Here’s something most homeowners don’t realize: by the time a roof problem shows up on your ceiling, the real mistake was usually made years earlier — often on the day the roof went on.

We’ve pulled up shingles on homes in Stratham and Exeter and found decking so soft you could push a thumb through it, hidden under a roof that looked perfectly fine from the driveway. We’ve climbed into attics in Laconia where the previous crew nailed a brand-new roof right over a ventilation problem nobody bothered to fix. None of that is bad luck. It’s a handful of common roofing mistakes that repeat themselves over and over — and almost all of them are preventable. Here are the seven we run into most, what they actually cost you, and how to make sure they don’t happen on your house.

1. Chasing the cheapest bid

We get it — a roof is a big check to write, and when one number comes in $4,000 under the others, it’s tempting. But a roof is one of those things where the price tells you what’s in it. When a bid is dramatically lower, the savings came from somewhere, and it’s never the part you can see from the street.

Usually it’s the boring stuff that does the heavy lifting: the underlayment, the ice & water shield at the eaves, new flashing, a proper ridge vent. Cut those and the roof still looks great on install day. It just doesn’t make it to year ten. The honest way to compare quotes isn’t the bottom line — it’s line by line. When two bids include the same tear-off, the same underlayment, the same flashing and ventilation, then price is a fair tiebreaker. Until then you’re comparing a roof to a haircut.

2. Roofing over the old shingles

An overlay — nailing new shingles right on top of the old ones — is the classic way to make a roof cheaper. And we understand the appeal. No tear-off means less labor and no dumpster. But it’s a shortcut that costs you on the back end every single time.

Two layers of asphalt hold more heat, which cooks the new shingles faster than they should ever wear. The bigger problem is what you’re covering up. The deck — the plywood or plank sheathing your shingles are nailed to — is where the real damage hides. If there’s rot, soft spots, or old water staining, an overlay buries it instead of fixing it. We tear off to the deck on every re-roof, specifically so we can walk it, find the spongy sections, and replace the bad wood before anything new goes down. You only get one clean look at that decking, and it’s during the tear-off.

3. Treating attic ventilation as optional

This is the one homeowners almost never think about, and it might be the most important item on the whole list. A roof isn’t just shingles — it’s a system, and air movement is part of it. When an attic can’t breathe, heat and moisture build up underneath the deck. In summer that heat ages your shingles from the inside out. In a New Hampshire winter, warm attic air melts the snow on your roof, the meltwater runs to the cold eave and refreezes, and you’ve got an ice dam pushing water back up under the shingles.

A balanced system needs two things working together: intake low at the soffits and exhaust high at the ridge. Get the ratio wrong — or block the soffits with insulation, which we see constantly — and the whole thing stalls out. Most shingle manufacturers, Owens Corning included, will actually reduce or void your warranty if the ventilation doesn’t meet their spec. Skipping it to save a couple hundred dollars on install day is one of the costliest “savings” in roofing.

4. Reusing old flashing

Shingles get all the attention, but flashing is what actually keeps water out at the tricky spots — around the chimney, where a roof plane meets a wall, down the valleys, around pipe boots and skylights. Those transitions are where roofs fail, not the wide-open field of shingles in the middle.

Here’s the corner that gets cut: instead of bending and installing fresh step flashing and counterflashing, a crew reuses the old metal because it’s already there and it’s faster. Problem is, that flashing has been through years of New Hampshire weather. It’s corroded, the sealant’s shot, and once new shingles are woven around it, nobody’s touching it again for 25 years. New step, valley, and counterflashing should be a line item on your quote. If you don’t see it spelled out, ask — vague language around flashing is one of the most reliable red flags we know of.

5. Skimping on underlayment and ice & water shield

Under every good roof is a layer most people never see. Synthetic underlayment covers the whole deck as a secondary water barrier. Ice & water shield is the self-sealing membrane that goes at the eaves, in the valleys, and around penetrations — the spots most likely to get hammered by ice dams and wind-driven rain. Around here, code requires that ice & water shield run from the eave edge to at least two feet inside the heated wall line, and it’s required for a reason.

The shortcut is using cheap felt instead of synthetic, or running the ice & water shield only along the very edge instead of where it actually needs to be. You’ll never spot it from the ground. You’ll spot it the first January when a brown ring shows up on the bedroom ceiling. This is the kind of detail that separates a roof built to handle a real New England winter from one built to look good in a driveway photo.

6. Hiring an uninsured or unverified crew

This is the mistake that turns a bad roof into a genuinely expensive problem. Roofing is dangerous work, and if a crew member gets hurt on your house and there’s no workers’ comp behind them, that liability can land on you. Same with property damage — if there’s no general liability policy, you’re the one paying.

It takes about twenty minutes to protect yourself. Ask for a certificate of insurance and confirm it’s current. Check that the company is established — a real address, a phone number that gets answered, reviews that go back more than a few months. And look for manufacturer certification: being an Owens Corning Preferred Contractor, for example, means the manufacturer has vetted the company and that better warranty coverage is even on the table. A truck with a magnet on the door and a too-good-to-be-true price is not the same thing as a contractor who’ll be around to honor the work.

7. Skipping a real, in-person inspection

A lot of outfits will give you a “quote” without ever setting foot on your property. They pull a satellite measurement, maybe fly a drone, and email a number. It’s fast, and it’s almost always incomplete. You can’t feel for spongy decking from a drone. You can’t see whether the soffits are blocked or the bath fan is venting straight into the attic from a photo. You can’t judge flashing condition from the street.

We don’t do drone-only or drive-by estimates, and it’s not for show — it’s because the things that decide whether your roof lasts are the things you have to get up there and put your hands on. When we quote a roof in Portsmouth or Wolfeboro or up in Meredith, somebody’s on the roof and in the attic first. That’s how you avoid the change orders and “well, it turns out…” conversations halfway through the job. The inspection isn’t a sales step. It’s the part that makes the quote actually mean something.