Most homeowners reading a St. Louis roof quote look at the shingle line, the color and the gutter detail. The membrane under the lowest few feet of shingle usually gets one short line, and it is the part of the roof that takes the most water in a hard winter. This page covers that one spec: how far up the deck the eave membrane should run, what sets the length, and what to ask for in writing before the crew starts tearing off.
Material and spec
The eave membrane is a self-adhered sheet that bonds directly to the roof deck. It is a different product from the felt or synthetic underlayment that covers the rest of the roof. It goes down first, along the lowest edge and up the slope, and its job is to stop water that has backed up behind the shingles from reaching the deck and the wall below.
Self-adhered membrane and ordinary underlayment are not interchangeable. Underlayment lets water run out through its laps. The membrane seals to the deck around the nail holes, so water that gets under the shingle has nowhere to go but out through the sealed layer. That difference matters only at the eave, which is why the membrane is usually run there and nowhere else on the roof.
Keys installs Atlas and Malarkey shingles. Whatever membrane goes under those shingles should be named on the quote by product name, not described only as an ice barrier or an eave membrane. A product name tells you what you are getting and gives you something to check on the roof while the work is open.
The 24-inch line
The International Residential Code, which many local permit desks reference, calls for an ice barrier at the eaves in areas where ice has a history of forming along the lower edge of a roof. In that case the code says the barrier should extend from the lower edge of the roof to a point at least 24 inches inside the exterior wall line of the building. The 24 inches is measured horizontally from the inside face of the exterior wall, so a wall with a deep overhang or a thick exterior wall changes the actual length of membrane on the slope.
Each jurisdiction adopts its own edition of the code, and St. Louis County municipalities do not all use the same one. Before you accept a membrane length on a quote, ask the permit desk for your address which edition applies and whether the ice barrier requirement is in it. A permit desk can answer that question in a phone call. A roofer who has pulled permits in your town should be able to answer it without looking it up.
Why the freeze-thaw cycle sets the length
St. Louis winters rarely stay below freezing for long. The usual pattern is a hard freeze, a warm afternoon, a cold night and then a warm day again. On a sunny day the upper part of the roof warms and snow on it melts. The meltwater runs down the slope until it reaches the overhang, where the roof is cold because it is outside the warm attic and the wall below. The water refreezes at the edge and builds a ridge of ice. The next melt backs water up behind that ridge, under the shingles, and onto the deck.
The membrane has to reach far enough up the slope that backed-up water is caught inside it. A short membrane that stops at the wall line will often protect the very edge and fail a few feet up, where the water actually travels. The 24-inch rule is a minimum built around a typical ice build-up, not a promise that every roof will stay dry. Low-pitch sections, north-facing slopes that stay shaded, and areas under a tree canopy tend to hold snow longer and need the membrane to run farther than a sunny south slope on the same house.
The water that gets past the shingle at the eave often shows up later on a wall or ceiling, not on the roof. A stain on an upstairs wall with no visible roof leak above it is a common result of a membrane that stopped short. That is one reason the length belongs in the quote rather than in the crew's judgment on the day.
What sets the length on your house
Three measurements decide how much membrane the roof needs. The first is the distance from the eave edge to the inside face of the exterior wall, plus the 24 inches the code uses as a minimum in ice-prone areas. The second is the set of places where the roof surface changes direction, such as valleys and the lower edges of dormer walls, because water collects there and runs back toward the eave. The third is the pitch, because a flatter section holds water and snow longer than a steep one.
Dormers and valleys are the places quotes most often leave the membrane short. A dormer wall sits above the main roof, and water that runs down its face can pool at the base where it meets the lower slope. If the membrane stops at the main eave and never turns up that dormer base, the water finds its way under the shingle at a point the quote never addressed. Ask the roofer to show the membrane on a sketch of your roof, with the valleys and dormers marked, before the material is ordered.
What it costs
The membrane is usually priced in one of two ways. Some quotes carry it as a linear-foot line along the eave and the rakes that need it. Others fold it into a square count for the whole membrane area. Either can be fair. What is not fair is a membrane line with no quantity at all, or a line that says the material will be installed as needed. A line like that is not a price, and it leaves the length to be decided on the roof, when the shingles are already off and the crew is under pressure to finish the day.
Ask for the quantity in both feet and the area the quantity covers, and ask what happens if the measured eave turns out longer than the quote assumed. That question is more useful than asking for a lower number, because the answer tells you whether the price moves with the roof or stays fixed regardless of what the crew finds.
What to ask
Bring these questions to the roofer before you sign. Each one has a specific answer that can be written into the scope.
- How far up the deck will the membrane run, measured from the eave edge to a point inside the exterior wall line, and does that include the 24-inch allowance?
- Which membrane product is quoted by name, and what is the install method for the laps and the seal to the deck?
- Does the membrane run into each valley and turn up at the base of each dormer wall, and is that shown on the sketch?
- Where does the metal drip edge sit relative to the membrane at the eave, and is that detail written in the scope?
- Will the crew photograph the membrane before the shingles or the underlayment cover it, with a tape measure in the frame?
- Does the quote state the membrane quantity in feet and in area, and what happens to the price if the measured eave is longer?
- Which edition of the residential code applies at your address, and has the permit desk confirmed the ice barrier requirement?
Keys puts a written scope with photos and measurements in the homeowner's hands, so the membrane photos and the measured lengths can be checked against the quote after the job is done. If the answers to these questions are not in the document you are holding, the membrane is not yet specified. Call the Keys Roofing office at (314) 220-2333 and ask for the eave membrane detail to be written out before you choose a start date.