Ice and water shield is a self-adhering, rubberized membrane that bonds to the roof deck and seals around every nail or screw driven through it. Under a metal roof it’s the backup layer for the spots where water backs up or sits: the eaves, the valleys, and around anything that comes through the roof.

If you build anywhere with real winters, including most of the Inland Northwest, you’ll almost always want it at the eaves, and your building code may require it. Here’s where it goes, why a metal roof needs a high-temperature version, and where regular underlayment does the job. No sales pitch, just the information you need.

What ice and water shield does

The classic problem is the ice dam. Heat from the house melts snow on the upper roof, the water runs down to the cold eave past the wall line, and it freezes again. More meltwater ponds behind that ridge of ice and gets pushed back up the slope, under laps and trim, and onto the deck.

Metal sheds snow better than shingles, but eaves can still dam, especially on lower slopes or where snow guards hold snow in place on purpose. The membrane is what keeps that backed-up water out of the house. Because it’s self-sealing, it closes around the fasteners that go through it instead of letting water follow them in.

You’ll also hear it called an ice barrier, ice water shield, or ice and water membrane. The name comes from one well-known brand, but people use it for the whole category.

Where ice and water shield goes on a metal roof

• Eaves. The International Residential Code calls for an ice barrier where there’s a history of ice forming along the eaves, running from the edge of the roof to at least 24 inches inside the exterior wall line (IRC Chapter 9). Your local building department decides whether it applies to you, and many builders in snow country run it farther up the roof than the minimum.

• Valleys. Two roof planes dump water into one channel here, so the membrane usually runs the full length of the valley.

• Penetrations. Around pipes, chimneys, and skylights. A membrane patch under a roof jack backs up the boot if it ever fails.

• Roof-to-wall joints. Where a roof meets a sidewall or headwall and water gets concentrated.

• Low-slope sections. Water moves slower on a flatter roof. Check the minimum slope for your metal roof panel, and treat anything near that limit with extra care.

Use a high-temperature ice and water membrane under metal

This is the part people miss. Metal panels get hot in summer sun, much hotter than the air. Standard granular-surfaced membranes made for shingle roofs can soften in that heat, stick to the back of the panels, and get in the way of the panels’ movement.

That movement matters. Steel grows and shrinks with temperature, and standing seam panels in particular need to slide on their clips. Here’s why standing seam installation depends on panels being free to move. Look for a membrane labeled high-temperature and approved for use under metal roofing, and follow both the membrane and panel manufacturers’ instructions.

Ice and water vs synthetic underlayment

Most metal roofs in snow country use both: the membrane at the trouble spots and a high-temperature synthetic over the field. Our guide to metal roofing underlayment covers the synthetic side and how to choose one.

FAQ

Do I need ice and water shield under a metal roof?

If you get snow and freezing temperatures, almost certainly at the eaves, and in the valleys and around penetrations too. Check with your local building department for what’s required where you build.

Can you put ice and water over the whole roof?

Some builders do, but the membrane is a vapor barrier. Covering the whole deck can trap moisture from below unless the roof is well vented. Here’s how a ridge vent for metal roofing works with intake at the eave.

Is ice water shield the same thing as ice and water shield?

Yes. It’s the same self-adhering membrane under a slightly different name.

How far up from the eave should it go?

Code sets the minimum at 24 inches inside the exterior wall line where an ice barrier is required. Deep overhangs, low slopes, and heavy snow are all reasons to go farther.

Bottom line

A self-adhering membrane is cheap insurance at the spots where a metal roof is most likely to see backed-up water. Put it at the eaves, valleys, penetrations, and wall joints, use a high-temperature version made for metal, and let a high-temperature synthetic cover the rest. If you’re planning a roof and want help picking panels for your snow load, give us a call or request a quote.