Every roof we replace has a moment nobody plans for. The shingles are off, the old underlayment’s peeled back, and for the first time in 25 years the roof decking is out in the open. Most of the time it’s fine. Sometimes it’s not — and when it’s not, it’s usually a lot worse than the homeowner expected, because rot works from the inside out. We’ve pulled up shingles on homes in Stratham and Newmarket and found sheathing so far gone it was spongy to the touch and the only thing holding the roof surface together was the shingle mat itself.

This post is about that layer. What rotted roof decking looks like from the attic and from the roof, why it rots in New Hampshire specifically, what it costs to replace in 2026, and how to tell the difference between a roof that needs a few sheets swapped and one that needs a complete redeck. We’ll also tell you how to read a roofing quote so a decking surprise doesn’t turn into a fight at the end of the job.

What is roof decking, and why does it matter more than the shingles?

Think of it as the subfloor of your roof. On most New Hampshire homes built after about 1970 it’s 1/2-inch CDX plywood or 7/16-inch OSB in 4×8 sheets. On older homes — and we’ve got a lot of them from Exeter to Wolfeboro — it’s board sheathing: 1×6 or 1×8 planks, sometimes rough-sawn, sometimes with gaps you could slide a hand through. Really old capes and farmhouses occasionally have skip sheathing left over from a wood shingle roof, spaced boards that were never meant to hold asphalt shingles at all.

The decking’s job is simple. It carries the snow load to the rafters, and it holds nails. That second part is the one homeowners underestimate. A roofing nail driven into solid 1/2-inch plywood has real pull-through resistance — that’s what keeps a shingle on the roof during a 60 mph nor’easter. Drive that same nail into plywood that’s delaminated into three soggy layers, and it’s holding nothing. The shingle looks fine on install day. It’s gone by February.

What does rotted roof decking actually look like?

We’ll split this up by where you’re standing, because the two views tell you different things.

From inside the attic

Get up there on a dry, bright day with a good flashlight. You’re looking at the underside of the roof between the rafters. Healthy plywood is light, uniform, and dry to the touch. Here’s what bad looks like:

• Dark staining. Brown, black, or rusty-colored patches, often tracking downhill from a chimney, plumbing vent, or valley. A single old stain with no active moisture can be a leak that was fixed years ago. Spreading stains, or stains that are damp, mean it’s ongoing.
• Mold or mildew. White, gray, or greenish fuzz across wide areas of the deck — not just one spot. This one almost always means an attic moisture problem, not a roof leak. We see it constantly on homes in Dover and Rochester where somebody blocked the soffit vents with insulation.
• Frost or drips on the nail tips in January. Shiny frost on hundreds of nail points means warm, wet house air is hitting a cold deck and condensing. Every thaw, that frost melts and soaks into the wood.
• Sagging or waviness between rafters. Plywood that’s been wet repeatedly loses stiffness and starts to dish. If you can see the rafter pattern from the ground as a series of shallow dips across the roof plane, that’s decking failure showing through the shingles.
• Daylight. Any pinhole of light through the deck is a hole, and holes mean water.
• The screwdriver test. Press a screwdriver tip into a stained area. Solid wood resists. Rotted wood lets it sink in with almost no effort, sometimes right through.

From on top, after tear-off

This is where we find out for real. Once the shingles and underlayment are off, decking either looks like wood or it doesn’t.

• Delaminated plywood. The plies separate and you can peel the top layer off in sheets with your fingers. It’s often gray-black and smells like a wet basement. This is the most common thing we replace.
• Swollen, crumbly OSB. OSB doesn’t peel — it puffs up. A sheet that started at 7/16 of an inch can be 3/4 of an inch thick and falling apart at the edges and around every nail. Once OSB has swelled, it never goes back.
• Spongy underfoot. Our crews walk every square foot of a stripped roof before the first sheet of underlayment goes down. Good decking feels like a floor. Bad decking has a little give — you feel it in your heel before you see it. We mark it with a lumber crayon and cut it out.
• Rot at the eaves. The bottom 2 to 3 feet of the roof is where ice dams live, and it’s the single most common spot for decking damage in this part of the country. We’ll often find a perfect roof deck except for a dark, punky strip right along the gutter line.
• Black, punky board sheathing. On older homes, individual planks can be rotted end to end while their neighbors are perfectly sound. Rain finds the gaps between boards, and the board edges take the damage.

Why does roof decking rot in New Hampshire?

Homeowners assume rot means a leak. Sometimes it does. But if we had to put a number on it, well over half the decking we replace never saw a drop of rain from above. It rotted from the attic side.

Here’s the mechanism. Your house makes a lot of water vapor — showers, cooking, breathing, a humidifier on the furnace. Warm, moist air rises, finds every gap in the attic floor, and lands in the attic. In January the underside of the roof deck is running close to outdoor temperature, sometimes 10°F. That moist air hits the cold plywood and condenses, then freezes. On a bad night you’ll see frost coating the entire underside of the roof. Then the sun comes out, the frost melts, and the water has nowhere to go but into the wood. Do that 60 or 80 times a winter for 20 winters. That’s how a roof deck rots without ever leaking.

The things that make it worse — and we see every one of these constantly across the Seacoast and the Lakes Region:

• Blocked or missing soffit vents. Somebody blew in insulation and buried the intake. No intake means no airflow, no matter how many ridge vents you’ve got.
• Bathroom exhaust fans vented into the attic. This one’s a killer. A fan that dumps into the attic instead of through the roof or wall is pumping a hot shower’s worth of steam onto the deck every single day. We’ll find a perfect circle of black rot directly above the fan.
• No ridge vent, or a ridge vent with no path to it. Common on older homes that got a re-roof from someone who skipped the ventilation entirely.
• Long-term slow leaks. A cracked pipe boot or a rusted step flashing leaks a cup of water per storm for a decade. Not enough to show on the ceiling. Plenty to rot a 3-foot radius of decking.
• Ice dams at the eaves. Water backs up under the shingles and sits on the deck for weeks. If the roof didn’t have ice & water shield — and most roofs over 20 years old don’t — it soaks straight in.
• Coastal moisture. Homes in Rye, Hampton, and York sit in salt-laden, humid air year-round. It doesn’t rot decking by itself, but it stacks on top of everything above.

The point of understanding this: if we replace your rotted decking and put on a beautiful new roof but don’t fix the ventilation and the bath fan, the new plywood starts rotting the first winter. A contractor who only talks about shingles is missing the actual problem.

How much does roof decking replacement cost in NH in 2026?

Decking is a line item, not a surprise — or it shouldn’t be. Here’s how the pricing works on our jobs and on most reputable contractors’ jobs in Southern New Hampshire and the Merrimack Valley:

A few things that move the number. Plywood prices bounce around — we’ve seen a sheet of 1/2-inch CDX swing by $15 in a single season — so per-sheet pricing on a quote written in the spring might be off by fall. Steep roofs cost more because cutting out and fitting sheets on a 10/12 pitch is slow, careful work. And disposal matters: rotted OSB is heavy and bulky, and it all goes in the dumpster.

For scale: on a $16,000 architectural shingle roof, a typical decking bill is $0 to $600. A full redeck can push the whole project past $24,000. That’s a big enough difference that you want to know which one you’re looking at before the crew shows up.

When does a NH roof need complete redecking?

We don’t redeck roofs that don’t need it. It’s expensive, it’s a full extra day, and if the wood is sound there’s no reason to throw it away. But there are four situations where patching in sheets stops making sense:

1. The rot is everywhere, not somewhere

Scattered rot has a cause you can point at — this chimney, that boot, the eaves. Rot across the whole plane means the attic’s been wet for decades. Once we’re cutting out every third sheet, the labor of piecing it in costs more than just stripping the roof to the rafters and sheeting it fresh. We hit this on a cape in Gilford last year where the bath fan had been venting into the attic since the ’80s. The whole north side was gone.

2. Swollen OSB across broad areas

Plywood can get wet, dry out, and be okay. OSB can’t. Once it’s swelled, it’s lost its structure permanently, and it’s uneven enough that the new shingles will telegraph every seam. If half the OSB on a roof is puffed up, the rest isn’t far behind.

3. Skip sheathing or wide-gapped boards

Homes that started life with cedar shingles often have boards spaced 2 to 4 inches apart. That’s fine for wood shingles, useless for asphalt — nails land in the gaps. Same story with board sheathing that’s shrunk and cupped so the gaps are over a quarter inch. On those roofs we overlay with new 1/2-inch plywood across the entire surface. It’s not technically “redecking” but it’s the same cost category and the same result: a solid nailing surface.

4. Thin or non-code decking

Occasionally we open up a roof and find 3/8-inch plywood, or worse, particle board. It might be dry, but it’s not enough to carry snow load or hold nails. It comes off.

On a 100-year-old New Hampshire farmhouse, expect to be in redeck territory more often than not. On a 1995 colonial in Bedford, expect a handful of sheets at the eaves and nothing else. Older isn’t automatically worse — some of the tightest board sheathing we’ve seen is on 1920s homes in Portsmouth where old-growth lumber has outlasted everything installed since. But the odds shift.

Plywood or OSB for replacement decking?

We’ll be upfront that this is an opinion, and some good contractors disagree. OSB is cheaper — usually $8 to $15 less per sheet — and it’s a perfectly fine product in a dry attic. The problem is that “dry attic” is doing a lot of work in that sentence. In our climate, attics get wet, and when they do, plywood forgives and OSB doesn’t. We’ve replaced 15-year-old OSB decks that were done for while 40-year-old plywood next door was still solid. That’s enough data for us.

If a contractor wants to use OSB to hit a lower number, that’s a reasonable conversation. Just make sure it’s at least 7/16-inch, it’s rated for roof sheathing (look for the APA stamp), and the ventilation is fixed so it stays dry.

How does decking replacement show up on a roofing quote?

This is the part where roofing jobs go sideways. A contractor writes a tight number to win the job, tears off the roof, “finds” rot, and hands the homeowner a bill for 30 sheets at $150 each with the roof already stripped and rain in the forecast. You’ve got no bargaining position at that point. It’s one of the oldest tricks in the trade.

Here’s what to look for instead:

• A per-sheet price written into the contract. If it’s not on paper, the price is whatever they say it is on tear-off day.
• An honest estimate based on an actual inspection. If somebody’s been in your attic and walked the roof, they can tell you “probably 2 to 4 sheets at the eaves” or “plan on a full redeck.” A drone photo or a drive-by can’t tell you either.
• Photos of every sheet replaced. We shoot each one before and after, with the sheet count. You should never be asked to pay for decking you can’t see in a photo.
• A conversation about ventilation. If your deck was rotted from the attic side and the quote doesn’t address why, you’re paying to rot a second deck.
• No overlays. Any quote that proposes shingling over an existing layer skips the decking inspection entirely. That’s the whole point of a tear-off.

One more thing. If a quote comes in $3,000 under everyone else’s and it’s vague about decking, that gap is probably going to show up on the change order. Ask about it directly. The good contractors — and there are plenty in Southern NH — won’t be offended by the question.

Can you prevent roof decking rot?

Almost none of this is expensive. Rerouting a bath fan through the roof with a proper cap is a couple hundred dollars. Clearing soffit vents and adding baffles is a morning’s work. Replacing a cracked pipe boot before it soaks the deck for five years is a $200 repair instead of a $900 decking bill. If you’re replacing your roof anyway, this is exactly the time to fix all of it — the deck’s exposed, the ridge is open, and adding ventilation costs a fraction of what it does as a standalone job.

Every fall, and again in late winter, go up in the attic and look. That’s it. Ten minutes a year with a flashlight will catch most decking problems while they’re still a sheet or two instead of a whole roof.