Shingles get most of the attention on a roof, but they are only the outer layer of a larger system. Underlayment sits between the roof deck and the shingles. It is there to help the roof manage water that gets past a shingle joint during wind-driven rain, to protect the deck while work is in progress, and to provide the material the selected shingle system calls for.

That does not make underlayment a substitute for good shingles, flashing, ventilation, or installation. It is one part of a complete roof. The useful question is not simply, “What is the cheapest roll?” It is, “What product belongs under these shingles, on this roof, with these details?” Starting there helps you compare materials without missing the pieces that make a reroof work.

What roof underlayment does

A properly installed shingle roof is designed to shed water from the top down. Shingles overlap so water moves toward the eaves. The underlayment below them is a secondary layer of protection, not the main weather surface. It helps the roof deck during temporary exposure and gives the system another layer behind the field shingles.

Underlayment also makes the work more manageable. On a reroof, the roof deck may be exposed after old materials come off and before the final shingle field is complete. A crew still needs dry, sound decking and the correct layout for the chosen products. The exact sequence, fastening, overlap, and exposure limits come from the underlayment and shingle manufacturer instructions, not from a generic online rule.

The roof measurement guide explains why each material needs its own unit. Field shingles are measured by squares or bundles. Underlayment is commonly sold by roll coverage. Starter and ridge materials often use linear feet. Keeping those units separate makes it easier to spot a mismatch before anyone starts loading a truck.

Roll of FeltBuster synthetic roofing underlayment

1. Asphalt-saturated felt

Asphalt-saturated felt is the traditional underlayment many people mean when they say “roofing felt” or “tar paper.” It has a long history under asphalt shingles and is still used on some projects. Felt is usually identified by its weight class, such as No. 15 or No. 30, although the exact product, coverage, and approved application matter more than a shorthand name.

Felt can be a sensible option when it matches the selected roof system and the installer is following its instructions. It is also familiar to many experienced crews. But it needs careful handling. It can tear more easily than some synthetic products, and it can wrinkle or absorb moisture if it is left exposed in poor conditions. That can affect how it lies beneath the shingles.

Do not choose felt just because a previous roof used it. The deck condition, roof slope, local weather, shingle manufacturer, and project schedule all matter. If the plan calls for felt, check the exact roll label for coverage and the written installation requirements before converting a roof measurement into rolls.

2. Synthetic roof underlayment

Synthetic underlayment is made from engineered materials rather than asphalt-saturated paper. It is a common choice for modern asphalt-shingle projects because many products are lighter to carry, resist tearing well, and cover more roof area per roll than traditional felt. Those advantages can make the roof easier to stage and can reduce the number of rolls that need to be handled.

The tradeoff is that “synthetic” is a category, not one universal product. One roll may have different fastening requirements, approved exposure time, surface grip, overlap markings, or coverage from the next. A roll that is suitable under one shingle system may not be the right choice for every roof. Treat its package data and manufacturer instructions as part of the order, not as fine print to sort out later.

Affordable Roof Supply currently lists synthetic felt and a FeltBuster synthetic roofing felt roll in its inventory. Listed coverage and yard availability can change, so use those pages to start the conversation, then call the yard to confirm the exact material and quantity before the trip.

When comparing synthetic options, ask for the roof-system requirements first. Then compare roll coverage, the installation method, and the materials that sit above and below it. For a shingle project, the selected shingle manufacturer’s instructions remain the standard to follow. For example, Owens Corning’s Duration Series installation instructions set out roof-deck and underlayment requirements for that specific shingle system.

Roll of self-adhered ice and water protection for roofing

3. Self-adhered weather protection

Self-adhered weather protection is a separate category from standard underlayment. It uses an adhesive backing to bond to the roof deck and is often used at vulnerable roof details or where a roof system, climate, or local requirement calls for added protection. You may hear it called ice and water barrier, membrane, or peel-and-stick protection.

It is not a blanket replacement for every other layer on every roof. The locations and amount depend on the roof design, the chosen products, and the applicable requirements. Valleys, eaves, walls, penetrations, and low-slope transitions are examples of details where a project may need additional attention. A qualified installer should decide the actual layout from the roof plan and manufacturer instructions.

The important buying point is that self-adhered protection is measured separately. A project might need a full field underlayment layer plus shorter sections of a self-adhered product at defined details. Do not fold both into one “felt” number. Record the roof areas and the linear details separately, then compare them with the coverage shown on each product package.

4. Specialty products for special roof systems

Some roofs use materials designed for specific systems, slopes, or conditions. Low-slope assemblies, metal roofing, tile, and other roof coverings can have different requirements from a standard asphalt-shingle reroof. A material that is appropriate beneath one covering may be wrong beneath another.

This is where broad product labels stop being useful. Ask what roof covering is being installed, what the roof pitch is, whether the deck has been repaired, and whether there are unusual transitions or penetrations. Those details steer the product decision more reliably than picking a roll by price or color.

If the project includes a deck repair, add that before finalizing the underlayment order. A roof deck must be addressed before the layers above it go down. The yard’s 7/16 OSB listing is one example of a material that may be part of a repair plan, but a qualified installer needs to evaluate the actual deck and any local requirements.

OSB roof sheathing panels stacked for a roof deck repair

Synthetic vs. felt: how to make the comparison

Synthetic underlayment and asphalt-saturated felt can both appear in roofing work, but they should not be compared as if the only difference is price. Start with compatibility. If the selected shingle system specifies approved underlayment choices, those instructions narrow the field. The next questions are coverage, handling, roof design, and the conditions the product may face before the shingles are installed.

Synthetic products often appeal because they can be lighter and more tear-resistant, with larger roll coverage. Felt can be familiar and appropriate for a compatible system, but it can be more sensitive to handling and temporary exposure. Neither label tells the whole story. The exact product instructions, not a general category comparison, decide whether it is suitable for a particular roof.

Avoid mixing components casually. A low price on one roll does not help if its coverage calculation, fastening method, or approved use does not fit the job. A better comparison is the complete installed system: the selected shingles, the underlayment, the protection at roof details, and the installation plan that ties them together.

Roof details change the material plan

A roof is not one clean rectangle. Valleys, eaves, hips, ridges, chimney walls, skylights, plumbing vents, sidewalls, and roof-to-wall transitions all change how materials are installed. These are also the places where water control depends on more than the field shingle pattern. Underlayment is part of that plan, but flashing and manufacturer-specific details matter just as much.

Valleys deserve special attention because they collect runoff from more than one roof plane. A valley may require a particular underlayment or weather-protection detail based on the shingle system and the selected valley method. Do not count the roof area and assume the valley is covered automatically. Put every valley on the roof sketch and confirm its material requirement as a separate item.

Eaves and roof edges should be recorded too. The appropriate detail depends on the local climate, the roof design, and the system instructions. The same goes for walls and penetrations. A pipe boot, chimney, or skylight is not simply a hole cut through underlayment. It needs the right flashing sequence so water is directed back onto the roof surface rather than behind the layers.

This is why a simple material list benefits from two columns. One column tracks roof areas in square feet. The other tracks linear details such as ridges, hips, eaves, and valleys. The first helps calculate field shingles and underlayment. The second helps the installer and yard identify starter, ridge cap, flashing, ventilation, and specialized protection that cannot be estimated from roof area alone.

Common underlayment mistakes to avoid

The most common mistake is treating every roll as interchangeable. Similar-looking products can have different approved applications, overlap instructions, fastening requirements, and exposure limits. A label that says synthetic does not tell you everything needed for a final order. Match the product to the specified roof system first, then compare price and coverage.

Another mistake is using house square footage as the roofing number. Interior floor space says little about the true roof surface on a sloped home with overhangs, porches, dormers, or a garage. A preliminary estimate can start from the building footprint, but the final material plan should use the adjusted roof surface area. For a safe way to organize that calculation, start with a roof sketch and measurement plan.

It is also easy to forget that a roll’s printed size may not be the same as its usable coverage. Overlaps are intentional. They help move water across the layer, but they use material. Cuts around valleys and penetrations use material too. Follow the actual product instructions for the coverage figure and installation layout, then add a reasonable allowance for the roof’s shape rather than rounding down to the nearest roll.

Finally, do not make an underlayment choice after the shingles are already being installed. The shingle package, starter, flashing, and ventilation plan should be settled together. A last-minute substitution can create a problem if the replacement product does not fit the written system requirements. When stock changes, ask whether an available alternative is approved for the same use before buying it.

How to estimate underlayment without guessing

Begin with the adjusted roof surface area, not the home’s interior square footage. A roof’s slope, overhangs, attached garage, porches, and dormers all affect the number. Use a sketch that shows each roof plane, then work from the total roof surface area. The roof measurement guide walks through that starting point in detail.

Next, read the coverage on the exact product being considered. A roll may list gross coverage, usable coverage after overlaps, or a recommended coverage figure for a specified application. Use the manufacturer’s stated figure for the planned installation. Do not assume two rolls with the same size have the same usable coverage.

Then account for what the roof layout actually requires. Overlaps, waste, valleys, ridges, and changes in direction use material. Areas with separate self-adhered protection should be calculated on their own. Round the final material quantity up after the right allowance is included. The goal is not to buy extra at random. It is to avoid a stop in the middle of a job because a necessary overlap or roof detail was never counted.

Read the roll label before you calculate quantity

The fastest way to turn a good roof measurement into a bad order is to calculate with an assumed roll size. Before working out quantity, read the exact package or manufacturer sheet for the product you plan to use. Note the stated coverage, approved roof covering, installation method, overlap directions, fastening requirements, and the maximum time it may remain exposed before the final roof covering is installed.

Those details matter because a product is being used in a real installation, not stored on a shelf. A wider roll can reduce seams, but it may have its own overlap markings and fastening pattern. A lighter roll may be easier to handle, but it may not be the right product for the selected roof system. The label and instructions turn a category name into an actual material decision.

Keep a photo of the label or product name with the roof sketch. That gives the installer and yard a common reference when checking stock. It also prevents an order from being based on a vague description remembered after a long day. For shingles, do the same with bundle coverage. The bundle guideexplains why package coverage is more reliable than a generic bundles-per-square rule.

When to pause and get a professional roof assessment

A material estimate is useful, but it cannot tell you whether the roof deck is sound or whether a leak has damaged framing, insulation, or flashing below the visible surface. Pause the purchasing decision when there are soft areas underfoot, sagging, repeated interior leaks, widespread missing shingles, damaged valleys, or signs that the deck has been wet for a long time. Those conditions may change the scope of the work before any underlayment goes down.

The same caution applies to steep, high, wet, or unfamiliar roofs. Do not climb up just to confirm a measurement or inspect a material detail. A qualified roofer can assess the condition, roof geometry, flashing, and system requirements safely. The better material choice is the one that fits a sound installation plan, not the one selected from a distance on an incomplete measurement.

If the roof is in good condition for a planned reroof, bring the finished measurement and product details to the yard. That keeps the material conversation focused on stock, coverage, and the items needed to complete the installer’s plan.

Keep product records with the roof file

Once the roof system is selected, save the shingle and underlayment product names, color, coverage notes, installation date, invoices, and written instructions in one place. Those records make future repairs, warranty conversations, and material matching much easier. A few clear notes now can save a lot of guessing years later when the packaging is no longer around.

This is especially useful for an overstock purchase or a repair where the exact color and product line matter. Record what was installed, not simply that “synthetic felt” or “gray shingles” were used. The specific brand and product details give a future installer a much better starting point when they need to assess a roof or find a compatible material.

What to bring when you call the yard

A good yard call starts with the numbers and the roof details, not just a product name. Have the adjusted roof area, the shingle brand and product line if it has already been selected, the roof pitch, and any areas that need special protection. A photo of the product instructions or the existing material can also help identify what is being matched.

Ask about the unit of sale and the listed coverage. For an underlayment roll, confirm how many square feet it covers for the intended application. For self-adhered protection, confirm whether the quantity is being calculated by area or by the detail where it will be used. Then ask whether the material is at the nearest yard and whether the quantity is still available.

Start with the current roofing supply inventory, then use the yard locations page to call the closest Affordable Roof Supply yard. The team can help match the listed unit to the material list. They cannot replace the installer’s design or manufacturer instructions, but a prepared list makes it much easier to confirm what is on hand.

Build the whole material list before buying

Underlayment works alongside the rest of the roof, so the final list should include more than the field shingles. Depending on the project, it may include starter material, ridge cap, flashing, fasteners, ventilation, sealants, weather protection, and deck repair materials. Each has a different measurement method and job on the roof.

The best time to catch an incomplete list is before material is picked up. Use the shingle bundle guide to convert the shingle quantity, review the underlayment coverage separately, and keep the product instructions with the measurements. That simple process reduces the chance that a job is delayed by one overlooked roll or accessory.

Roof materials move quickly, especially with overstock shingles and select items. Call before driving over, confirm the listed product and quantity, and keep the plan flexible enough to use an approved alternative when it truly fits the roof system.

Common questions

Roof underlayment FAQ

What is the best type of roof underlayment?

The best underlayment depends on the shingle system, roof slope, weather exposure, local requirements, and the manufacturer instructions for the roof being installed. Synthetic underlayment is a common choice for many asphalt-shingle reroofs, while self-adhered weather protection is used where the roof design or project requirements call for it.

Can I use synthetic underlayment instead of felt?

Often, yes, but only when the selected synthetic product is approved for the roof system and is installed according to its instructions. Do not assume every roll is interchangeable. Confirm the product coverage, fastening method, exposure limits, and compatibility before buying.

How many rolls of underlayment do I need?

Start with the adjusted roof surface area, then compare it with the coverage printed on the exact roll. Add enough allowance for overlaps, cuts, waste, and the roof details that need separate weather protection. The package label and installation instructions should guide the final quantity.

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