Most homeowners never see their roof underlayment once the shingles are installed. That makes it easy to ignore. But underlayment is one of those hidden roof-system details that can separate a basic roof from a better-built roof. It sits between the roof deck and the shingles, helping protect the plywood or OSB if wind-driven rain, ice, snow, or temporary exposure gets past the top layer.

What does roof underlayment actually do?

Roof underlayment is the protective layer installed over the roof deck before shingles, metal, or another roof covering goes on. It is not the main roof surface. Your shingles or metal panels do the primary job of shedding water. But underlayment provides an important secondary layer of protection beneath the finished roof.

Think of it as the backup layer. If a shingle is damaged, if wind-driven rain gets under the laps, if the roof is temporarily exposed during installation, or if water reaches the deck during a storm event, the underlayment helps protect the wood structure below.

Underlayment is not the part of the roof you brag about from the driveway. It is the part that quietly helps protect the deck when the weather gets ugly. West Country Roofing

Underlayment is not a replacement for ice and water shield

This is important. Synthetic underlayment is a major improvement over tar paper in many ways, but it is not the same thing as self-adhered ice and water shield. A proper roof system should still use ice and water protection in critical areas like eaves, valleys, penetrations, skylights, low-slope transitions, and other vulnerable details where required or recommended.

Synthetic underlayment is excellent as a field underlayment across the roof deck. Ice and water shield is the more aggressive waterproofing layer used where water has a higher chance of backing up, sitting, or forcing its way into the system.

What is synthetic underlayment?

Synthetic underlayment is a modern roof underlayment made from engineered polymers rather than asphalt-saturated paper or organic felt. Different brands and grades vary, but most synthetic underlayments are designed to be lighter, stronger, more tear-resistant, and more weather-stable than traditional felt.

It usually comes in larger rolls, lays flatter, resists wrinkling better, and is easier for roofers to handle on the roof. For homeowners, that means a cleaner installation and a stronger hidden layer beneath the shingles.

Why synthetic underlayment is usually better

Synthetic underlayment is not just a fancy upgrade for the sake of sounding modern. It solves real problems that roofers commonly see with older felt products. Tar paper can wrinkle, tear, absorb moisture, dry out, become brittle, and get damaged more easily during installation. Synthetic underlayment is designed to reduce many of those issues.

Better for modern shingle systems

Modern architectural shingles are not the same as the old-school roofing products from decades ago. Roof systems have improved, manufacturer requirements have become more specific, ventilation matters more, and homeowners expect better long-term performance. Synthetic underlayment fits that modern standard better than tar paper.

At West Country Roofing, we look at underlayment as part of a full-system approach. The goal is not just to get shingles on the roof. The goal is to build a roof with proper layers, proper water management, and fewer weak points.

What is tar paper or asphalt felt?

Tar paper, often called asphalt felt or roofing felt, is the older traditional underlayment used under shingles for many decades. It is made from a base mat saturated with asphalt. You will commonly hear terms like 15-pound felt and 30-pound felt, although modern product weights do not always match the old naming exactly.

Tar paper was the standard for a long time because it was affordable, familiar, widely available, and better than installing shingles directly over bare wood. It served a purpose. But being the old standard does not mean it is the best choice today.

The issue with tar paper

The main issue with tar paper is that it is easier to damage and less stable under real jobsite conditions. It can wrinkle when it gets damp. It can tear during installation. It can become brittle over time. It is heavier to handle, comes in smaller rolls, and often creates more seams across the roof deck.

That matters because underlayment is supposed to be a reliable secondary layer. If it is torn, wrinkled, poorly lapped, or damaged before the shingles even go on, the system is already starting from a weaker place.

Is tar paper still allowed?

In many situations, traditional felt may still be allowed depending on the product, code requirements, manufacturer requirements, and roof assembly. But “allowed” and “best choice” are not the same thing. A roof can meet a minimum requirement and still not be the roof system we would recommend if the goal is stronger long-term performance.

Tar paper is not automatically useless. It can still function when installed correctly in the right application. But if the choice is between a quality synthetic underlayment and basic tar paper for a modern residential roof replacement, synthetic is usually the more professional and better-performing option.

Water resistance: why synthetic has the advantage

Both tar paper and synthetic underlayment are designed to help shed water beneath the roof covering. But synthetic underlayment generally handles moisture exposure better. It does not absorb water like felt can, it usually stays flatter, and it is less likely to wrinkle after getting damp.

Wrinkling matters because a roof should be installed over a smooth, stable base. If tar paper wrinkles beneath the shingles, it can affect the finished appearance and create small irregularities under the roof surface. Synthetic underlayment is usually more dimensionally stable, which helps keep the roof deck cleaner and more consistent before the shingles are installed.

Temporary exposure

During a roof replacement, underlayment may be exposed briefly before shingles are installed. Weather can change quickly. Wind can pick up. Rain can move in. Synthetic underlayment is generally much better suited to temporary exposure than traditional felt, provided it is installed correctly and within the manufacturer’s exposure limits.

That last point matters: every product has limits. Synthetic underlayment is better, but it still needs to be installed according to the manufacturer’s instructions. Fastener type, lap direction, cap nails or cap staples, exposure time, slope, and weather conditions all matter.

Installation: synthetic is cleaner, faster, and tougher

Roofing is physical work. Materials get carried, rolled, walked on, pulled tight, cut, lapped, fastened, and exposed to wind. A product that handles the jobsite better often leads to a better finished roof.

Synthetic underlayment is lighter and typically comes in larger rolls than tar paper. That means fewer roll changes, fewer seams, easier handling, and a cleaner installation process. It also tends to resist tearing better when crews are working across the roof.

Fewer seams can mean fewer weak points

Every seam in a roof system needs to be overlapped correctly. Larger synthetic rolls can reduce the number of seams compared with traditional felt. Fewer seams does not remove the need for proper installation, but it can make the system cleaner and more efficient.

Better in windy conditions

Anyone who has installed felt in wind knows it can be frustrating. Tar paper can tear, fold, flap, and rip if conditions are not ideal. Synthetic underlayment is generally tougher and easier to keep controlled, especially when installed with proper cap fasteners.

Cost: why synthetic is worth the upgrade

Tar paper is usually cheaper upfront. That is the main reason it still gets used. But underlayment is not the place where most homeowners should chase the lowest possible number. The cost difference between basic felt and synthetic underlayment is usually small compared with the total cost of a roof replacement.

When you consider the whole roof system—shingles, labour, disposal, flashing, ventilation, metal edging, valleys, ridge cap, starter, ice and water shield, and workmanship—the underlayment upgrade is a relatively minor part of the total project. But it can make a meaningful difference in installation quality and secondary protection.

Saving a little on the hidden layer beneath your shingles rarely makes sense if the goal is a stronger roof system. Synthetic underlayment is one of those upgrades that fits the roof, not just the sales pitch. West Country Roofing

The cheap layer can become the weak layer

A roof is only as strong as its weakest details. If the shingles are good but the underlayment tears easily, wrinkles badly, or gets damaged during installation, the system is not as strong as it should be. That does not mean underlayment carries the whole roof. It means the hidden layers deserve respect.

For homeowners planning to stay in their home, synthetic underlayment is usually a simple, sensible upgrade. It improves the roof system without needing to jump into luxury-product territory.

When might tar paper still make sense?

There are situations where tar paper may still be used. A detached shed, very budget-sensitive project, temporary structure, or certain repair applications may not justify the same specification as a full residential roof replacement. Some roofers may also use felt in specific assemblies where it is acceptable and compatible with the roofing product.

But for a proper residential shingle replacement, especially when the homeowner wants long-term value, cleaner workmanship, and a more modern system, synthetic underlayment is usually the better call.

Synthetic underlayment vs tar paper comparison

The difference between synthetic underlayment and tar paper is not just “new vs old.” It is strength, stability, installation quality, moisture resistance, roll size, jobsite durability, and long-term confidence in the hidden roof system.

What about 15-pound felt vs 30-pound felt?

Traditional felt is often discussed as 15-pound or 30-pound. In general, 30-pound felt is heavier and more durable than 15-pound felt. If someone insists on using felt, 30-pound is usually the stronger option.

But even 30-pound felt does not change the bigger picture. Synthetic underlayment is still usually easier to install cleanly, more tear-resistant, more moisture-stable, and better suited to modern roof replacements. Choosing heavier felt may improve an old-school felt system, but it does not make it equal to a quality synthetic underlayment.

Synthetic underlayment in a complete roof system

The best way to think about synthetic underlayment is not as a standalone miracle product. It is part of a complete system. A roof still needs the right shingles, proper starter, ridge cap, eave protection, valley protection, ventilation, flashing, drip edge, gutter apron, fasteners, and clean workmanship.

That is why synthetic underlayment makes sense. It strengthens the middle layer of the system without pretending to replace the other critical pieces.

Final thoughts

Tar paper had its place. It was the standard for a long time, and it can still function in certain applications. But roofing has moved forward. Materials are better, homes are more expensive, weather expectations are higher, and homeowners deserve roof systems that are built with modern protection in mind.

For most residential shingle roofs, synthetic underlayment is the better choice. It is stronger, lighter, cleaner, more stable, more tear-resistant, and better suited to a professional full-system roof replacement. It does not replace ice and water shield, and it does not make flashing or ventilation less important. But as the main field underlayment beneath shingles, it is the option we would push heavily over tar paper.

If a quote still includes basic tar paper, it is worth asking why. Sometimes the answer is simply cost cutting. And when it comes to the hidden layers protecting your roof deck, cost cutting is not usually where the best roofs are built.

Synthetic underlayment vs tar paper FAQ