Felt for Shed Roof: Choosing and Installing the Right Underlayment

Felt for shed roof is a common, cost-effective underlayment that protects sheds from moisture and helps roofing materials perform better.

Felt Type	Typical Weight	Best For
15# Organic Felt	15 lb per 100 sq ft	Budget projects, short-term protection
30# Organic/Saturated Felt	30 lb per 100 sq ft	Standard shingle roofs, moderate durability
Synthetic Underlayment	Lightweight, varied	High durability, long-term waterproofing

What Is Roofing Felt And Why It Matters For Sheds

Roofing felt is a sheet underlayment traditionally made from organic mat saturated with asphalt or newer synthetic materials designed to act as a secondary barrier under shingles or metal panels.

For sheds, felt provides an inexpensive layer of protection against wind-driven rain, ice dams, and moisture migration, and it helps extend the life of the finished roofing material.

Types Of Felt And Underlayment Options

The main options are organic asphalt-saturated felt (commonly 15# or 30#) and synthetic polypropylene or polyester underlayments that mimic felt but with improved performance.

15# felt is lighter and cheaper but less durable, while 30# felt offers better tear resistance and longer service life.

Synthetic underlayment is more tear-resistant, lighter, and often more water-resistant; it resists wrinkle and uplift better in windy conditions, making it popular for exposed or higher-end sheds.

Breathable Underlayments

Some modern underlayments are breathable membranes that allow water vapor to escape while blocking liquid water, reducing condensation risk in enclosed sheds.

Breathable membranes are useful when sheds are poorly ventilated or contain moisture-generating activities like woodworking or storage of damp materials.

Key Factors When Choosing Felt For A Shed Roof

Important considerations include roof slope, climate, roof covering, budget, and expected lifespan of the shed roof assembly.

Roof slope determines minimum underlayment and roofing choices; low slopes often need specialized underlayments or full-coverage membranes, whereas steeper slopes can use standard felt under shingles.

In cold climates with snow and ice, thicker felt or synthetic underlayment combined with ice-and-water shield at eaves reduces leakage risk from ice dams.

Thickness, Weight, And Performance Specifications

Felt is often specified by weight: 15-pound and 30-pound rolls are standard for asphalt-saturated felt, while synthetics are rated by tensile strength, slip resistance, and exposure time.

30# felt provides greater tear resistance and longer service life than 15#, but synthetic underlayments often outperform both in strength-to-weight ratio and UV exposure tolerance.

Installation Steps For Felt On A Shed Roof

Proper installation ensures felt does its job. Steps include deck inspection, underlayment selection, layout, fasteners, overlaps, and detailing around penetrations.

Start by ensuring roof decking is clean and securely fastened; replace damaged boards and install drip edge where required before underlayment goes on.

Roll out felt horizontally from the eave to the ridge with the printed or smooth side down, maintaining manufacturer-recommended overlaps (commonly 2–4 inches horizontally and 4–6 inches at end laps).

Fasten felt with roofing nails or cap nails spaced per guidelines; avoid excessive fastener penetration that could create leaks and ensure felt lays flat without buckling.

Install flashing, pipe boots, and ridge venting components on top of the felt or as specified, and follow shingle or metal panel manufacturer instructions for final roofing layers.

Common Installation Mistakes To Avoid

Frequent errors include insufficient overlap, stapling in wind-prone areas without caps, exposing felt to long-term UV without final roofing, and skipping ice-and-water shield where needed.

Avoid leaving felt exposed to sun for extended periods, as organic felt degrades quickly; most synthetics allow longer exposure but should still be covered promptly.

Maintenance And Repair For Felt Underlayment

Once the finished roof is installed correctly, felt usually requires little maintenance, but inspections after storms and seasonal checks help catch issues early.

Look for lifted shingles, flashing failures, or water stains inside the shed—these signs often indicate underlayment or flashing problems that need attention.

For localized damage, remove roofing material in the affected area, replace the damaged felt section with proper overlaps and sealant, and reinstall roofing material per manufacturer instructions.

Alternatives To Traditional Felt For Shed Roofs

Alternatives include self-adhered ice-and-water shields, peel-and-stick membranes, underlayment shingles, synthetic underlayment, and direct-to-deck metal flashings for metal roofs.

Self-adhered membranes at eaves and valleys offer superior leak protection and are highly recommended for sheds in freeze-thaw regions or where low slopes increase leakage risk.

Compatibility With Different Roof Coverings

Felt works with asphalt shingles, roll roofing, and some metal roofing systems but may be less appropriate under some metal panel installations that require specialized slip sheets or direct-fastening details.

For metal roofs, consult panel manufacturer guidelines—they may require synthetic underlayment or specific fastener spacing and sealing methods to prevent condensation and creasing.

Cost, Lifespan, And Value Comparison

Material costs vary: 15# felt is cheapest, 30# mid-range, and synthetic underlayment costs more but offers longer life and better performance.

For a small shed, the incremental cost of synthetic underlayment is often justified by easier installation, longer UV exposure allowance, and improved moisture resistance.

Option	Typical Cost Per 100 Sq Ft	Expected Lifespan
15# Felt	Low	5–10 Years
30# Felt	Medium	10–20 Years
Synthetic Underlayment	Higher	15–30+ Years

Best Practices For Different Climates And Roof Slopes

In humid climates, breathable membranes can reduce condensation risk; in cold climates, combine felt with ice-and-water shield at eaves and valleys for better protection.

For low-slope sheds (below 3:12), consider full-coverage self-adhered or fully welded membranes instead of traditional felt, which may not provide adequate protection on near-flat surfaces.

Environmental And Safety Considerations

Organic felt contains asphalt and fiberglass reinforcement in some products; synthetic underlayments are often recyclable but check local facilities and manufacturer guidance for disposal options.

When cutting and installing felt, use proper PPE including gloves and eye protection and follow safe ladder practices on shed roofs to reduce fall risk.

Cost-Saving Tips And Where To Buy

Buying felt or synthetic rolls in bulk, using local supplier promotions, or combining a shed roofing project with neighbors can reduce unit costs significantly.

Big-box home centers, roofing supply houses, and online retailers all carry felt and synthetic underlayments; compare specs like exposure time and tensile strength, not only price.

Frequently Asked Questions About Felt For Shed Roofs

Is felt necessary for a shed roof? Felt is highly recommended as a secondary barrier beneath roofing materials to reduce leaks and protect decking, though some specialized roofing systems may allow alternatives.

Can felt be installed over plywood decking? Yes; felt installs readily over plywood and OSB when the deck is dry, flat, and secured.

How long can felt be exposed before shingles go on? Organic felts should be covered as soon as practical—ideally within a few days; synthetics may allow longer exposure (check manufacturer specs for exposure limits).

Resources And Manufacturer Guidance

Always consult roof covering manufacturer installation instructions and local building codes to ensure underlayment choice and installation meet warranty and code requirements.

Manufacturer technical datasheets provide exposure limits, fastening patterns, and overlap requirements, which are essential to follow for warranty compliance and optimal performance.

For complex situations like low slopes, heavy snow loads, or metal roofing, consulting a licensed roofing contractor or local building official ensures the right underlayment choice and roof details.