Most synthetic underlayment sold for residential roofs is a woven or spunbond polymer sheet. Once the shingles are on, it keeps wind-driven rain and melt water away from the sheathing, and it is light enough to carry up a ladder by the roll. What it does not do is tolerate sunlight indefinitely. Each manufacturer publishes a maximum exposure period, the number of days or months the sheet may sit uncovered before ultraviolet light breaks down the polymer. That number is on the product data sheet, and it is the number your spec should carry. A deck opened on a Monday and left bare through a week of spring storms can run past it.
This page is for the homeowner who has two or three proposals in hand and wants to know what the underlayment line commits the crew to. It does not rank products or name a winner. It explains what to write down before the tear-off begins.
Material and spec
Roofers have used asphalt-saturated felt for generations, and some older St. Louis houses still show it in the attic as a dark, brittle layer on the sheathing. Felt tears more easily under foot traffic, absorbs moisture, and has a shorter practical window of exposure than most synthetic sheets. Synthetic underlayment costs more per roll, but it covers more square per roll and handles the walking that happens during a tear-off with less damage. Neither material replaces correct flashing, a proper drip edge, or a deck that is sound. Tolerances differ from one product to the next, so the useful comparison is the data sheet for the specific product, not the category name.
Keys installs Atlas and Malarkey shingles. The shingle spec and the underlayment spec are separate decisions, and the proposal should name both. A written scope that says underlayment without a manufacturer, a product line, and a stated exposure limit leaves the most important part of the deck protection open until the crew arrives.
When the spec is written, it should name three things. The first is the product, by manufacturer and product line. The second is the stated exposure limit taken from that product's data sheet. The third is the underlayment type at each location on the roof. Eaves, valleys, and low-slope sections often receive a different layer or an additional membrane than the field of the main slope. A bid that lists underlayment as one line for the whole roof hides those differences, and you will not see them until the bill arrives.
What the exposure limit actually measures
The exposure limit is a clock that starts when the sheet is laid, not when the shingles go on. It is set by the manufacturer and measured against sunlight, not against rain. A sheet can be completely dry and still be close to its limit. Crews often lay underlayment on the first day of a tear-off to close the deck. The shingles then follow days or weeks later because of weather, a late delivery, or a short crew. That gap is the part to write down and ask about.
Ask the roofer to state what the data sheet says in plain numbers, and what the manufacturer expects to happen if the limit is reached. Some manufacturers state a calendar limit. Others require the sheet to be inspected before shingles go on, and some ask that damaged areas be replaced. Each answer is useful because it tells you what the crew must do on the day the clock is running. A general statement that the product is weather resistant is not useful, because it gives you no date and no inspection step.
Where the St. Louis calendar makes the gap longer
Spring tear-offs in the metro often run into a stretch of rain, gusty wind, and short workdays. A roof opened in late March can wait through several storm fronts before the shingle truck arrives. Summer is the other hazard. Direct sun on a dark synthetic sheet can run well above the air temperature, and ultraviolet load is highest from late May through July. A sheet that has a generous calendar limit in February may use up a much larger share of it in July, which is why the season of the tear-off belongs in the conversation.
Fall work has its own problem. A run of cool, damp mornings in October and November can keep the deck wet and push the start of the shingle install back by a day or two at a time. A wet deck is not a reason to skip the underlayment. It is a reason to confirm that the crew has a plan for the sheet while the deck dries, and that the plan fits inside the stated window.
Winter adds a separate issue. Ice dams at the eave often form over decks where the eave membrane was not in place when the snow arrived, and the underlayment is the layer that keeps melt water from reaching the sheathing. The freeze and thaw cycle in St. Louis can move water a long way up a roof once it finds a path under the shingles. For the underlayment spec, the point is simple: the eave membrane has to be on the deck and in its correct position before the first hard freeze, and the rest of the field should not be left open through the cold months.
What it costs
Underlayment is a small share of a shingle roof's total, and it is one of the lines that quotes describe most loosely. A synthetic sheet typically costs more per square than felt, and a proposal should show that difference as its own line rather than folding it into a general materials figure. The square count of underlayment depends on the roof's area, the eave length, and the number of valleys, and that count should match the measurement on the written scope. If the measurement is wrong, every underlayment figure built on it is wrong too.
Ask whether the figure includes the eave membrane, the valley material, and any ice and water shield at the eaves and valleys. These are often separate products with separate prices, and a proposal that names them only in passing may not include them at all. Look also for a line on the drip edge and whether the underlayment is lapped over it or tucked under it, since that sequence affects both the material count and the labor.
A quote that prices underlayment by the square with no product named is a number you cannot check later. A quote that names the product, gives its exposure limit, and shows the quantity per square lets you compare the same material across bids. If two proposals differ on underlayment, the difference is worth asking about. One may use a heavier synthetic or an extra membrane at the eave while the other uses the standard product, and the two numbers are not the same roof.
Do not accept a price that moves underlayment into a general allowance without a product named. An allowance is a dollar figure that stands in for a choice that has not been made yet, and that choice is the one that controls the exposure limit. If the allowance is the only thing in the proposal about the deck, ask for the product and the limit in writing before the contract is signed, not after the first delivery.
What to ask
Bring these questions to each roofer and write down the answers. Each one asks for a fact you can check, not a promise.
- Which underlayment product will go on the deck, by manufacturer and product line, and which type goes at the eaves, valleys, and field?
- What is the exposure limit on that product's data sheet, and can you send me the sheet before I sign?
- If the shingles cannot go on the same day as the underlayment, what is the plan for the open deck, and who checks the sheet before the shingles arrive?
- Does the price include the eave membrane, the valley material, and any ice and water shield, and where does each one appear on the written scope?
- What does the spec say happens if the sheet reaches its exposure limit before the shingles go on, and who pays for a replacement sheet if it does?
The answer to the first question should match the proposal line for line. If the roofer names a product that does not appear in the written scope, correct the scope before you sign. Keys puts a written scope with photos and measurements in the homeowner's hands before the tear-off, and the underlayment product and its exposure limit belong on that page with the shingle line. To ask which underlayment goes with a particular roof, call Keys at (314) 220-2333.
One last check before the crew arrives is to read the underlayment line against the calendar. Count the days between the planned tear-off and the planned shingle delivery, and compare that count with the exposure limit on the data sheet. If the count is larger than the limit, the schedule needs to change or the spec needs to change, and the conversation is simpler to have in the week before the tear-off than on the morning the deck is open.