What Goes Under Roof Shingles? 7 Layers of a Roof Explained
Your roof shingles are just the top layer of a multi-component system designed to keep water out of your home. Underneath those shingles are 6 additional layers, each serving a specific purpose: structural support, water resistance, wind protection, edge sealing, and ventilation. If any single layer is missing, damaged, or incorrectly installed, the entire system is compromised.
This guide walks through every layer of a residential shingle roof from bottom to top, explains what each layer does, and covers the specific requirements for homes in Myrtle Beach and coastal South Carolina where hurricane codes demand higher standards for nearly every component.
Want a deep dive on underlayment? See our synthetic underlayment vs felt paper guide for a detailed comparison of the two main underlayment types and why coastal SC homes need synthetic.
Layer 1: Roof Decking (The Foundation)
The roof deck is the structural base of your entire roofing system. It consists of sheets of plywood or oriented strand board (OSB) nailed to the roof trusses or rafters. Every other roofing component is attached to or laid on top of the decking. If the deck is compromised, nothing above it will work correctly.
Standard residential roof decking uses 7/16-inch or 1/2-inch OSB or 1/2-inch CDX plywood. In coastal South Carolina, some builders and code officials prefer plywood over OSB because plywood handles moisture exposure better. OSB can swell at the edges when exposed to water, creating bumps that show through shingles.
Coastal SC requirement: During a roof replacement, every section of decking must be inspected for rot, delamination, and structural integrity. Damaged sections must be replaced before new underlayment is installed. In Myrtle Beach, humidity and occasional water intrusion make decking damage more common than in drier climates.
Layer 2: Drip Edge
Drip edge is a narrow strip of metal flashing installed along the eaves (bottom edges) and rakes (side edges) of the roof. It serves three purposes: it directs water away from the fascia board and into the gutter, it prevents wind-driven rain from getting under the roofing materials at the edges, and it provides a clean, straight line for the first row of shingles.
Drip edge is installed directly over the decking at the eaves (before the underlayment) and over the underlayment at the rakes. This specific layering order ensures water flows over the drip edge and into the gutter rather than behind it.
Coastal SC requirement: Drip edge is required by code in South Carolina. In coastal areas, use aluminum or galvalume drip edge — not plain galvanized steel, which corrodes in salt air within a few years. The added cost for corrosion-resistant drip edge is minimal (a few dollars per linear foot) and prevents premature failure.
Layer 3: Ice and Water Shield
Ice and water shield is a self-adhering rubberized asphalt membrane applied to the most vulnerable areas of the roof deck: along the eaves, in valleys where two roof planes meet, around chimneys and vent pipes, around skylights, and at any other penetration or transition point.
Unlike regular underlayment which is mechanically fastened with nails or staples, ice and water shield sticks directly to the deck, creating a waterproof seal. It is specifically designed to seal around the nails that penetrate it when shingles are installed above, preventing water from wicking down the nail shanks into the deck.
The name references ice dams, which are less of a concern in Myrtle Beach. But the wind-driven rain protection that ice and water shield provides is critical on the coast. During a hurricane, wind can push rain uphill against gravity, forcing it under shingles and past regular underlayment. Ice and water shield at the eaves and valleys is the last defense against this water intrusion.
Coastal SC requirement: Building code requires ice and water shield along the eaves for a minimum of 24 inches past the interior wall line. In Wind Zone III, many contractors extend coverage to 36 inches or the full first 3 feet from the roof edge. Valleys and all penetrations also require ice and water shield. In Myrtle Beach, this is non-negotiable — it is the primary defense against wind-driven rain intrusion.
Layer 4: Underlayment
Underlayment covers the entire roof deck (over the ice and water shield at the eaves and valleys, and directly over the bare deck everywhere else). It is the secondary water barrier for the full roof surface. If any shingle is missing, cracked, or lifted by wind, the underlayment prevents water from reaching the deck.
There are two main types: traditional felt paper (asphalt- saturated felt) and modern synthetic underlayment. In coastal South Carolina, synthetic underlayment is the clear choice. It is more water resistant, more tear resistant, tolerates heat and UV exposure better, and meets Wind Zone III code requirements that felt paper often cannot satisfy.
Synthetic underlayment costs $0.15 to $0.25 per square foot versus $0.05 to $0.10 for felt — a difference of roughly $200 to $300 on a 2,000 square foot roof. That small investment buys a secondary water barrier that lasts 25 to 50 years instead of 15 to 20 years for felt. Read our complete synthetic underlayment vs felt paper comparison for the full breakdown.
Coastal SC requirement: The underlayment must meet wind uplift and water penetration standards for Wind Zone III. Premium synthetic underlayment meets these requirements. Standard 15-pound felt does not. Most building inspectors in Myrtle Beach require synthetic underlayment for permit approval on new roof installations.
Layer 5: Starter Strip Shingles
Starter strip shingles are a specialized adhesive-backed strip installed along the eaves and rakes before the first course of regular shingles. They serve two critical functions: they provide a continuous adhesive bond at the most wind-vulnerable edge of the roof, and they fill the gap beneath the first row of shingles where the cutouts between shingle tabs would otherwise leave exposed underlayment.
Without starter strip, the first row of shingles has no adhesive backing it. Wind can catch the lower edge and peel shingles off starting from the bottom. This is one of the most common failure points during windstorms, and it is entirely preventable with proper starter strip installation.
Coastal SC requirement: Starter strip is required at all eaves and rakes. In Wind Zone III, use a high-wind starter strip with enhanced adhesive (most major manufacturers offer a coastal or high-wind version). The starter strip adhesive line should face the eave edge, not the ridge, to prevent wind uplift.
Layer 6: Shingles (The Outer Shell)
The shingles are the outermost layer and the first line of defense against weather. They take the direct impact of rain, wind, hail, and UV radiation. Asphalt shingles are the most common choice, but options include architectural asphalt, composite (synthetic), metal panels, concrete tile, clay tile, and natural slate.
Shingles are installed from the bottom of the roof upward, overlapping each course to create a layered water-shedding surface. Each shingle is fastened with 4 to 6 nails (coastal codes typically require 6 nails per shingle instead of the standard 4) placed in a specific nailing zone. Incorrect nail placement is one of the most common installation defects and can void the manufacturer's warranty.
Coastal SC requirement: In Wind Zone III, shingles must be rated for the design wind speed (typically 130 to 150 mph for Myrtle Beach). A 6-nail pattern is required instead of the standard 4-nail pattern. Nails must be corrosion-resistant (stainless steel or hot-dipped galvanized). These requirements add 10 to 20% to the cost of shingle installation but are essential for hurricane survivability.
Layer 7: Flashing and Ventilation
Flashing and ventilation are not a single flat layer like the others — they are components integrated throughout the roofing system at every transition, joint, and penetration point.
Flashing
Flashing is sheet metal (aluminum, galvanized steel, or copper) installed wherever the roof plane meets a vertical surface or changes direction. Common flashing locations include:
- Chimney flashing — Step flashing along the sides and counter flashing against the chimney face
- Valley flashing — Open or closed metal in the valley where two roof planes meet
- Vent pipe boots — Rubber and metal collars around plumbing vent pipes
- Skylight flashing — Step and counter flashing around skylight frames
- Wall flashing — Where the roof meets a vertical wall (dormer sides, second-story walls)
- Headwall flashing — Where a sloped roof meets a wall above it
Flashing failures are the number one source of roof leaks. The flashing itself does not fail often, but the sealant between flashing pieces and around penetrations dries out, cracks, and separates over time. During a roof replacement, all flashing should be replaced or re-sealed.
Ventilation
Proper roof ventilation prevents moisture buildup and excessive heat in the attic. A balanced ventilation system includes:
- Soffit vents — Intake vents in the eave overhangs that draw in outside air
- Ridge vent — Exhaust vent along the peak of the roof that lets hot, moist air escape
- Attic baffles — Channels that keep insulation from blocking the soffit-to-ridge airflow path
Without proper ventilation, moisture condenses in the attic and rots the decking from the inside. In Myrtle Beach's humid climate, poor ventilation can cause decking failure in as little as 5 to 8 years, even on a roof with new shingles. Good ventilation also reduces attic temperatures by 20 to 40 degrees in summer, extending shingle lifespan and reducing cooling costs.
Coastal SC requirement: Flashing must be corrosion-resistant material in coastal areas. Aluminum is standard; galvanized steel corrodes in salt air. Ventilation must meet the 1:150 or 1:300 net free area ratio (depending on vapor barrier presence) per South Carolina building code. Ridge vent should be rated for the design wind speed to prevent rain intrusion during storms.
All 7 Layers at a Glance
| Layer | Material | Purpose | Coastal SC Note |
|---|---|---|---|
| 1. Decking | Plywood or OSB | Structural base | Inspect for rot at every replacement |
| 2. Drip edge | Aluminum or galvalume | Water direction, edge protection | Use corrosion-resistant metal only |
| 3. Ice & water shield | Self-adhering membrane | Waterproof seal at vulnerable areas | 24” min past interior wall at eaves |
| 4. Underlayment | Synthetic (preferred) or felt | Secondary water barrier | Synthetic required for Wind Zone III |
| 5. Starter strip | Adhesive-backed shingle strip | Wind seal at eaves and rakes | High-wind rated starter required |
| 6. Shingles | Asphalt, composite, metal, or tile | Primary weather protection | 6-nail pattern, wind-rated material |
| 7. Flashing & ventilation | Aluminum/copper flashing, ridge/soffit vents | Seal joints, control moisture/heat | Corrosion-resistant, wind-rated vents |
Frequently Asked Questions
What are the layers under roof shingles?
From bottom to top, the layers under roof shingles are: roof decking (plywood or OSB boards that form the structural base), drip edge (metal flashing along the roof edges), ice and water shield (self-adhering waterproof membrane at eaves, valleys, and penetrations), underlayment (synthetic or felt moisture barrier covering the entire deck), starter strip (adhesive shingle strip along the eaves), and then the shingles themselves. Flashing and ventilation are integrated throughout.
Do you need underlayment under roof shingles?
Yes. Roof underlayment is required by building code in every state, including South Carolina. It serves as a secondary water barrier that protects your roof deck if wind-driven rain gets past the shingles, if a shingle is missing or damaged, or if ice dams form. In coastal South Carolina, synthetic underlayment is the standard because it outperforms felt paper in water resistance, heat tolerance, and wind zone compliance.
What is the most important layer of a roof?
Every layer serves a specific purpose, but the roof decking is arguably the most critical because every other component attaches to it. If the decking is rotted or structurally compromised, nothing above it will perform correctly. The underlayment is the most important protective layer because it is the true water barrier. Shingles shed most water, but the underlayment handles everything that gets past them.
How many layers should a roof have?
A properly installed shingle roof has 7 distinct layers: decking, drip edge, ice and water shield (at critical areas), underlayment, starter strip, shingles, and flashing at all joints and penetrations. Ventilation components are also part of the system. In coastal South Carolina, code requires specific types or grades of several of these layers, particularly the underlayment, ice and water shield coverage, fasteners, and drip edge material.
Can you put new shingles over old shingles without replacing underlayment?
While some building codes allow a second layer of shingles over an existing layer, it is generally not recommended. The old underlayment beneath the first layer of shingles is already aged and degraded, and you cannot inspect or replace it without removing the old shingles. In coastal South Carolina, most professional roofers recommend a full tear-off to new deck on every replacement, allowing inspection and repair of decking, fresh underlayment installation, and proper installation of all waterproofing layers per current code.
Get Every Layer Right on Your Myrtle Beach Roof
A roof is only as good as its weakest layer. At WeatherShield Roofing, we inspect, repair, or replace every layer during a roof replacement — from decking through final shingle. We use synthetic underlayment, corrosion-resistant fasteners, and high-wind rated components as standard, because cutting corners on the layers you cannot see is how roofs fail prematurely on the coast.
We are GAF Certified contractors (SC License #124773) with 82 five-star Google reviews, serving Myrtle Beach homeowners since 2022. Request a free inspection to see exactly what shape your roof layers are in.