Valley to low-slope

Hey folks, I’m looking for some advice. I’ve pulled off my old roof and I’m about to install shingles but I’ve got a weird roof design to deal with. I’ve basically got a horseshoe of ridges that all lead down to a low-slope roof area. I’d really like to use W-flashing in the valleys because a) it looks good and b) when we get water or snow I’d like to direct the water flow as much as possible.

But what do I do at the bottom of the valleys? I can’t just stop the metal at the bottom of the valley, water/snow/ice will back right up inside of it. Should I do a closed valley for the bottom 12-18 inches then lay the metal over that? Is there a good option to plug the bottom of the metal?

Panorama shot of the roof

Or is a metal valley just a lost cause in this case?

Job stats:
Location: Pacific northwest, but the dry part. 300 days of sun a year, but plenty of rain or snow when it does fall. Also on a valley wall, so winds can get into the 40s at times.
Underlayment: GatorSkin
Low-slope option: Polyglass Elastoflex SA P
Ice/water: Polyglass Polystick IR X (at rake, eaves and down valleys)
Shingles: Certainteed Patriot AR (Because they were super cheap)

Thanks for any advice!

I dont really think w-flashing goes well in this situation.
Just use the galvanized rolled valley.
Install all underlayment.
Install cap sheet.
After you go up the shingle slopes about a foot or so with your Self-adhered cap sheet
Install 18-20 inch wide galvanized steel down the valley. Stop it just short of the transition so it will lay smooth and flat.
Speaking of installing valley metal…or shingling a valley… Not a rookie job.

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You need to run the low slope 18" Up onto the sloped section, so the little bit of backup won’t mean much. I would still do a course of closed valley before I started the metal. No risk of the metal cutting the low slope roof.

I agree with patchap and roof lover. W-flashing in the low sloped valleys is not a good idea (it is fine on the steeper ones though). What you want is to properly run your pollyglass up the low slope valleys and then bring your shingles down on top of it.

Thanks for the input, gentlemen. To clarify, I wasn’t thinking about running w-flashing in the transition from regular slope to low slope (circled w/ red), just in the valley between the decks of similar pitch (circled in blue).

@roof_lover Thanks for watching out for me; it’s not my first roof but it’s been a while. I put on a couple dozen roofs in my early college summers but that’s been some time ago and most were pretty basic. I may call a current roofer buddy to back me up on the valley metal from regular to low-slope (red areas).

@patchap and IslandRoofing Thanks for the input, guys. If I go with w-flashing in the blue areas how many courses of closed valley should I do before the w-metal? Patchap, you say one course?

If you like the splasher valley, put it in. I’m not a fan of the open valley, but some like the look. All you will be sealing is the raised section at the bottom. I would be more concerned at the pitch change. Since you are using the SA modified, I would make sure to run some up the valley, pretty far, just to be sure you don’t get backup. I would go 24", at least. I would then put my watershield in the valley, overlapping the modified. SA’s have a selvage edge, which if you don’t use it, you will have leaks. Kind of crazy, but I would measure from the height I wanted run under the shingles, all the way to the eave. You will be sure to hit the selvage edge on every sheet. SA’s don’t like adhering to the field. I don’t use them anymore, but even when we would prime them and seal with field adhesives, they would fail. Just my thoughts. Take your time. Keep those fasteners back away from the pitch change on the valley, and I would nail the bottom row of shingles high. Keep the bottom row of shingles up high as possible, away from the pitch change. This will also help you with the splasher valley.