Roofing Underlayment: Synthetic vs Felt vs Ice-and-Water
Underlayment is the layer nobody sees and almost nobody asks about. It sits between your roof deck and your shingles, it is usually a line item on an estimate that gets skimmed past, and it is one of the few places where the cheap option genuinely costs you more.
Shingles are not waterproof. They are water shedding. They move the great majority of water off the roof, and the underlayment is what handles everything that gets past them: wind-driven rain, water backing up under a lifted shingle, condensation, and the period between tear-off and the new roof going on.
The three types, and what each is for
Asphalt-saturated felt
The traditional option, sold as fifteen pound and thirty pound. It is organic or fibreglass mat saturated with asphalt, and it has been used for the best part of a century.
It works. Its weaknesses are that it tears easily during installation, it wrinkles when it gets wet and those wrinkles telegraph through the shingles above, it degrades reasonably quickly when exposed to sun, and it is heavy for what it delivers. Thirty pound felt is meaningfully better than fifteen pound and the price difference is small.
Synthetic underlayment
Woven or spunbonded polypropylene or polyethylene. Lighter, far stronger, does not tear when a boot catches it, does not wrinkle when it gets damp, and tolerates weeks of sun exposure rather than days.
The practical advantages matter more than the specification sheet suggests. A synthetic underlayment stays intact through installation, which means the layer actually performs as designed rather than being full of small tears nobody noticed. It also stays watertight if the job is interrupted by weather, which on a multi-day roof is not a hypothetical.
It costs more per square than felt. The difference on a typical house is small relative to the roof as a whole, and it is the upgrade we would recommend on almost every job.
Ice-and-water shield
This is a different product doing a different job, and it is often confused with the other two. It is a self-adhering rubberised asphalt membrane that sticks directly to the deck and seals around nails driven through it.
That self-sealing property is the whole point. Ordinary underlayment is penetrated by every nail. Ice-and-water shield closes around the shank, so water sitting on top of it cannot get through the fastener holes. It is not a whole-roof product on most homes because the roof needs to breathe, and it is considerably more expensive. It goes where standing water is a genuine risk.
Where each one belongs
Most well-built roofs use two products, not one:
- Eaves and the bottom courses. Ice-and-water shield. This is where ice dams form and where water backs up under shingles.
- Valleys. Ice-and-water shield. Valleys concentrate the runoff from two roof planes into one channel and carry more water than anywhere else on the roof.
- Around penetrations. Ice-and-water shield at chimneys, skylights, and where roofs meet walls.
- Low-slope sections. Ice-and-water shield, because shallow pitches drain slowly and water has time to find a way in.
- The rest of the field. Synthetic underlayment.
What this looks like across our markets
- Denver and Parker. Ice dams are a real and recurring risk, so eave protection is not optional. This is the market where ice-and-water shield earns its cost most obviously.
- Myrtle Beach, Charleston, Savannah and Jacksonville. The risk is wind-driven rain rather than ice. Water gets pushed sideways and uphill under shingles during a storm, and the underlayment is the only thing standing behind them. Synthetic holds up where felt has often already degraded.
- Oklahoma City and Springfield. Both. Ice at the eaves in winter, wind-driven rain and hail in spring.
- Everywhere. Heat. An attic that runs hot cooks felt from below over the years, which is one more argument for synthetic.
Questions worth asking about your estimate
Underlayment is frequently described on estimates in terms vague enough to mean anything. Specifics to ask for:
- Which product, by manufacturer and name, not just “synthetic underlayment”.
- How much of the roof gets ice-and-water shield, and measured how — one course at the eaves, or two?
- Are the valleys getting ice-and-water shield or just underlayment?
- What happens around penetrations?
- If felt is specified, is it fifteen pound or thirty pound?
A contractor who answers all five precisely is telling you something useful about the rest of the job. Vague answers here usually mean the cheapest available product.
If you are weighing up systems, our guide to roofing types covers what goes above the underlayment, and the GAF roofing system pages explain how the layers are specified to work together as one assembly. A free roof inspection is the way to find out what is currently on your deck.
Underlayment questions
On almost every roof, yes. It does not tear during installation, it does not wrinkle when damp, and it tolerates weeks of sun exposure rather than days. The price difference on a typical house is small compared with the roof as a whole, and the layer actually performs as designed rather than arriving full of small tears.
Ordinary underlayment, felt or synthetic, is a water shedding sheet that every nail penetrates. Ice-and-water shield is a self-adhering rubberised membrane that seals around the nail shank, so water sitting on it cannot get through the fastener holes. They do different jobs and a good roof uses both.
Yes, though for a different reason. In warm coastal markets the risk is wind-driven rain pushed sideways and uphill under shingles during a storm, plus slow drainage on low-slope sections. Valleys and penetrations benefit from it in any climate.
No. It is removed with the old shingles during tear-off and replaced. Any estimate that leaves the existing underlayment in place is either an overlay, which is a different job entirely, or cutting a corner.
It depends on the product. Felt begins to degrade in days and wrinkles if it rains. Quality synthetics are rated for weeks of exposure. This matters on larger roofs and in unsettled weather, which is exactly when a job is most likely to be interrupted.
Thirty pound is noticeably more durable and resists tearing better, and the cost difference is small. If felt is being used at all, there is little reason to specify fifteen pound.