Your shingles are only the top inch or so of your roof. They are the part you see from the driveway and the part you pick a color for, but they are not the part doing most of the work keeping water out of your house. That job belongs to the layers underneath, and they are the layers a quick roofing quote tends to skip over. This guide walks through every layer of a typical asphalt shingle roof, from the wood deck up to the shingles, in plain language. You will learn what each layer does, why New Hampshire’s winters make some of them far more important than they would be in a milder climate, and how to spot them on an estimate so you can compare quotes fairly.
The Layers of a Roof at a Glance
A modern asphalt roof is a system, not a single product. Each layer has a specific job, and the roof only performs well when all of them are installed correctly. Working from the inside out, here is the order:
| Layer | What it does | Where it goes |
|---|---|---|
| Roof decking | Structural base the whole roof is fastened to | Over the rafters |
| Ice and water shield | Self sealing barrier against water backed up by ice | Eaves and valleys, plus other vulnerable spots |
| Underlayment | Second line of defense that sheds water under the shingles | Over the rest of the deck |
| Drip edge | Metal strip that guides water off the roof edge | Eaves and rakes |
| Flashing | Seals joints and penetrations | Walls, chimneys, vents, valleys |
| Shingles | The weather surface that sheds rain, snow and sun | Over everything |
| Ventilation | Moves air through the attic to control heat and moisture | Soffits and ridge |
Layer 1: The Roof Decking
Everything starts with the deck. Decking is the layer of plywood or similar sheet material fastened across the rafters. It gives the roof its flat, solid surface, and every other layer is nailed to it.
Decking does not usually fail on its own. It fails because water got to it. A slow leak around a chimney, an ice dam that pushed water under the shingles for several winters, or poor ventilation that left the underside damp can all soften and rot the wood over time. The trouble is that you cannot see the deck from the ground or from under the shingles, which is why a proper replacement starts with a full tear off. Once the old roofing is gone, the deck is exposed and can be inspected board by board. Soft, rotted, or damaged boards get replaced before anything new goes on top.
This is one of the most common surprises on older New Hampshire roofs. A homeowner expects a shingle swap and finds out the wood underneath needs work. It is much better to learn that during the tear off than to have it covered over and discover it years later.
Layer 2: Ice and Water Shield
Ice and water shield is the layer that matters most in a climate like ours, and it is also the least understood. It is a self adhering membrane, usually a rubberized asphalt product, that sticks directly to the deck and seals itself around the nails driven through it.
Here is why that matters. Ice dams form when heat escaping from the house melts snow on the roof. The meltwater runs down to the cold edge of the roof and refreezes, building a ridge of ice that blocks drainage. Water backs up behind that ridge and gets pushed up and under the shingles. Ordinary underlayment is not designed to stop standing water. A self sealing membrane is, because it forms a watertight skin that water cannot work its way through.
On a well built roof, ice and water shield goes along the eaves, where ice dams form, and in the valleys, where water from two roof slopes concentrates. Some roofs call for more coverage, especially low slope sections, where water drains slowly and sits longer. Model residential building codes call for an ice barrier in cold regions with a history of ice forming at the eaves, extending from the roof edge to a point inside the exterior wall line. Local rules can vary, so your town building department can confirm what applies to your address.
If you have ever seen brown water stains on a ceiling along an exterior wall after a snowy stretch, or peeling paint and rotted fascia at the eaves, that is often the result of a roof without enough ice protection at the edge.
Layer 3: Underlayment
Over the rest of the deck, the roofer installs underlayment. Think of it as the roof’s second line of defense. Shingles shed most of the water, but wind driven rain, snow that melts and refreezes, and the occasional damaged shingle will let some moisture through. Underlayment catches that water and sends it down the slope and off the roof instead of letting it reach the wood.
It also protects the deck during installation. A roof is rarely finished in the same hour it is torn off, and a deck left exposed to a sudden rain shower can be damaged before the shingles ever go on.
There are two main types:
- Synthetic underlayment. Made from woven or spun polymer. It is strong, resists tearing, does not absorb water, and holds up well to weather exposure while the roof is going on. It is the type used on our asphalt roofs.
- Asphalt saturated felt. The traditional option. It costs less and is still widely used, but it can wrinkle when it gets wet and is generally less durable than synthetic.
Neither is a substitute for ice and water shield at the eaves and valleys. The two work as a team, with the self sealing membrane in the places water is most likely to back up and underlayment covering the rest.
Layer 4: Drip Edge
The drip edge is a thin strip of metal flashing installed along the edges of the roof, at the eaves and along the sloped rake edges. It is one of the cheapest components on the roof and one of the easiest to skip, which is why it shows up as a line item on quotes that some homeowners do not recognize.
Its job is simple. It guides water off the roof edge and into the gutter instead of letting it cling to the underside of the shingles and run back onto the fascia board and deck edge, where it causes rot. It also gives the outermost shingles a firm edge to rest on. We cover this part in detail in our guide on what a drip edge is and why every NH home needs one.
Layer 5: Flashing
Flashing is the metal that seals the places where the roof meets something else. Wherever a roof surface is interrupted by a wall, a chimney, a plumbing vent, or the intersection of two slopes, there is a joint, and joints are where leaks start.
The main types you will hear about are:
- Step flashing along walls where a roof meets vertical siding, installed in overlapping pieces so water always runs over the top of the next piece down.
- Chimney flashing that wraps the base of the chimney, often paired with counterflashing set into the masonry.
- Vent flashing around plumbing pipes and other roof penetrations.
- Valley flashing where two roof slopes meet and channel water downward.
Flashing is where a lot of roof leaks originate, even on roofs with good shingles, because it is the area most exposed to movement from the freeze and thaw cycle. Old flashing that has rusted, lifted, or been sealed with caulk instead of replaced is a common culprit. On a full replacement, new flashing goes in at walls, chimneys, and vents rather than reusing the old.
Layer 6: The Shingles
Finally, the layer everyone sees. Architectural asphalt shingles are the standard choice for most New Hampshire and Massachusetts homes. They are made from a fiberglass mat coated in asphalt and topped with mineral granules that protect against sun and weather. The dimensional look comes from multiple layers laminated together.
With proper attic ventilation, architectural shingles from a major manufacturer are expected to last roughly 25 to 30 years in our climate. We install complete Owens Corning systems, and you can see exactly what goes into each job on our asphalt shingle roofing page. A complete system also includes starter shingles along the edges and ridge cap shingles at the peaks, and using matching components from one manufacturer is generally what makes a stronger system warranty possible.
The key point to take from this section is that shingles are the part of the roof most likely to be marketed to you and the part least likely to determine whether it leaks. A great shingle over a poorly built base will underperform a good shingle over a well built one.
The Layer You Cannot See: Attic Ventilation
Ventilation is not a layer on the roof itself, but it determines how long every layer above it lasts. Air should flow in through the soffits at the bottom of the roof and out through vents at or near the ridge. That airflow keeps the attic close to the outside temperature in winter, which reduces ice dam risk by keeping the roof deck cold, and it carries away heat and moisture in summer.
A roof with poor ventilation can cook its shingles from underneath, trap moisture in the decking, and fail years ahead of schedule. It is worth understanding on its own, and our article on why attic ventilation might be the reason your shingles are failing early explains the signs to look for.
Why a Full Tear Off Matters
Everything above explains why we remove the old roofing down to the deck on every replacement. A tear off lets the roofer do four things that layering new shingles over old ones makes impossible:
- Inspect the decking for soft or rotted boards and replace them.
- Install ice and water shield directly on a clean deck, where it can seal properly.
- Replace the flashing instead of covering over worn or failing pieces.
- Avoid hiding problems and adding the weight of an extra layer to the roof structure.
Building codes also limit how many layers of asphalt shingles a roof can carry, and the rules vary by state. We explain the Massachusetts version in our guide to re shingling over an old roof and the two layer rule. Most homes are finished in a single day, even with a full tear off.
How to Spot These Layers on a Roofing Estimate
This is where understanding the layers pays off. Two quotes can look similar at the bottom line while including very different amounts of work. When you compare estimates, look for these line items and ask about anything that is missing:
- Tear off and number of layers removed. Is the old roofing coming off completely?
- Decking allowance. How are replaced boards priced, and what happens if more rotted wood turns up than expected?
- Ice and water shield coverage. How far up from the eaves does it go, and is it included in the valleys?
- Underlayment type. Synthetic or felt, and over what area?
- Drip edge. Is new drip edge included at both the eaves and the rakes?
- Flashing. Will walls, chimneys, and vents get new flashing, or is existing flashing being reused?
- Ventilation. Has the attic been assessed, and is any ventilation work included?
- Warranty. What is covered, for how long, and by whom?
A quote that leaves these out is not automatically bad, but a lower price that omits layers is not really lower. For a sense of what homeowners actually pay when all of these are included, our New Hampshire roof replacement cost guide uses real signed jobs. Most asphalt replacements we sign in NH run between $12,500 and $22,000, and that price includes the tear off, ice and water shield, cleanup, and our 25 year installation warranty.
Frequently Asked Questions
What are the layers of a roof?
From the inside out, an asphalt shingle roof is built from the roof decking, an ice and water barrier along the eaves and valleys, synthetic or felt underlayment over the rest of the deck, drip edge along the edges, flashing at walls, chimneys and vents, and the shingles on top. A ventilation system in the attic works alongside these layers.
What is roofing underlayment?
Underlayment is a water resistant layer installed over the roof decking and beneath the shingles. It acts as a second line of defense, shedding any water that gets past the shingles, and it protects the deck while the roof is being installed.
Do I need ice and water shield in New Hampshire?
For most New Hampshire homes, yes. Ice and water shield is a self sealing membrane that blocks water driven back under the shingles by ice dams. Model residential building codes call for an ice barrier in cold regions with a history of ice at the eaves, and many roofers install it along the eaves and in valleys on every job. Confirm the specific requirement with your town building department.
Is synthetic underlayment better than felt?
In general, synthetic underlayment is stronger, resists tearing, and holds up better to weather exposure during installation than traditional asphalt saturated felt. Felt is still used and costs less, so the right choice depends on the roofing system, the budget, and the manufacturer’s warranty requirements.
Can a new roof go over old shingles?
It is sometimes allowed by code, but a full tear off is the better practice. Removing the old roofing lets the roofer inspect the decking for rot, install ice and water shield correctly, and get the flashing right. Layering over old shingles hides problems and adds weight to the roof.
Want to See What Is Under Your Roof?
You cannot judge the layers of a roof from the ground, and a quote that does not spell them out leaves you guessing. Nick Tracey Roofing is based in Bedford and offers free, no obligation inspections and written, line item estimates across New Hampshire and Massachusetts. We will show you photos of the decking and explain exactly what goes into your roof. Contact us to schedule a visit.
