3 Types of Roofing Underlayment: A Complete Guide

Roofing underlayment is an important component of any roofing system. It is installed underneath the primary roof covering to provide an extra layer of protection. There are three main types of roofing underlayment used in residential and commercial roofing projects: felt paper, synthetic underlayment, and self-adhering underlayment. Each type has its own advantages and ideal applications. This comprehensive guide will examine the three major types of roofing underlayment, their key features, proper installation methods, and the best uses for each type.

Felt Paper Underlayment

Felt paper, also known as tar paper, is the most traditional and commonly used roofing underlayment material. It is composed of wood pulp or other fibers saturated with asphalt and formed into sheets.

Key Features and Benefits

Cost-effective – Felt paper is typically the most affordable underlayment option. It provides a budget-friendly base layer.
Absorptive – The fibrous and porous nature of felt allows it to readily soak up moisture. This helps prevent condensation buildup.
Durable – Multiple layers of heavy-duty felt can withstand foot traffic and provide a stable base for roofing materials.
Versatile – Felt comes in a range of weights suited for various roof designs and environments.

Proper Installation

Proper installation is crucial to achieve the full benefits of felt underlayment:

Decking – Felt paper should be installed atop a clean, dry, and well-secured roof deck. Apply over solid sheathing or spaced boards.
Overlap – Run horizontal overlapping courses of felt up the roof with 2-4” vertical seams and 4-6” horizontal seams. Overlaps should face downhill.
Fastening – Use corrosion-resistant fasteners. Staples are common for spaced decking. Use roofing nails on solid decking along seams and at the edges.
Weather lapping – Upper courses should overlap lower ones by at least 2″ to shed water safely down the roof. Don’t rely on sealants or adhesives.
Ventilation – Ensure proper attic ventilation to prevent humidity buildup from damaging the underlayment over time.

Best Uses for Felt Paper

Felt paper works well as underlayment in the following scenarios:

Under asphalt shingles – The most common use. Multiple layers enhance durability.
Beneath metal roofing – Felt helps inhibit corrosion and condensation issues.
On sloped roofs – The layered install and absorption properties work well on inclines.
Warm, dry climates – The ventilation felt allows performs better in low humidity environments.
Budget roof projects – The affordability makes felt a top choice for cost-conscious jobs.

Synthetic Underlayment

Synthetic underlayment offers a lighter, thinner, and more energy-efficient alternative to traditional felt paper. It is typically composed of polypropylene, polyethylene or a composite fiber blend.

Key Features and Benefits

Lightweight – Synthetic underlayment weighs far less than felt, easing installation.
Thinner – At around 1/16”, synthetics install with less bulk and protrusions.
Excellent traction – The woven or grid-patterned surface provides sure footing for roofers.
Reflective – Reflective coatings shield against heat and UV rays, improving energy efficiency.
Resists moisture – Water resistant materials prevent water saturation and leakage.

Proper Installation

Like felt paper, proper synthetic underlayment installation is key to unlocking its advantages:

Deck prep – Ensure the roof deck is clean, dry, structurally sound and meets code requirements.
Seam overlaps – Overlapping seams with 4-6” of coverage provides water protection redundancy.
Fastening – Use plastic cap nails or roofing nails with 1” plastic caps along seams as needed. Avoid unvented metal fasteners.
Sealing – Carefully seal around any penetrations, protrusions, valleys and edges prone to uplift. Use compatible and approved sealants.
Ventilation – Make sure to allow for proper airflow to prevent moisture issues in the synthetic material over time.

Best Uses for Synthetic Underlayment

The lightweight and reflective properties of synthetic underlayment make it ideal for:

Hot climate installations – Reflectivity and breathability allow it to withstand extreme heat.
Metal roofing jobs – Prevents condensation corrosion issues common with metal roofing.
Windy regions – Synthetics offer enhanced wind uplift resistance compared to felt.
Low-slope roofs – Sheds water and resists leakage more readily on flat or shallow-pitch roofs.
Eco-friendly projects – Greener than felt paper and reduces energy consumption.

Self-Adhering Underlayment

Self-adhering underlayments provide premium protection with a peel-and-stick application. The rubberized asphalt layer bonds securely to the roof deck.

Key Features and Benefits

Waterproof barrier – Provides a fully water-tight base layer that guards against leaks.
Self-sealing – The peel-and-stick installation seals seams and renders fasteners unnecessary in most cases.
High tack adhesion – Bonds tightly to the roof deck without the need for fasteners or adhesives.
Extreme durability – Advanced synthetics and rubberized asphalt withstand harsh weather, impacts, and foot traffic.
Easy installation – Self-adhesive application is simple, fast, and requires fewer tools/materials.

Proper Installation

Despite the ease of peel-and-stick application, proper techniques are still important:

Surface prep – Ensure the roof deck is clear, dry, and free of debris, dust, and protrusions.
Priming – Prime concrete, masonry, and metal surfaces with asphalt primer for optimal adhesion.
Seam overlaps – Overlap seams by 3-6”. Stagger end joints between overlapping rows.
Sealing – Fully seal around vents, edging, and protrusions through the underlayment.
Flashing – Coordinate with step flashing, sidewall flashing, and roof-wall intersections per code.

Best Uses for Self-Adhering Underlayment

The premium protection and waterproofing abilities make self-adhering underlayments ideal for:

Low-slope and flat roofing – The self-sealing coverage excels on shallow angles and flat surfaces.
High-exposure regions – Wind, storms, and heavy rain make the durability worthwhile.
High-traffic roofs – Withstands foot traffic and damage far better than felt or synthetics.
Problematic areas – Provides dependable coverage on eaves, valleys, seams and protrusions prone to leaks.
Premium roofing jobs – Justifies the added cost for high-end roofing projects or buildings with sensitive interiors.

How to Choose the Right Underlayment

With an understanding of the different underlayment types and their ideal applications, you can make an informed decision based on several factors:

Roof type and pitch – Felt excels on sloped roofs while self-adhering works better on low slopes. Synthetics are more middle-ground.
Local climate – Felt suits dry climates. Synthetics withstand heat and humidity. Self-adhering resists heavy precipitation.
Noise considerations – Felt and synthetics insulate against noise better than self-adhering types.
Foot traffic – Self-adhering underlayment holds up to foot traffic far better than the alternatives.
Budget – Felt paper is the most economical choice while self-adhering costs significantly more upfront.
Project complexity – Self-adhering underlayments make sense for intricate roofs with lots of protrusions. Felt can suffice for simpler roof structures.
Energy efficiency goals – Reflective synthetic underlayments or sealed self-adhering help minimize energy consumption.

Installing Underlayment Correctly

To fully capitalize on the benefits of your chosen underlayment, proper installation is critical. Here are some key best practices to follow:

Roll out horizontal overlapping rows, not vertical columns. Overlaps should face downwards.
Stagger end joints between rows and overlap seams sufficiently – 4-6” for synthetics and felt, 3-6” for self-adhering.
On sloped roofs, upper rows should overlap lower rows by at least 2” for adequate water flow.
Fasten sufficiently along seams, edges, and protrusions according to manufacturer specifications – more is better.
Take extra care sealing around vents, edging, valleys, chimneys, and other penetrations.
Allow for adequate ventilation to prevent moisture issues down the road. This is especially key for felt and synthetics.
Use compatible, high-quality primers, nails, caps, and sealants as needed. Don’t cut corners with cheaper substitutes.
Follow all local building codes and manufacturer application instructions. Seek professional guidance for unfamiliar roofing scenarios.

The Impact of Proper Underlayment Coverage

Installing the right underlayment for your roof is only half the battle – proper coverage is equally important. Here’s how underlayment coverage impacts roof performance and longevity:

Moisture protection – Lap seams sufficiently so water flows over them without seeping underneath into vulnerable areas.
Leak prevention – Don’t leave open gaps, holes, or unsealed nails/staples that provide a direct path for water intrusion and leaks.
Uplift resistance – Proper underlayment fastening prevents wind uplift that can damage roof coverings and sheathing.
CONDENSATION PREVENTION – Adequate ventilation prevents humidity buildup within the roof structure than can damage decking and sheathing.
Energy efficiency – Correct overlapping and sealing prevents heat loss while reflective synthetics boost energy efficiency even further.
Roof system stability – Underlayment provides a stable base layer for the roof covering to adhere and perform optimally over the long term.
Longevity – When installed correctly, quality underlayments enhance durability, lifespan and weather resistance – saving you from costly premature repairs.

Common Mistakes to Avoid

While the roofing underlayment itself makes a big difference, flawed installation can undermine even premium materials. Be vigilant against these common mistakes:

Improper overlapping – Inadequate overlap allows moisture seepage into vulnerable areas. Always meet manufacturer overlap recommendations.
Insufficient fastening – Skimping on nails/staples along seams invites wind uplift damage. Fasten at the prescribed intervals or closer.
No weather lapping – On sloped roofs, the upper underlayment row should overlap the lower row by several inches to encourage proper downhill water flow.
Poor penetration sealing – Any unsealed nails, gaps, tears, or holes provide an entry point for leaks. Take time to seal thoroughly.
Lack of primer – Primer promotes adhesion on metal, masonry and concrete. Don’t skip priming prescribed surfaces.
Substandard preparation – Debris, dust, and protrusions compromise underlayment bonding. Always prep surfaces adequately before installing.
Improper ventilation – Moisture buildup underneath can result in mold, rot, and degradation if airflow is insufficient.
Rushing the job – Moving too quickly inevitably leads to mistakes, improper sealing, and subpar coverage. Take your time.

Protecting Vulnerable Areas

Certain parts of the roof are especially prone to leakage and moisture damage. Take extra care when installing underlayment in these critical areas:

Valleys

Cut underlayment strips to fit fully along the valley length without gaps or bunching.
Run valley strips before the main underlayment rows. Make the strips at least 36 inches wide.
Overlap horizontal seams by 12 inches and vertically by 6 inches. Stagger overlaps.
Seal underlayment tightly into the valley with compatible sealants.

Eaves

Run eave strips at least 24 inches up from the edge prior to the main underlayment.
Overlap eave strips by 4-6 inches. Make sure fasteners are no more than 3 inches from the edge.
Fold over and seal the edge using metal drip flashing. Integrate properly with gutter apron.

Penetrations

Cut around pipes, vents, and ducts in an “X” shape rather than a straight line.
Seal gaps around penetrations thoroughly with polyurethane caulk.
Use prefabricated flashings when possible for easier integration.
Leave release paper in place until overlapping with next row to keep adhesion sound.

Skylights and hatches

Run underlayment at least 9 inches up the curb sides.
Install self-adhesive underlayment along the base of the curb.
Counterflash properly with metal flashings integrated with the skylight or hatch.
Seal the intersection of the underlayment and curb thoroughly.

Integrating Underlayment With Other Roof Components

Roofing underlayment is just one part of a complete roofing system. Make sure to integrate properly with other roof components:

Step flashing

Coordinate step flashing with underlayment to overlap each row.
Install step flashing before final row of underlayment to overlap properly.
Use compatible sealants between the flashing and roof underlayment for water tightness.

Drip edge flashing

First install drip edge along the eave. Run underlayment on top to divert water off the edge.
Along rakes, underlayment should overlap on top of drip edge to protect the edge.
Secure with nails 3-4 inches apart and seal with asphalt roofing cement.

Gutters and downspouts

Coordinate underlayment terminations into any gutter aprons.
Check integration with gutter hangers – seal any fastener holes through underlayment.
Slope underlayment towards downspouts and create end-dams for proper drainage.

Vents and ductwork

Make underlayment cuts around pipes, ducts, and venting in an “X” shape then seal.
Use prefabricated flashings when possible for straightforward integration.
For custom flashings, seal carefully with roof cement or butyl tape.
Ensure proper clearance around heat emitting ducts or flues.

Best Practices for Underlayment Maintenance

Proper underlayment installation isn’t the end of your responsibilities. Follow these maintenance best practices to maximize longevity:

Inspect underlayment when inspecting general roof condition. Look for tears, missing granules, and other degradation.
Repair small tears quickly with compatible sealant and membrane.
Have a roofer address any major underlayment damage, tears, lifted seams, or moisture spots.
Remove debris and leaves regularly to prevent clogs, moisture buildup, and other issues.
Clean out gutters and downspouts regularly to maintain proper roof drainage and runoff.
Ensure adequate attic/roof ventilation year-round to avoid harmful humidity buildup.
Replace old or degraded underlayment when re-roofing to restore full moisture protection.
Plan on replacing worn underlayment after around 20-30 years for optimal roof performance.

Working With a Roofer

Installing roofing underlayment on anything beyond a simple small roof repair is a job for professional roofers, not DIYers. Here’s how to work effectively with roofing pros:

Choosing a Roofer

Select roofers who:

Have years of proven experience, especially with your roof type and underlayment materials.
Maintain proper up-to-date licensing, insurance, and workers’ comp.
Have worked extensively with reputable underlayment brands you’re considering.
Can provide manufacturer certifications for full warranty eligibility.
Offer detailed quotes clearly outlining scope, materials, timeline, and procedures.

Planning the Project

Provide a detailed roof plan marking trouble areas needing extra underlayment protection.
Request they follow manufacturer instructions precisely, get necessary approvals, permits, and inspections.
Agree on a communication plan should any issues arise requiring input or change orders.

Overseeing Progress

Perform periodic walkthroughs to inspect workmanship without disrupting workflow.
Look for proper overlapping, fastening, integration with other roof components, and penetration sealing.
Address any concerns immediately before they are covered up and irreversible.
Avoid constant micro-management. Trust their expertise unless you have valid concerns.

Finalizing Payment

Once the work is complete, perform a final comprehensive inspection to ensure satisfactions.
Request manufacturer warranty documentation when applicable.
Withhold final payment until all work and cleanup meets contractual obligations.

Key Takeaways

The three main types of roofing underlayment are felt paper, synthetic, and self-adhering underlayments. Consider the roof, climate and budget when selecting.
Proper underlayment installation with adequate fastening, sealing, and overlap is crucial to realize the benefits. Don’t cut corners.
Pay special attention to vulnerable areas like valleys, eaves, skylights, vents and penetrations which are prime candidates for leaks