Roof underlayment is the sheet that sits between your roof deck and your shingles, and it is the part of the roof doing the work when a storm strips shingles away. Most homeowners first hear the word on a quote, as a line item they cannot see or compare. This guide explains what roof underlayment is, how felt, synthetic and self-adhered membranes differ, what the residential building code says about it, and how it shows up on a Columbus, Georgia estimate. Costs are third-party national ranges, clearly labelled. Nothing here asks you to go up on a roof.
Roof Underlayment, the Second Line of Defense
Roof underlayment is a water-resistant layer fastened over the wooden roof deck before shingles go on. Shingles shed most rain. Underlayment catches the water that gets past them, from wind-driven rain, a cracked tab or a shingle that blew off, and steers it down to the eaves instead of into the deck and attic.
Think of the roof as three layers working together:
| Layer | What it is | What it does |
|---|---|---|
| Deck (sheathing) | Plywood or OSB boards nailed to the rafters | Gives the roof its structure and a surface to nail into |
| Underlayment | A felt, synthetic or self-adhered sheet over the deck | Second line of defence against water; protects the deck during installation |
| Roof covering | Shingles, metal panels or tile | First line of defence; sheds rain and takes the sun |
Underlayment is not meant to be the roof on its own. When shingles are lost in a wind event, it is often the reason a ceiling stays dry until a repair crew arrives, but an exposed underlayment is protecting the home on borrowed time.
If the deck under the underlayment is already soft or rotted, the underlayment cannot fix that. Damaged decking has to be replaced before new layers go on, and our guide to decking rot found during tear-off explains how that is priced.
The Main Underlayment Types on Homes
There are three main types of roof underlayment on homes: asphalt-saturated felt, often called tar paper; synthetic underlayment, a woven or spun polymer sheet; and self-adhered membranes, sometimes called ice-and-water membrane, which stick directly to the deck. Each has a place, and many roofs use more than one type.
| Type | What it is | Where it tends to be used | Points to know |
|---|---|---|---|
| Asphalt-saturated felt (“tar paper”) | Paper or fibre mat soaked in asphalt; sold in weights such as #15 and #30 | Field of the roof under shingles | Listed in the code under ASTM D226 and D4869; Fixr calls it the most economical type |
| Synthetic underlayment | Polymer sheet | Field of the roof under shingles or metal | Fixr describes it as thinner, lighter and more durable than felt; check which standard or evaluation the product meets |
| Self-adhered membrane | Rubberized asphalt (polymer-modified bitumen) with a sticky back | Valleys, eaves, around chimneys and penetrations, low-slope areas | Seals around nails; costs more per square foot; the code allows it as an alternative under ASTM D1970 |
The building code table that lists underlayment for asphalt shingles names felt-type standards (ASTM D226 and ASTM D4869) and an inorganic glass-fibre standard (ASTM D6757), and it allows self-adhered membranes under ASTM D1970 as an exception. It does not list synthetic underlayment by name. If your quote specifies a synthetic product, it is fair to ask the roofer which standard or product approval it meets.
The kind of roof covering matters too. Metal panels and asphalt shingles sit differently on the deck, and our existing article comparing a metal roof vs asphalt shingles for Columbus homes covers the covering side of that choice.
Building Code Minimums for Underlayment
Georgia’s state minimum standard for one- and two-family homes is the 2018 International Residential Code with Georgia amendments, as published on UpCodes’ Georgia IRC 2018 roof assemblies chapter. Section R905.1.1 requires underlayment under asphalt shingles, metal, tile and other coverings, conforming to the standards in that chapter.
The parts of Chapter 9 that matter most for a shingle roof, as shown on that page:
- R905.1.1 Underlayment. Underlayment for asphalt shingles and the other listed coverings “shall conform to the applicable standards listed in this chapter.”
- Table R905.1.1(1). For asphalt shingles where the design wind speed is under 140 mph, it lists ASTM D226 Type I, ASTM D4869 Types I to IV, or ASTM D6757. At 140 mph or more it lists the heavier D226 Type II, D4869 Types III or IV, or D6757.
- Table R905.1.1(2). On slopes of 4:12 or steeper, one layer is applied “shingle fashion,” starting at the eave and lapped 2 inches. Slopes from 2:12 up to 4:12 need two layers.
- R905.2.2 Slope. Asphalt shingles may only be used on slopes of 2:12 or greater.
- R905.1.2 Ice barriers. An ice barrier is required only “in areas where there has been a history of ice forming along the eaves,” extending at least 24 inches inside the exterior wall line.
Two practical points follow. First, a low-slope section of a shingle roof, such as a porch tie-in, has a stricter underlayment rule than the main roof. Second, the ice-barrier rule depends on local history, so it is not a blanket requirement. Which edition and amendments the city enforces today is a question for the Columbus Inspections and Code office. Confirm with them, not with a blog post, before relying on any code detail.
Tar Paper on Today’s Roofs
Yes. Asphalt-saturated felt, the product people call tar paper, is still one of the underlayment options the residential code lists for asphalt shingles, under ASTM D226 and ASTM D4869. Synthetic underlayment is the main alternative. Whether your roofer uses felt or synthetic is a product choice, and a good quote names which one, along with the standard it meets.
The question usually comes up when one quote says “synthetic” and another says “felt.” The type alone does not show corner-cutting. What matters is that the product meets a recognised standard and is installed to the required laps and fastening. A line that just says “underlayment” with no product named is the bigger warning sign.
Reading Underlayment on a Roofing Quote
On a re-roof quote, underlayment is usually its own line item, priced by area, sometimes with a separate line for self-adhered membrane in valleys and around penetrations. Two quotes can name different products for this layer, and that is one reason totals differ even when the shingles are the same.
Homeowners comparing quotes often cannot tell why the totals are far apart. Line items such as layers torn off, decking allowance and underlayment make two quotes comparable, not the bottom number.
When you read the underlayment line, check for:
- The product type. Felt, synthetic or self-adhered, and ideally the product name.
- Where the self-adhered membrane goes. Valleys, eaves, around chimneys and pipes, or the full deck.
- What happens to the old layer. A full tear-off removes the old underlayment with the shingles; the quote should say so.
- The decking allowance. Underlayment goes on after any rotten sheets are replaced, so the quote should price a per-sheet allowance in advance.
- Low-slope sections. If part of the roof is between 2:12 and 4:12, the quote should reflect a two-layer application there.
A change order for decking found during tear-off can be legitimate, because rot under old underlayment is invisible until it is opened. A contract that names the per-sheet price before work starts avoids an argument mid-job. For more on reading the whole estimate, see what drives the gap between roofing quotes.
Underlayment Prices, by the Square Foot
Fixr, a cost-estimating site, puts the national average for roof underlayment replacement at about $900, within an $800 to $1,200 range, and $0.20 to $2 per square foot installed. These are Fixr’s national figures, last updated January 2025. They are not Emergency Roofing Columbus prices and do not include shingles or labour for the whole roof.
Fixr’s roof underlayment replacement cost page breaks material-only costs down by type:
| Underlayment type | Fixr material cost (per sq ft, national) |
|---|---|
| Felt (asphalt-saturated) | $0.05 to $0.50 |
| Synthetic | $0.17 to $0.55 |
| Rubberized asphalt (self-adhered) | $0.55 to $0.65 |
Read these as a guide to the gap between types, not as a Columbus quote. Local labour, roof pitch, valley membrane and decking replacement all move the real number, and we do not publish our own prices without seeing the roof.
Longest-Lasting Underlayment Options
Fixr’s cost page says most underlayments need replacing every 20 to 30 years, that 50-year underlayments exist, that synthetic materials can last “up to 50 years,” and that felt lasts the least. That is a third-party summary, not a manufacturer warranty. A product’s own data sheet is the source to trust for any lifespan figure.
In practice, underlayment is replaced when the roof covering is replaced, because it sits under the shingles and cannot be renewed on its own without stripping them. So the useful question is less “which lasts longest” and more “will this layer last as long as the shingles over it.” If you are paying for a long-life shingle or metal roof, ask the roofer to show you the underlayment manufacturer’s published data for the product on your quote.
Columbus Weather Puts the Layer to Work
Columbus gets rain in every month of the year, summer highs above 90 °F, and repeated tropical-storm declarations. Those conditions test the layer under the shingles more often than a drier climate would, especially after wind lifts or removes shingles and the underlayment becomes the only barrier against the next storm.
NOAA’s 1991-2020 climate normals for the Columbus Metropolitan Airport station show no month averaging under 2.78 inches of rain. March is the wettest at 4.92 inches, and July’s average high is 92.9 °F. Summing the monthly normals gives about 49 inches a year; that total is our arithmetic, not a NOAA figure. There is no long dry season for a damaged roof to wait out.
Wind is the other half. The OpenFEMA disaster declarations data for Muscogee County lists hurricane-related declarations for Opal in 1995, Irma in 2017, Michael in 2018 and Helene in 2024. A declaration is not a damage count, but it shows the county keeps finding itself in the path of storm remnants. After that kind of wind, a roof can look fine from the street while shingles have lifted and the underlayment is taking water. Our guide to hidden roof damage after tropical-storm winds covers what you cannot see from the ground.
Age matters as well. Census Reporter’s republication of the American Community Survey table B25034 for Muscogee County counts 93,004 housing units, and adding up the year-built rows gives about 46,650 built before 1980.
Four Questions to Ask Before a Re-Roof
Ask four things: which underlayment product is on the quote and what standard it meets, where self-adhered membrane will go, how damaged decking will be priced, and how low-slope sections will be handled. The answers tell you whether two quotes describe the same roof or two different ones.
Useful questions to put in writing:
- Which underlayment product is included, and can you share its data sheet?
- Where does self-adhered membrane go: valleys, eaves, chimneys, pipe and vent openings?
- Are any parts of the roof below 4:12, and how is underlayment applied there?
- Who confirms the permit with Columbus Inspections and Code, if one is required?
For a shingle roof, our team covers shingle roof systems and what goes under them. Whoever you hire, ask for a quote that names the underlayment rather than leaving it as one word. Underlayment is one piece of a roof that holds up through storm season; the companion pillar on protecting your Columbus home from storm and roof damage puts it alongside insurance, storm preparation and safety.
If shingles are already missing after a storm, keep off the roof. Photograph the damage from the ground and call a roofer to cover the exposed area.
Frequently asked questions
Is synthetic roof underlayment better than felt?
Neither is automatically better. Fixr describes synthetic as thinner, lighter and more durable, and felt as the most economical material. What matters most is that the product meets a recognised standard and is installed to the required laps and fastening.
Does the code require ice and water shield in Georgia?
The Georgia edition of IRC 2018 requires an ice barrier only in areas with a history of ice forming along the eaves. A roofer may still specify self-adhered membrane in valleys and around penetrations. Confirm local requirements with the Columbus Inspections and Code office.
How many layers of roof underlayment do I need?
It depends on the roof slope. Under the Georgia edition of IRC 2018, asphalt shingles on slopes of 4:12 or steeper need one layer applied shingle fashion and lapped 2 inches. Slopes from 2:12 up to 4:12 need two layers. Confirm the rule in force with the Columbus Inspections and Code office.
What thickness roof underlayment should I use?
For felt under asphalt shingles, the Georgia edition of IRC 2018 lists ASTM D226 Type I or ASTM D4869 Types I to IV where the design wind speed is under 140 mph, and the heavier D226 Type II or D4869 Types III or IV at 140 mph or more. Ask which one your quote uses.
What are the disadvantages of synthetic roof underlayment?
Synthetic underlayment can cost more than felt for materials: Fixr’s national ranges are $0.17 to $0.55 per square foot for synthetic against $0.05 to $0.50 for felt. The Georgia IRC 2018 underlayment table also does not list synthetic by name, so ask your roofer which standard or product approval it meets.