Standard roofing felt is water-resistant, not permanently waterproof, and it works as an underlayment beneath a finished roof covering. Even heavier felt isn't a substitute for a sealed waterproof membrane.
If water can run off felt, why can an unfinished roof still leak through it? The gap is between shedding water downslope and blocking water at seams, fasteners and penetrations. For Utah homeowners, that distinction matters when snow melts, temperatures drop again, or a storm interrupts a reroof.
Is Roofing Felt Waterproof? The Short Answer
Roofing felt provides secondary drainage, not permanent exposed waterproofing. Its job is to intercept water that reaches beneath the shingles and direct it toward the roof edge. Whether it succeeds depends on the material, the installation and the conditions of exposure.
Traditional felt combines a fibrous sheet with asphalt. Asphalt saturation limits liquid-water transmission, but the reinforcement isn't transformed into an impermeable sheet. Calling the material waterproof without describing the roof assembly leaves out the most important qualification.
ASTM International's D226/D226M specification recognizes asphalt-saturated organic felt for steep-slope underlayment and waterproofing applications, identifying Type I, commonly called No. 15, and Type II, commonly called No. 30. The heavier grade generally withstands handling damage better, but neither designation makes ordinary felt an indefinitely exposed, watertight roof.
What the grade actually tells you
A stronger sheet gives installers more resistance to tearing while unrolling, fastening and working over it. That's useful, but strength and watertightness are different properties. A durable sheet can still leak through an incorrectly arranged seam.
The application matters, too. Non-perforated felt belongs in steep-slope underlayment applications, while perforated products serve certain built-up roofing applications. A roll labeled roofing felt isn't automatically suitable for every roof system.
The word waterproofing in a standard's scope also doesn't mean the material can serve alone as the finished roof. A component may contribute to waterproofing when installed within a designed assembly without providing that performance independently.
Practical rule: Read a felt designation as a material specification, not a promise that the unfinished roof can withstand any weather.
Why asphalt saturation matters
The history of the material explains why the label alone is insufficient. The industry publication Buildings traces ASTM D226 to 1925 and describes historical minimum asphalt saturation of approximately 140% to 150% of dry felt weight, later reduced to about 120% as fiber sources changed.
Those percentages compare asphalt content with the dry reinforcement's weight. They aren't percentages of water stopped, and they aren't a guarantee of leak-free service. Lower saturation can mean lower water resistance, which is why the quality of impregnation matters alongside the sheet's grade.
Saturation efficiency, expressed through the Kerosine Number, is another measure: the percentage of theoretical asphalt saturation achieved. The same Buildings account describes the later steep-slope standard's type classifications and the saturation-efficiency benchmarks summarized below.
Felt grades and saturation benchmarks
| Grade or type | Standard | Saturation benchmark | What it signals |
|---|---|---|---|
| Type I, No. 15 felt | ASTM D226 | Historical minimum about 120% of dry felt weight, not a current grade-specific guarantee | Lighter grade; less handling resistance |
| Type II, No. 30 felt | ASTM D226 | Historical minimum about 120% of dry felt weight, not a current grade-specific guarantee | Heavier grade; more handling resistance |
| Type 1 steep-slope felt | ASTM D4869 | Minimum 75% saturation efficiency | Higher listed saturation-efficiency benchmark |
| Types 2, 3 and 4 | ASTM D4869 | Minimum 70% saturation efficiency | Lower listed saturation-efficiency benchmark |
These measures shouldn't be treated as interchangeable. Asphalt content relative to dry weight describes a different relationship from saturation efficiency. Neither tells you, by itself, how an installed seam will perform.
For a reroof specification, confirm the applicable standard, product grade and manufacturer's installation instructions. A meaningful product submittal identifies what was tested and how the product must be installed. A marketing claim that says waterproof doesn't answer either question.
Felt, Synthetic and Self-Adhered Underlayments Compared
The useful comparison isn't old material versus new material. It's mechanically fastened drainage layers versus a bonded waterproofing membrane. Asphalt felt and conventional synthetic underlayment generally serve the former role; self-adhered ice and water barriers serve the latter at designated locations.
Synthetic underlayment is often selected when handling strength or a longer construction exposure rating matters. However, synthetic describes a broad category, not a uniform performance rating. Don't assume that a product's resistance to sunlight establishes its ability to protect an unfinished building during rain.
Felt, synthetic and self-adhered underlayments by criterion
| Criterion | Asphalt felt | Synthetic underlayment | Self-adhered membrane |
|---|---|---|---|
| Primary role | Temporary water-shedding beneath shingles | Water-shedding beneath shingles | Waterproofing at eaves, valleys and penetrations |
| Seals around nail holes | No | No, for conventional mechanically fastened products | Yes, when designed for this purpose and installed correctly |
| Exposure before covering | Short; cover promptly | Follow the manufacturer's rating | Follow the product rating |
| Relative cost | Usually the lowest material-cost option | Commonly higher than felt; verify the project quote | Commonly the highest material-cost option; verify coverage and labor |
| Best use | Standard steep-slope underlayment | Field underlayment where the specified exposure rating suits the schedule | Eaves, valleys, approved low-slope details and penetrations |
Choose by the performance that changes the job
For felt, the deciding question is whether the installation schedule allows prompt covering without damage. For synthetic underlayment, it's whether the specific product's exposure and fastening requirements fit the project. For self-adhered membrane, it's whether the location calls for fastener sealing and a bonded water barrier.
There's test evidence available for felt, rather than just a descriptive label. ASTM's D4869/D4869M specification includes a water-shower exposure test for resistance to liquid-water transmission. That evaluates a defined material property; it doesn't certify an entire installed roof against every possible leak.
A membrane also doesn't correct unsuitable roofing geometry. A low-slope section must be compatible with the roof covering and the complete approved assembly. Adding a waterproof underlayment beneath shingles doesn't automatically make those shingles suitable for that slope.
Treat the cost ranking as a starting point, not a purchasing decision. Compare the installed scope, including preparation, transition details and repair access. Match the product to slope, exposure and assembly, rather than choosing by habit or roll price.
How to Lay Felt Correctly and Know Its Exposure Limits
Installation should create an uninterrupted drainage path. The National Roofing Contractors Association's underlayment guidance describes temporary weather protection and notes that organic felt can absorb water even when substantially saturated. That's why installation quality and the covering schedule belong in the same plan.
Use this installation sequence as a jobsite review, not an invitation to work on a roof without appropriate fall protection.
Install for continuous drainage
Prepare the deck. Confirm that the sheathing is sound, dry and free of protruding nails or debris. Covering soft decking or raised fasteners hides defects that can damage the new underlayment.
Choose the specified felt grade and verify its standard on the packaging. If the specification calls for synthetic underlayment instead, verify that product's applicable approval rather than assuming a felt standard applies.
Start at the eaves and work upward. Roll courses horizontally, placing each upper course over the course below using the manufacturer's required overlap. Pulling the sheet tight across uneven decking can create bridging instead of flat contact.
Arrange end laps and transitions so runoff travels over them, not into them. Shortcuts around valleys or adjoining roof planes can interrupt an otherwise correct drainage path.
Fasten without tearing. Use the specified fastener type and spacing, keeping the sheet flat. Overdriven fasteners and wrinkles leave damage that shingles won't repair.
Install required self-adhered protection at vulnerable locations in the sequence specified for the assembly. Coordinate it with flashing so that adjoining layers discharge water onto the layer below.
Install the finished covering promptly. Inspect the underlayment before covering it and replace damaged portions. Don't bury tears, wet wrinkles or deteriorated material to preserve the schedule.
Judge exposure by conditions, not the calendar alone
An overnight delay isn't automatically safe or unsafe. Review these conditions together:
- Slope: Can water drain freely, or can it collect?
- Overlaps: Are courses secure and arranged for downslope drainage?
- Penetrations: Are openings weather-protected and properly integrated?
- Fastening: Are sheets intact and secure against wind?
- Exposure time: Is covering still within the product's stated limits?
- Product type: Is this ordinary felt or a membrane rated for the intended conditions?
A product exposure allowance isn't permission to leave the building's contents dependent on underlayment alone.
If construction will remain unfinished, specify a separate temporary weather-protection plan. Ordinary felt isn't a temporary roof for extended construction.
Where Felt Fails and How to Fix It
Fastener penetrations change the installed material's performance. The NRCA's roofing guidance notes that underlayments may be waterproof before shingles are installed yet may not remain waterproof after shingle nails penetrate them. A leak-free sheet and a leak-free nailed assembly aren't the same thing.
The fastener detail below shows why the completed assembly matters more than the appearance of the sheet alone.
Diagnose the entry path before choosing a repair
Consider a storm arriving during a delayed reroof. Rain reaches exposed laps, wind moves an unsecured edge, and water finds a fastener penetration. The resulting ceiling stain doesn't prove that every sheet of felt is defective; it identifies a failure somewhere in the temporary assembly.
Water can also travel before it becomes visible indoors. A stain around a vent may originate at its flashing, while moisture at a deck seam may have entered farther upslope. Use the symptoms below as diagnostic leads, not final conclusions.
Felt symptoms, causes and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Drips at nail lines | Nail penetrations through felt | Restore the roof covering; use self-adhered membrane at designated high-risk areas and repair associated flashing defects |
| Wet seams after storms | Laps too short or misaligned | Re-lay the affected section with manufacturer-required overlaps |
| Torn or wrinkled felt | Handling damage or wrinkles beneath shingles | Replace torn sections; flatten or replace wrinkled material before fastening and covering |
| Staining around vents | Inadequate or damaged penetration flashing | Install or repair flashing; check its edges and specified sealant |
| Mold smell in attic | Possible trapped moisture, leakage or inadequate drying | Find the moisture source, assess ventilation and check the assembly's drying path |
| Brittle or shrinking felt | Possible degradation after prolonged exposure | Replace degraded sections and install the covering promptly |
Resealing is appropriate only when the defect is a sealant joint designed to be sealed. It won't correct reversed laps, torn sheets or missing flashing. Those defects require rebuilding the affected detail, sometimes including removal of overlying shingles.
The video below provides additional visual context for reviewing roof work.
After a storm, inspect accessible attic areas for new staining along deck seams, nail lines and penetrations. Stay on supported walking surfaces and avoid wet electrical components. Record the location and photograph changes before assuming the visible shingles are the only possible source.
Where Felt Works in Utah and When to Upgrade
Utah roof decisions need to account for snowmelt, freezing conditions and wind-driven precipitation. When water can back up or remain against a vulnerable detail, a drainage layer alone is a weaker choice than a properly integrated waterproof barrier.
For a conventional steep-slope reroof, correctly installed felt can remain a reasonable field underlayment. The more important upgrade is often targeted protection at vulnerable locations, rather than replacing every square of felt with the least permeable material available.
Underlayment choice by Utah roof situation
| Roof situation | Recommended underlayment | Reason |
|---|---|---|
| Standard steep-slope reroof, moderate snow | Heavier asphalt felt or an approved, ASTM-compliant synthetic | Provides secondary shedding when correctly lapped and promptly covered |
| Eaves and valleys | Self-adhered ice and water barrier where required by code or design | Provides fastener sealing at high-risk zones |
| Low slopes or heavy snow areas | Self-adhered membrane over the designated risk area, within an approved assembly | Improves resistance to backed-up water; doesn't authorize an unsuitable roof covering |
| Unfinished roof overnight or storm forecast | Appropriately rated synthetic or self-adhered product, plus a weather-protection plan | Product rating and installed details must suit the forecast exposure |
| Energy upgrade with tighter air sealing | Selection coordinated with the assembly designer and checked for vapor permeance | Changes to insulation and air movement can alter drying |
Waterproofing and drying are separate requirements
A less permeable membrane can improve liquid-water protection while restricting outward drying. The University of Waterloo building-science paper distinguishes drainage behavior from vapor transmission and explains why moisture control depends on the complete enclosure.
Moisture may already be present in construction materials, enter through a leak, or arrive from indoor humidity. The underlayment can't be selected independently of ventilation, insulation, decking condition and vapor-control layers.
Assembly check: Before specifying a less permeable layer, identify where moisture could enter and how the deck will dry.
This doesn't mean waterproof membranes always cause rot. It means neither maximum vapor openness nor maximum impermeability is universally correct. Tighter air sealing and added insulation warrant an assembly review, not an automatic material substitution.
Compare the upgrade's installed cost with the location's water exposure and the consequences of opening the finished roof later. Spend on the detail that addresses the actual risk, while preserving a workable drying strategy and meeting local requirements.
Confirm Your Underlayment Before Your Next Reroof
An underlayment decision should appear in the contract, not emerge after tear-off. A description such as roofing paper leaves too much unresolved: the product, installation requirements and vulnerable-area protection all need to be identifiable.
Bring the proposal and any photographs of existing leaks to the consultation so the discussion addresses your roof's details.
Put the material and weather plan in writing
Use this checklist before signing with any roofing contractor:
- Product identification: Record the manufacturer, product name, felt grade where applicable, and the standard or approval it meets.
- Installation details: Include the required overlaps, fastener type and fastening pattern, with attention to transitions between materials.
- Vulnerable locations: Identify where self-adhered membrane will be installed, including applicable eaves, valleys and penetration details.
- Weather protection: Establish what happens if the roof remains unfinished overnight or a storm interrupts work. Include who checks and replaces damaged underlayment.
- Assembly compatibility: Confirm that the roof covering, slope, underlayment, ventilation and insulation form a compatible system.
- Warranty documents: Keep the installation warranty and applicable product requirements with the signed scope. Our roofing installations come with extended warranties.
Keep a useful roof history
Set an annual inspection reminder and inspect again after major storms. Focus on attic stains, penetration areas and damaged flashing, using ground-level observations or qualified roof access rather than climbing onto a wet or icy roof.
Keep dated photographs, inspection notes and repair records together. Note whether a stain is new, unchanged or growing; that distinction helps separate an active leak from evidence of a repaired problem.
Our roof inspection service includes detailed reports and maintenance recommendations. Customers can request an inspection through our website, making it easier to establish the existing condition before deciding on a reroof.
Before signing, get a written answer to this question: Which underlayment goes on, under which standard, and what happens at the eaves and valleys?
Superior Home Improvement offers a free roofing consultation and a free estimate to help you evaluate underlayment, vulnerable roof details and the complete roofing assembly. Visit Superior Home Improvement to request a roof inspection and discuss a reroofing scope that addresses both water protection and roof drying.
