much mirror the Old Man's way of doing things but lately I've been running into some roofs that are done the way Johnson says to do it and I'm wondering what the prevailing mindset is on this - smaller roof uncut and larger roof cut, or lesser-pitched roof uncot and more steeply pitched roof cut.

I'd appreciate everyone's input.

Thanks.

ONE TEAM - ONE FIGHT!!!

Mike

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If the roof pitch is equal on both sides, I run the shingles up the one with the larger area.

If the roof pitch is steeper on one side, I run the shingles up the steeper side, regardless of area.

There are also these things called metal valleys . . .

- Jim Katen, Oregon

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If the roof pitch is equal on both sides, I run the shingles up the one with the larger area.

If the roof pitch is steeper on one side, I run the shingles up the steeper side, regardless of area.

There are also these things called metal valleys . . .

- Jim Katen, Oregon

Yeah, except some manufacturer's - Certainteed for instance - don't like open valleys.

Anybody else?

ONE TEAM - ONE FIGHT!!!

Mike

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I'm a little confused with the terminology. The terms that I'm familiar with are:

Open cut valley: The shingles terminate away from the valley. Valley is lined with metal.

Closed cut valley: Shingles on one side of the valley run up beneath the opposing side. Shingles from the opposite side run to the center of the valley and are cut at that point with adhesive applied along the undeside of the cut end.

Woven valley: Neither side is cut. The courses continue through the valley. Pitches must the same on both sides of the valley for this method.

On closed cut valleys, I'm with Certainteed, and your old man. It's more about the volume of rainwater streaming down a roof surface than a perceived velocity.

Marc

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In my area they are called cut valley. I write them up if they are not cut where the greater water slope or greater pitch.

I had 2 of them on the same day last week.

The first one is where they repaired around the chimney.

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The other one was on a 4 year old town house. The cut was 4 inches from the center line and someone had patched on it.

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. . . On closed cut valleys, I'm with Certainteed, and your old man. It's more about the volume of rainwater streaming down a roof surface than a perceived velocity.

I don't endorse it because of the perceived velocity. It's the angle. If I have the choice between running the lower layer up a shallower roof plane or a steeper roof plane, I choose the steeper one, regardless of the watershed on either.

- Jim Katen, Oregon

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Why would the angle matter, if not because of it's influence on the flow of rainwater?

Because water prefers to not run uphill. Water flowing down the valley will be less likely to run up between the shingles on the steeper side than on the shallower side.

- Jim Katen, Oregon

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If there's decent valley liners of 40 mil IWS, I don't think any of this matters particularly.

A very good point; and one I recently made to someone where a very steep roof met a narrow shallow pitched triangular roof wedged in between the steep roof plane, the side of a second story dormer and the front wall of the second story.

The upper end of that lower roof plane was about 4ft. wide and has a valley where the two roof planes meet and it narrows from 4ft. to 10 inches in the space of about 8ft. The only thing under that roof was felt and I was concerned that even a moderate snowfall was going to create a dam that was going to create a mess below that area in the foyer of that home.

This is what I'm talking about

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ONE TEAM - ONE FIGHT!!!

Mike

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I see your point. This is sort of a drift but down here, I have occasionally seen something similiar to that on new construction, except that the valley that you refer to actually runs into the dormer wall instead of directly to the eaves. I write it up as an error in roof framing design. The builder generally responds by replacing a half square or so of shingles with 2 to 4 pieces of soldered copper at the point of 'convergence'.

Marc

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I base it on volume of water, along with velocity of water that drains in the wrong direction (what Jim said I think). Water should not be running perpendicular or diagonally to shingles any more than absolutely necessary.

I probably write up a good half of the cut valleys I run into for a number of reasons. Most often due to the underlying course not running far enough up the opposite roof line (12" min)., fasteners placed too close to the valley center line,underlying edges of shingles not trimmed, or overlying course joints too close to valley centerlines.

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The roof plane with the higher water volume gets cut last. The watershed/water volume is a combination of roof area and pitch. The charts I have seen to calculate the run off volume have been with gutter sizing. I googled this table for calculation and I think it is consistent with what I read some years ago.

http://www.finehomebuilding.com/how-to/ ... stems.aspx

http://books.google.com/books?id=IqTmIG ... of&f=false

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. . . This is what I'm talking about. . .

I call those kind of roof sections "funnels." I see at least one a week.

The better roofers install them with mod bit membranes that run a couple of feet up the adjacent steep roof and at least a foot up the opposite sidewall.

The bozos don't treat them any differently than a regular roof. They do fine until a tennis ball gets lodged in the narrow end.

- Jim Katen, Oregon

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The other one was on a 4 year old town house

On that one I would have cut the other roof plane, since it extends above the one that they chose to cut. And, what are we looking at there, it appears to be Cor-a-vent nailed over the top of the cap shingles?

That the way they install ridge vents here.

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So they cut the felt open at the ridge, install the Cor-a-vent, and just skip the cap shingles? I bet the mfr. of the vent wants shingles installed over their product. We install caps so that they overhang the vent by 1 inch, to reduce the chance that water wicks into the vent, which it might do if the rain is falling directly on it. Bizarre...

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