Meta description: Learn when underlayment is needed for roll roofing, which materials work best, how roof slope and climate affect the choice, and what to check before installation.
Roll roofing can be a practical covering for sheds, garages, porches, and other low-slope roofs. But the roofing material is only one part of a durable roof assembly. The right underlayment can help protect the roof deck from moisture during installation and from water that gets past the outer covering. The best choice depends on the roll roofing product, roof pitch, local building requirements, and manufacturer instructions. Here is how to evaluate those factors and install an appropriate underlayment.
What Underlayment Does Beneath Roll Roofing
Roofing underlayment is a protective layer installed over the roof deck and beneath the finished roofing material. Depending on the product and roof design, it can provide temporary weather protection during construction, help limit moisture exposure at the deck, and create a separating layer between the deck and roof covering.
Underlayment is not a substitute for correctly installed roll roofing, sound flashing, or proper drainage. It is also not automatically a waterproofing system. The finished roof and its details must work together to shed water. On low-slope roofs in particular, seams, edges, penetrations, and transitions deserve careful attention because water drains more slowly than it does on a steep roof.
Some roll roofing products are designed to be installed over a specific underlayment; others may require a base sheet, a particular membrane, or a different assembly. Requirements vary by product. The product’s current installation instructions should be treated as the primary guide, subject to local code and inspection requirements.
Is Underlayment Required for Roll Roofing?
Whether underlayment is required depends on the roofing product, the roof’s slope, the building code adopted in the area, and the approved roof assembly. There is no single underlayment rule that applies to every product sold as roll roofing. Some systems call for underlayment over the deck, while others use a specified base sheet or are installed according to a different method.
Roof pitch matters because water moves more slowly on low-slope roofs. A covering that performs adequately on a steeper roof may need a different installation method or additional protection at a lower pitch. Manufacturers commonly state a minimum slope and may specify different materials or fastening patterns for different slopes. Those limits should not be guessed at or replaced with general advice.
In the United States, the locally adopted building code and the authority having jurisdiction determine applicable requirements. The International Residential Code (IRC), where adopted and as amended locally, includes provisions for roof coverings and underlayment. Local rules can differ, and code adoption dates vary. A homeowner or contractor should check with the local building department and consult the exact product documentation before starting work.
Types of Underlayment Used With Roll Roofing
Asphalt-saturated felt
Traditional asphalt-saturated felt is made in different weights, commonly identified by designations such as No. 15 or No. 30. It is familiar to many installers and may be specified for certain roof assemblies. Felt can wrinkle or absorb moisture if it is exposed to weather, and it should be installed with the laps, fastening, and exposure limits required by its manufacturer. It is not interchangeable with every synthetic or self-adhered product.
Synthetic underlayment
Synthetic underlayments are made from polymer-based materials and are sold in a range of designs. Many are lighter and more resistant to tearing than traditional felt, but performance varies by product. Some are intended for use beneath particular roof coverings and some are not approved for a given roll roofing system. Check compatibility, permitted exposure time, fastening instructions, and any required slip-resistant or fire-performance characteristics.
Self-adhered membranes
Self-adhered membranes have an adhesive backing and are sometimes used where extra protection from water intrusion is needed, such as vulnerable eaves, valleys, or transitions. Certain products are intended as ice-barrier membranes; others serve as components of low-slope roof systems. They can be sensitive to installation temperature, deck condition, and proper surface preparation. A self-adhered membrane should not be assumed to be a suitable full-roof underlayment unless the roll roofing manufacturer approves that assembly.
Base sheets and system-specific materials
Some low-slope roofing systems use a compatible base sheet beneath the visible roof covering. A base sheet may be mechanically fastened, adhered, or installed by another approved method. It is part of a system, not simply a generic substitute for felt. Using materials from different systems without written compatibility guidance can create problems with adhesion, movement, or warranty coverage.
How to Choose Underlayment for Roll Roofing
Choosing underlayment starts with identifying the exact roof covering. “Roll roofing” can refer to several asphalt products, including mineral-surfaced roofing and other roll-applied membranes. Their installation requirements can differ substantially. Locate the product label and installation guide, and confirm the manufacturer’s recommended deck, underlayment or base sheet, slope range, and fastening method.
Use these checks before purchasing materials:
- Confirm the roof pitch. Measure the rise over a horizontal distance or have a roofing professional verify it. Compare the result with the product’s minimum-slope requirements.
- Check the roof deck. The deck must be structurally sound, dry, clean, and suitable for the selected system. Replace deteriorated or damaged sheathing before covering it.
- Verify compatibility. Confirm that the underlayment, primer, fasteners, and roll roofing are approved for use together.
- Review local requirements. Ask the building department whether a permit, inspection, or particular code-compliant assembly is required.
- Consider exposure during construction. Check how long the underlayment may remain uncovered and whether rain protection is needed during installation.
- Check warranty terms. Follow the required components and installation details; an unapproved material or method may affect coverage.
Climate also affects planning, though it does not override product instructions. In regions with freezing conditions, ice-dam protection may be required in specified locations under the locally adopted code. In hot weather, adhesive products may become difficult to handle, while cold conditions can reduce adhesion. The membrane manufacturer’s temperature and storage instructions should guide installation.
Underlayment and Roof Slope
Roof slope is one of the most important factors for roll roofing. A low slope gives water more time to find a weak seam or poorly sealed penetration. The underlayment can add a layer of protection, but it cannot make an unsuitable roof covering appropriate for a slope below its listed minimum.
Before work begins, confirm that the roof drains freely and that gutters, edges, and transitions do not trap water. The deck should not have depressions that hold standing water. If the existing structure has persistent ponding, sagging, or drainage problems, those conditions need correction rather than concealment beneath new roofing.
Do not apply rules for steep-slope shingles to roll roofing by assumption. Underlayment overlap distances, number of layers, and fastening patterns can vary by material and slope. The product instructions and local code establish the applicable details.
Preparing the Roof Deck
Good preparation helps the underlayment lie flat and adhere or fasten as intended. Remove old roofing only as appropriate for the project and local rules. Expose and inspect the deck, then replace rotten, delaminated, or loose sections. Protruding nails, splinters, dust, and debris can puncture a membrane or interfere with adhesion.
Check that the deck material and thickness meet the roofing manufacturer’s requirements. The surface should be dry before installing underlayment, especially self-adhered products. Moisture trapped beneath a roof covering can damage wood and compromise adhesion. If the deck has mold, active leaks, or structural damage, resolve the underlying issue before covering it.
Plan the installation so that water sheds over the layers below it. At eaves and other roof edges, flashing and drip-edge details must be coordinated with the underlayment and roofing instructions. The correct sequence can differ by assembly, so there is no universal rule for whether one layer goes above or below another at every edge.
Installing Underlayment: General Best Practices
The following steps describe general planning principles, not a substitute for the exact instructions supplied with the roofing system. Product-specific directions determine roll orientation, lap width, fastener type and spacing, adhesive use, and treatment at edges and penetrations.
- Read the full installation guide. Confirm the permitted deck, slope, underlayment, weather conditions, and details for edges, valleys, walls, and penetrations.
- Prepare and measure the deck. Remove debris, repair defects, and make sure the surface is dry. Stage materials so the roof can be covered within permitted exposure limits.
- Lay the material in the specified direction. Follow the manufacturer’s instructions for starting at the eave and progressing across the roof. Keep the material aligned and flat.
- Make the required laps. Overlap sheets or membrane edges by the specified amount. Incorrect laps can direct water beneath the underlayment instead of toward the eave.
- Fasten or adhere as specified. Use approved fasteners and spacing, or prepare the deck and apply adhesive membrane according to its instructions. Avoid wrinkles, fish-mouths, and unsealed edges.
- Detail transitions carefully. Install flashing and integrate the underlayment around vents, walls, valleys, and other penetrations using the approved sequence.
- Inspect before covering. Correct loose areas, punctures, wrinkles, and missed fasteners before installing the roll roofing.
Roof work involves fall hazards, and low-slope roofs can still be dangerous. Use appropriate fall protection and safe access. In wet, icy, or windy conditions, postpone work when safe handling or secure footing is not possible.
Common Underlayment Mistakes
- Using a generic product without checking compatibility: A material that works under shingles may not be approved beneath a particular roll roofing system.
- Ignoring minimum slope: Adding underlayment does not correct a roof that falls below the covering’s permitted slope.
- Leaving underlayment exposed too long: Sun, wind, and rain can damage products beyond their stated exposure limits.
- Installing over a wet or damaged deck: This can trap moisture and hide structural problems.
- Making incorrect laps or edge details: Water can move beneath the layers, especially near eaves, walls, and penetrations.
- Mixing incompatible products: Unapproved combinations of membranes, primers, adhesives, and roof coverings may fail to bond or may affect warranties.
- Assuming underlayment fixes drainage: Standing water and poor roof geometry need to be addressed directly.
When to Consult a Roofing Professional
Professional advice is especially useful when a roof has a very low slope, multiple penetrations, a history of leaks, damaged decking, or a transition between different roofing materials. A contractor can identify the existing roof assembly, evaluate drainage, and confirm whether the selected covering is suitable for the structure.
Before hiring, ask for the proposed product names, manufacturer installation details, and a written description of the underlayment and flashing assembly. Verify licensing and insurance requirements in the area, and confirm whether a permit is needed. A clear scope helps prevent a generic underlayment from being substituted for a required system component.
Frequently Asked Questions
Can felt paper go under roll roofing?
It may be suitable for some roll roofing products, but it is not automatically correct for every system. The roll roofing manufacturer must approve the felt type and installation method, and local code requirements still apply.
Is synthetic underlayment better than felt?
Neither material is universally better. Synthetic products may offer handling or tear-resistance advantages, while felt is specified in some traditional assemblies. The appropriate choice depends on product compatibility, exposure limits, roof conditions, and installation instructions.
Does roll roofing need ice-and-water shield?
Not in every location or assembly. Ice-barrier requirements depend on the locally adopted code, climate, roof details, and product instructions. A self-adhered membrane should be used only where it is required or approved as part of the roofing system.
Can underlayment be installed over old roofing?
That depends on the condition of the existing roof, the new product’s instructions, building-code limits, and whether the deck can be inspected. Installing over old material can hide damage and may be prohibited or unsuitable for the chosen assembly. A qualified roofer or local building official can help determine the permitted approach.
Sources and Further Guidance
For project-specific requirements, consult the current installation instructions from the roll roofing and underlayment manufacturers, the locally adopted building code, and the local building department. The International Code Council’s code resources provide access to model-code information; the code adopted in a particular jurisdiction may include amendments. The National Roofing Contractors Association offers additional roofing guidance and resources for locating professional help.