Almost every homeowner planning a new roof asks the same question. Will the price change once you start?
The honest answer is that it can, and there is one reason above all others. Decking. The layer of plywood or board sitting under your shingles is the only part of the roof nobody can fully inspect until the old roof comes off. On a New Hampshire house that has been through twenty five winters, what is under there is not always what the surface suggests.
Here is what we find, why it happens, and what it means for your estimate.
What roof decking actually is
Your roof is built in layers. Rafters carry the structure, decking is nailed across them, underlayment goes over the decking, and shingles go on top.
The decking is usually half inch plywood or OSB on newer houses, or plank board on older ones. Everything above it fastens into it. If the decking is soft, the nails holding your new shingles have nothing solid to grip, and no shingle warranty covers a roof installed over failed sheathing.
That is the whole reason this matters. You are not paying to replace decking because a contractor found something extra to sell. You are paying because without it, the roof above it cannot do its job.
Why roof decking rots in New Hampshire
Three things cause most of the damage here, and all three are seasonal.
Ice dams
Warm attic air melts snow in the middle of the roof. That meltwater runs down to the cold eave, refreezes, and backs up under the shingles. Instead of running off, the water sits on the decking.
Repeat that every February for fifteen years and the sheathing along the eaves goes soft first, which is exactly where we find it most often.
Poor attic ventilation
An attic without balanced intake and exhaust holds moisture. Through the winter that moisture condenses on the underside of the decking.
This one is quiet. The roof surface can look perfectly serviceable while the sheathing is delaminating from below, which is why a proper roof inspection has to include the attic and not just a walk on the shingles.
Small leaks nobody knew about
A cracked pipe boot or a lifted piece of flashing lets in a trickle rather than a flood. It never shows as a stain on a ceiling, so nobody calls anyone. Meanwhile the water tracks along a rafter and soaks a patch of decking for years.
Salt air, if you are near the coast
On the Seacoast there is a fourth cause. Salt corrodes fasteners and flashing years before the shingles themselves wear out, and once those fail the water has a route in. We see this constantly on roofs in Hampton, Seabrook and the surrounding coastal towns.
How you know before the roof comes off
Mostly, you do not. Any contractor who guarantees on a quoting visit that your decking is fine is guessing.
There are signs worth taking seriously though:
- A roof deck that dips or waves when you sight along the ridge line
- Soft or spongy movement underfoot when a roofer walks the roof
- Daylight visible between boards when you look from inside the attic
- Dark staining on the sheathing above the wall plate
- Insulation that feels damp to the touch
- Repeated ice damming at the same section of eave every winter
Any one of these is worth an inspection. Several together usually means the decking is already compromised.
What we do when we find it
Cut it out and replace it. There is no other correct answer, and anyone offering one is telling you what you want to hear.
The part that matters is how. We work section by section rather than stripping the entire roof at once. Opening one area, replacing the failed sheathing, and getting that section dried in before moving to the next is how a house stays watertight across a job that runs several days.
Tearing the whole roof off to save time and hoping the forecast holds is how ceilings get ruined. In New Hampshire, where a clear morning turns into a squall by mid afternoon, that is not a risk worth taking with somebody’s house.
Once new decking is down, everything else proceeds normally. Ice and water shield along the eaves and through the valleys, synthetic underlayment across the rest, then the shingles.
What this looks like in practice
We recently completed a roof replacement on a colonial in Hampton where this played out exactly.
From the ground the roof looked worn but not alarming. Once it was open, years of water intrusion had gone through the sheathing in multiple areas. We replaced decking, and while we were on site we also repaired siding where water had been getting behind the wall at the roof to wall junctions.
That job ran eight to ten days rather than the two or three a straightforward thirty two square roof would take. Every one of those extra days went into work that had to happen. Nailing new shingles over rotten wood would have looked identical on day one and failed within a few winters.
What decking replacement costs
Replacement decking typically runs between 70 and 120 dollars per sheet installed. That covers the material, the labor to cut out the failed section, and disposal.
The number of sheets is what varies, and it varies enormously. Some roofs need none at all. Some need half a dozen. Occasionally an entire eave line has to come out.
Because nobody can know that count in advance, we handle it the same way on every estimate. A decking allowance is written into the quote up front with a per sheet price stated clearly. Find nothing, and you are not charged for it. Find twelve sheets, and you already knew the rate before anyone climbed a ladder.
Be cautious of any quote that does not mention decking at all. It rarely means the contractor expects to find none. More often it means that conversation is being saved for the day the roof is open and you have no realistic option but to agree to whatever number comes next.
Fixing the decking is only half the job
If ice dams or poor ventilation caused the rot, and nothing about the attic changes, the new decking will eventually go the same way as the old.
This is the part most quotes skip. We check attic ventilation on every roof replacement and correct the balance between intake and exhaust where it is wrong. A ridge vent with blocked or missing soffit intake is not a ventilation system, it just looks like one from the outside.
Getting that right is usually a small part of the overall cost, and it protects everything above it for the next twenty five years.
Frequently asked questions
Can rotted roof decking be repaired instead of replaced?
No. Once sheathing has taken on enough water to go soft or delaminate, it cannot be dried out or reinforced back to structural condition. The failed section is cut out and new decking is installed in its place. Only the damaged area needs replacing, not the entire roof.
How many sheets of decking usually need replacing?
It ranges from none to a dozen or more, and there is no reliable way to know before the old roof is off. Roofs under fifteen years old with good ventilation often need none. Older roofs with a history of ice damming frequently need several sheets along the eaves.
Will my insurance cover rotted decking?
Usually not. Homeowners insurance covers sudden accidental damage such as a storm or a fallen limb. Decking that has deteriorated gradually from age, ice damming or long term moisture is treated as wear and maintenance, which policies exclude. Decking damaged as part of a covered storm event is a different matter and worth documenting.
How can I tell if my roof decking is bad?
Look for a roofline that dips or waves, spongy movement underfoot, daylight between boards in the attic, dark staining on the sheathing, or damp insulation. An inspection that includes the attic will usually confirm it, though the full extent is only visible once the shingles are removed.
Not sure what is under your shingles?
Our free roof inspection covers the roof surface and the attic, which is where decking problems show up first. You get photographs and a written condition report whether or not you ever hire us, and if your roof has years left in it, that is what the report will say.
Book a Free Inspection or call (603) 502-7080