Stand in a Decatur driveway on reroof day and you will see the old shingles come off in the first hour. What goes back on next is not shingles at all. It is a layer of underlayment rolled across the bare deck, with a sticky, rubbery membrane pressed into the valleys and along the edges. Homeowners rarely see these layers again once the shingles are down, yet roof underlayment and ice-and-water shield do much of the work of keeping water out of the house. This guide explains what each one is, how they differ, what Georgia’s residential code asks for, and where they matter most on the older rooflines common around DeKalb County.
What is roof underlayment?
Roof underlayment is the water-resistant sheet installed directly over the roof deck, beneath the shingles. It is the backup layer: shingles shed most of the rain, and underlayment catches the water that gets past them, whether it is wind-driven rain pushed up under a tab or a leak at a damaged shingle.
It also protects the deck during the install itself. Once the old roof is stripped, underlayment goes down quickly so the plywood is not exposed if a summer shower rolls through before the shingles are on.
Underlayment is not optional on a shingle roof in Georgia. The state’s one- and two-family dwelling code is built on the 2018 International Residential Code with Georgia amendments, and its roof assemblies chapter, viewable on UpCodes, sets out which underlayment materials are allowed under asphalt shingles and how they must be applied.
Felt or synthetic: what is the difference?
Felt underlayment is an asphalt-saturated paper or fiber mat, the traditional material named in the code through standards such as ASTM D226 and ASTM D4869. Synthetic underlayment is a woven or spun polymer sheet that does the same job, usually lighter per roll and more resistant to tearing.
Each has a place, and the difference shows up mostly during installation:
| Feature | Asphalt-saturated felt | Synthetic underlayment |
|---|---|---|
| Material | Paper or fiber mat soaked in asphalt | Woven or spun polymer sheet |
| Handling | Heavier rolls, can tear under foot | Lighter rolls, tougher under foot |
| Exposure before shingles | Can wrinkle if it gets wet | Generally holds up better if left exposed briefly |
| Code basis | Named directly in the code’s underlayment table | Accepted through the product’s own approval documents |
| Fastening | Nails or staples per the shingle and code rules | Fastened to the manufacturer’s pattern |
Neither material is fully waterproof at the fasteners. That is the gap ice-and-water shield is meant to close.
How must underlayment be installed under shingles?
On most Decatur roofs, those with a slope of 4 inches of rise per foot or steeper, the code calls for a single layer of underlayment applied shingle fashion: starting at the eave, running parallel to it, with each course lapped 2 inches over the one below and end laps of 4 inches offset by 6 feet.
Shallower shingle roofs get more. Asphalt shingles are only allowed on slopes of 2:12 or steeper, and between 2:12 and 4:12 the code requires two layers: a 19-inch starter strip along the eave, then full 36-inch sheets lapped 19 inches over each other. Porch roofs and shed dormers on older bungalows often fall in that range, which is one reason a porch can leak when the main roof does not.
The code also sets the order at the edges. A drip edge is required at eaves and rakes, and the underlayment goes over the drip edge at the eaves and under it along the rakes. Get that sequence wrong and water running down the underlayment ends up behind the metal instead of in the gutter.
What is ice-and-water shield?
Ice-and-water shield is a self-adhered membrane underlayment, usually rubberized asphalt with a peel-off backing, that sticks directly to the roof deck. In code language it is a self-adhering polymer-modified bitumen sheet. Its key property is that it seals around the nails driven through it, so water sitting on top cannot follow a fastener into the wood.
The name comes from cold climates. When heat escaping from a house melts snow on the roof, the water runs down and refreezes at the cold eave, forming an ice dam. The FORTIFIED program run by the Insurance Institute for Business & Home Safety explains that water trapped behind an ice dam has nowhere to go except under the shingles, and that ice-and-water shield offers some protection. The same article notes that most roofers install it only over roughly the first three feet at the bottom of the slope.
Does a Decatur roof need an ice barrier?
The code ties the ice barrier requirement to local history: an ice barrier is required where the jurisdiction designates a history of ice forming along the eaves and backing water up. Where it applies, it must be either two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet, carried from the lowest roof edge to at least 24 inches inside the exterior wall line. Whether that designation applies to a particular address is a question for the local building department.
Decatur does not see ice dams the way northern towns do, but the membrane earns its keep for other reasons here. It is commonly used on Georgia roofs in the places water concentrates or lingers:
- Valleys, where two roof planes pour into one channel during heavy rain
- Around vent pipes and other penetrations, where flashing meets the deck
- Where a roof meets a wall, such as the side of a dormer or a second-story wall
- Along the eaves, especially under trees where leaves slow drainage
- On shallow porch roofs, where water moves slowly across the surface
Valleys are the clearest case. On the cross-gabled bungalows and cottages found across the city, several valleys can carry a large share of the roof’s runoff, and a membrane-lined valley gives that water a sealed path even if a shingle shifts.
Is a fully sealed roof deck worth considering?
A sealed roof deck goes a step beyond spot use of membrane: the seams between deck panels are taped or the whole deck is covered in self-adhered membrane, so the roof stays watertight even if shingles are lost. The FORTIFIED Roof standard requires a sealed deck for exactly that reason, because when shingles come off, water can pass through the gaps in the exposed wood. The program says sealing the seams can reduce water intrusion by up to 95 percent, a figure from its own research.
For a homeowner in Decatur, the question is mainly about storm exposure and budget. Spring and summer thunderstorms bring wind that can strip shingles from an exposed slope, and a sealed deck limits what follows. It adds material and labor, so it is worth pricing as an option on a replacement rather than assuming it either way.
What to ask before your new roof goes on
The layers under your shingles are decided once, on replacement day, and nobody looks at them again until the next reroof. That makes the quote the moment to ask. Find out which underlayment is planned for the main slopes and for any porch roof under 4:12, whether the valleys and wall junctions get self-adhered membrane, and how the drip edge and underlayment are lapped at the eaves and rakes. If storm exposure worries you, ask what a taped or fully sealed deck would add. A clear answer to each of those tells you more about the finished roof than the shingle color does.
If you would like those choices laid out for your own house, the team at Roofing Decatur covers every layer in our roof replacement quotes, and our architectural shingle roofing page explains how the shingles above them are chosen.