How to do valley underlayment?

When I am doing the valley, how should this be done?
Do I put the ice and water on bare decking the go the past edge but few inches back from the valley with the synthetic underlayment?
or
Ice and water down first on bare decking then go over it with the underlayment but over lap to the other side of the valley?

The roof will be open for at least a month while I strip the rest of the house and shingle it. Since the roof is a step pitch (14/12) I know that water will come down fast in the valley and worry it run under the valley and past the ice and water.

Also I know it says not to nail down valley metal and only use the nail heads to hold them down for expansion and contraction. What I don’t get is that it seem pointless when you are going to be putting nails for the shingle right in it any ways.

Thanks, Marshal

Its better to cover you ice and water shield with underlayment imo. It’s gonna make a world of difference for next time the roof gets replaced. Yeah I know what you mean about not nailing through the valley metal. If I am doing some type of long term roof (slate/tile/ect) I won’t nail through the valley metal, but a temporary roof (any type of asphalt shingle no matter what super duper warranty they have) I have no issues nailing through the valley metal itself.

And I’d recommend not leaving your roof relying on underlayment that long. It would be better to strip the roof in sections and shingle as you go in my opinion.

Ice down in valley first then you can either cut the underlayment or lap across, lapping across will probably provent the water from getting under. If it’s going to be open for a long period of time I would use 30# felt opposed to synthetic, less likely to leak and holds down better in wind.

I never put any nails through metal valleys shingle, slate or otherwise. We bend a 1/2" hem on the outter edges, known as a slaters edge, then secure it with clips made out of scraps of metal every 2’. Bend 1/2" fold in a 2x3" piece of similar metal and crimp the clip onto the slaters edge. In on the top or bottom sets of clips put an upward cut into the outside hem and slide the clip into it to prevent the metal from sliding down. Valleys have been done that way on slate roofs for hundreds of years, it A- allows for expansion and contraction and B- the skaters edge makes a channel that prevents the water from washing over the valley metal. You don’t want to nail your shingles through the valley metal either.

So am I to leave a large gap then? The valley metal is 20 in wide.

I thought the whole point of synthetic is it’s water proof and can hold up to winds when using the proper securement method. Mine has 3 month exposure I seen other have 6 months.

We nail down valley metal here to. It makes me cringe when I think too hard about it but it holds up fine. Just put enough nails in it to hold it. Overlap the underlay 3-4" onto the metal.
If your gonna leave it open that long double the overlap and make sure you use plastic cap nails.

Synthetic is much better for a longer term dry in, felt doesn’t hold up very well when exposed.

Make sure you use plastic cap nails to secure the synthetic underlayment, staple by themselves will leak.

The no nails in the valley thing is for cedar & slate when the valley is held in by clips and is not as wide as the valley used with asphalt shingle.

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Axiom just said what I meant but better lol. IMO not nailing through valley metal on a shingle roof is a LOT of extra work (esp for an inexperienced diyer) for no real gain.

To each there own. Might take an extra 5 minutes a section to clip vs nailing through. Makes me cringe seeing guys shooting cheap e.g. Coil nails through a metal valley, epically copper, that if insralled right will last next 2 generations of shingles.