Ice and Water Shield and Underlayment — What Goes Under the Shingles
Your roof quote lists ice and water shield and synthetic underlayment and you want to know what they are, where they go on a Minnesota roof, and whether the whole roof should get the membrane.
Ice and water shield and underlayment are the two layers between the shingles and the deck. Ice and water shield is a self-adhered rubberized membrane that seals around nails; JMB Builders runs it from the drip edge up past the warm wall line the Minnesota code calls for, in every valley and around every pipe, vent, chimney and skylight. Synthetic underlayment covers the rest of the deck. Both go on every roof we replace across the Twin Cities metro.
- Eave membrane to the Minnesota code line, every valley
- Synthetic underlayment on the rest of the deck, not felt
- Membrane around every pipe, vent, chimney, skylight
- Send photos, reply within one business day
- Licensed MN contractor BC809005
Send photos and hear back within one business day — or book a free in-home estimate.
Shingles shed water. They do not seal it. Wind drives rain under the tabs, an ice dam backs meltwater up the slope until it finds a nail, and a valley carries two roofs' worth of water in a channel. What stops that water at the deck is the layer under the shingles, and on a Minnesota roof it is two different products doing two different jobs.
Ice and water shield is a rubberized asphalt membrane with a peel-off backing. It sticks to the deck, and when a nail goes through it the rubber closes around the shank. It goes where water sits or backs up: along the eaves from the drip edge up to and past the line of the inside wall below, so the ice dam that forms at the cold overhang is sitting on membrane, not on bare deck; up the full length of every valley; around every pipe, vent, chimney and skylight; and over any low-slope section. On a shallow roof with a deep overhang that eave band gets wide.
Synthetic underlayment covers everything else. It is a woven polymer sheet that replaced asphalt felt: lighter, it does not tear in wind or wrinkle when wet, you can walk on it without slipping, and it can sit exposed for weeks if the weather turns. Felt still meets code; we do not use it. Some homeowners ask for membrane over the entire deck. It can be done and we will quote it, but a fully sealed deck needs the attic ventilation to be right or moisture that used to escape through the felt has nowhere to go.
What you supply
- Photos of the eaves and any valleys from the yard, and the attic underside if you can reach it
- The depth of your overhang if you know it, or a photo of the soffit; it sets how wide the eave membrane goes
- A note about where the roof has leaked before, especially after a snowy winter
What JMB supplies
- Ice and water shield at the eaves to the coverage the Minnesota code calls for, measured from the inside wall, not guessed
- Membrane the full length of every valley, around every penetration and over any low-slope area
- Synthetic underlayment over the rest of the deck, lapped and capped-nailed
- Drip edge under the membrane at the eaves and over the underlayment at the rakes, so water drains onto metal
- New step flashing, kick-outs and pipe boots set into the membrane, not over it
- Photos of the dried-in roof before shingles in your job folder
How it is priced: per project
Quoted in writing from your photos, with a reply within one business day. What moves the number:
- Eave length and overhang depth, which set how many rows of membrane the eaves need
- Number and length of valleys
- Count of penetrations: pipes, vents, chimneys, skylights, each wrapped in membrane
- Low-slope sections that get full membrane coverage
- Whole-deck membrane if you ask for it, as its own line
- The rest of the roof: tear-off, squares, pitch, decking, shingle line
Both layers are included on every roof we replace and listed on the scope. If a competing quote does not list them, ask where the membrane goes.
How long it takes
Dry-in happens the same day as tear-off: membrane and underlayment go on as each section of deck is bared so the house is never open overnight. Shingles follow the same or next day.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized reply within one business day. Prefer a look in person? A free in-home estimate is always an option.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Eaves to the code line
Membrane from the drip edge up past the line where the heated wall below meets the roof. On a deep overhang or a low pitch that is several rows. This is the ice dam defense.
Valleys and penetrations
A full-width run of membrane down every valley before the shingles, and a square around every pipe, vent, chimney and skylight with the flashing set into it. Where most roof leaks start.
Low-slope sections
A porch or addition roof too shallow for shingles alone gets full membrane coverage under the shingles, or a membrane roof on its own if it is nearly flat. Covered on its own page.
Whole-deck membrane
Every square foot of deck sealed. Quoted as its own line on request. Only worth it with ventilation that is already right, because a sealed deck traps attic moisture the felt used to let through.
Ice and Water Shield and Underlayment — What Goes Under the Shingles: questions, answered
How far up the roof does ice and water shield need to go in Minnesota?
From the drip edge at the eave to a line past where the inside face of the heated wall below meets the roof; the Minnesota code sets that distance and it is what we install to. On a house with a deep overhang or a low pitch that works out to several rows of membrane, because the point above the warm wall is far up the slope. We measure it from the wall line at the estimate, not from a rule of thumb.
Should ice and water shield cover the whole roof?
It can, and we quote it when asked, but it is not the default. The eaves, valleys, penetrations and low-slope areas are where water sits or backs up; the rest of a pitched roof sheds water and synthetic underlayment handles it. A fully sealed deck also traps attic moisture that used to escape through felt, so it only makes sense with soffit intake and ridge exhaust that are already working. The membrane is also the most expensive layer per square.
What is the difference between synthetic underlayment and felt?
Felt is asphalt-soaked paper; synthetic is a woven polymer sheet. Synthetic is lighter, does not tear when the wind catches it, does not wrinkle when it gets wet, is safer to walk on, and can sit exposed for weeks if the weather turns. Felt still meets code and costs a little less per roll. We use synthetic on every roof.
Does ice and water shield stop ice dams?
It stops the leak, not the dam. The dam forms because heat from the house melts snow that refreezes at the cold eave; the membrane keeps the water that backs up behind it from getting through the deck. The fix for the dam itself is air sealing, insulation and ventilation in the attic. The membrane is the backstop for the winter when the dam forms anyway.
Does ice and water shield go under or over the drip edge?
At the eave the drip edge goes on the deck first and the membrane laps over it, so water running down the membrane lands on metal and drips clear. At the rakes the drip edge goes over the underlayment. Getting this backwards sends water behind the drip edge onto the fascia, which is how fascia boards rot behind a gutter.
Why does my new roof need membrane in the valleys?
A valley carries the runoff of two slopes in one channel and it is where snow and ice sit longest. Shingles alone in a valley rely on overlaps that wind and ice can get under. A full-width run of membrane down the valley before the shingles go on means a leak in the shingle valley does not reach the deck. It is included on every roof we replace.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners, builders and dealers. Counts come straight from our job log; no customer names.
Get a written quote for this job
Three photos and a sentence is enough: a reply within one business day, or ask for a free in-home estimate. Or text the photos to 763-333-4066.
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Read morePage reviewed October 2, 2026 · JMB Builders LLC, MN license BC809005 · All roofing pages
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