What Goes on a Roof Before Shingles: Layers Explained

The roofing process involves multiple critical layers installed before shingles to protect the structure, manage moisture, and extend roof life. This article outlines those layers, materials, and best practices to help homeowners and contractors make informed decisions. Understanding each layer helps prevent leaks, ice dams, and premature shingle failure.

Layer	Purpose	Common Materials
Decking	Structural Base	Plywood, OSB
Underlayment	Secondary Water Barrier	Felt, Synthetic
Ice & Water Shield	Leak Protection At Eaves/Rakes	Self-Adhesive Membrane
Starter Strip	Edge Support And Wind Seal	Starter Shingles Or Modified Strip
Flashing	Seal Around Penetrations	Metal Flashing, Roof Boots
Ventilation	Moisture And Heat Control	Ridge Vents, Soffit Vents

Roof Decking: The Structural Foundation

The roof decking is the flat surface that supports all subsequent layers and the shingles themselves. Plywood and oriented strand board (OSB) are the most common decking materials in the U.S.

Decking must be properly fastened, level, and free of rot or damage before any underlayment is installed. Typical thicknesses are 7/16″, 1/2″, or 5/8″ depending on local code and rafter spacing.

Underlayment: The Secondary Water Barrier

Underlayment is installed directly over the decking to act as a secondary moisture barrier in case shingles fail. Traditional felt underlayment (tar paper) and modern synthetic underlayments are the leading options. Synthetic underlayments are lighter, stronger, and more water-resistant than felt.

Installation is typically done in horizontal courses from the eave up to the ridge with a recommended overlap to direct water away. Some manufacturers and local codes specify particular underlayment types or wind rating requirements.

Felt Versus Synthetic Underlayment

Felt underlayment is asphalt-saturated and cost-effective, while synthetic underlayments offer improved tear resistance and UV tolerance. For long-term performance and fewer callbacks, many professionals favor high-quality synthetic underlayment.

Ice And Water Shield: Critical In Vulnerable Areas

Ice and water shield is a self-adhesive membrane that bonds to the decking and seals against water infiltration at eaves, rakes, valleys, and around penetrations. It is particularly important in colder climates where ice dams are a risk.

Code and best-practice guidelines often require ice and water shield in the first several feet of the eave and up vulnerable valleys. Installation demands clean, dry decking and precise application to avoid wrinkles or gaps.

Starter Strips: Where Shingles Begin

Starter strip shingles or a specially cut starter course provides a secure edge and helps prevent wind uplift for the first row of shingles. A properly installed starter strip increases wind resistance and creates a uniform edge for the shingles above.

Starter strips are usually applied along the eaves and rakes and should align with the shingle exposure to ensure proper water shedding and aesthetic lines.

Flashing: Sealing Around Penetrations And Edges

Flashing directs water away from roof openings like chimneys, skylights, vents, and where roof planes intersect. Common flashing materials include galvanized steel, aluminum, copper, and lead-coated copper. Properly installed flashing is one of the most important factors in preventing roof leaks.

Flashing types include step flashing for sidewalls, counterflashing for chimneys, and continuous metal at roof edges. Integration with underlayment and shingles is crucial to create a continuous drainage plane.

Ventilation: Managing Heat And Moisture

Roof ventilation balances attic temperatures and removes moisture that can degrade roof sheathing, insulation, and shingles. Common systems use a combination of intake vents at the soffits and exhaust vents at the ridge or roof plane. Proper ventilation extends roof life and improves home energy efficiency.

Ventilation requirements depend on attic insulation and roof slope. Professionals calculate net free ventilation area and ensure intake vents are not blocked by insulation or other obstructions.

Moisture Barriers And Breathable Underlayments

In certain applications, breathable membranes are used to allow vapor to escape while blocking liquid water. These are particularly useful on low-slope roofs or where condensation risk is high. Selecting the right membrane depends on climate, roof assembly, and ventilation strategy.

Builders must consider vapor drive, roof sheathing drying potential, and HVAC moisture sources when choosing whether to use a breathable underlayment.

Single-Ply And Specialty Roof Systems

For low-slope and commercial roofs, the substrate before the final membrane can include insulation boards, tapered insulation for drainage, and a cover board. These systems differ significantly from steep-slope shingle roofs in materials and installation techniques.

Single-ply membranes like TPO, PVC, and EPDM require precise substrate preparation, fasteners, and in some cases, adhesive or heat-welding to ensure watertight seams.

Common Installation Mistakes To Avoid

• Skipping Ice And Water Shield In Cold Climates: Increases risk of ice-dam leaks.
• Poor Deck Repair: Installing underlayment over rotted or uneven decking leads to premature failure.
• Improper Flashing Integration: Gaps or backward lapping can create pathways for water.
• Insufficient Ventilation: Causes heat build-up and condensation damage.
• Underestimating Wind Requirements: Failing to use the correct starter strip or fastener schedule reduces wind resistance.

Choosing Materials For Longevity And Performance

Material selection should match the local climate, roof pitch, and budget while complying with building codes and shingle manufacturer requirements. Investing in quality underlayment and flashing often results in fewer repairs and longer shingle warranty eligibility.

Homeowners should verify that the chosen underlayment is compatible with the shingle warranty and that installers follow manufacturer instructions for fastener type, nail placement, and layer sequencing.

Inspection And Preparation Before Shingle Installation

Before shingles go on, the roof should be inspected for truss or rafter issues, deck moisture, and flashing needs. An attic inspection helps identify ventilation problems and previous leak evidence that must be addressed.

Walkable underlayment or temporary roofing can protect interiors during a long re-roofing project. Installers should clean the roof of debris and ensure all substrates are dry for adhesive membranes.

Cost Considerations And Value-Added Upgrades

Upfront costs vary by material quality and scope. Synthetic underlayment and premium flashing add to material costs but reduce long-term maintenance. Ice and water shield and proper ventilation are common upgrades that deliver significant value in risk reduction.

Homeowners should request itemized bids that separate decking repair, underlayment, ice and water shield, flashing, and ventilation so that decisions can be made based on both price and performance benefits.

How Professionals Sequence The Layers

A typical installation sequence is: decking inspection and repair, installation of ice and water shield at eaves and valleys, full-field underlayment, flashing installation, starter strip placement, ventilation components, and then shingles. Following a consistent sequence ensures each layer performs its intended role.

Experienced roofers also follow manufacturer-specific requirements for nail placement, shingle alignment, and ridge cap installation to maintain warranties and optimize durability.

Questions To Ask A Roofer Before Work Begins

• Which underlayment will be used and why?
• Will ice and water shield be installed and in which areas?
• How will flashing be integrated with the underlayment and shingles?
• What ventilation strategy will be used for the attic?
• Will decking be inspected and replaced as needed?

Maintenance Tips After Shingles Are Installed

Periodic inspections of flashing, vents, and shingles can catch problems early. Clear gutters and downspouts to prevent water backup, and trim overhanging branches to reduce debris and moss growth. Addressing minor issues promptly preserves underlying layers and prevents costly interior damage.

If leaks occur, identify whether water is entering at seams, flashing, or due to compromised underlayment; a moisture meter and attic inspection help locate wet sheathing and insulation.

Resources For Further Reading And Code Guidance

Residential codes (IRC) and shingle manufacturers provide detailed guidance on underlayment, ventilation, and flashing requirements. Local building departments can clarify code amendments and necessary permits for re-roofing projects.

Industry organizations such as the National Roofing Contractors Association (NRCA) and major manufacturers publish best-practice guides and installation manuals beneficial to homeowners and contractors alike.