Everybody wants to inspect a roof by standing in the driveway and squinting. It is the least informative angle available. From out there you are looking at the weather face of the covering from fifty feet away and at a bad angle, and the failures that actually matter, the ones happening in the deck and at the flashing, are all on the other side.
The attic is where the diagnosis lives. It is unpleasant, it is hot, and it is where a roof tells the truth. Go up on a bright day, and then go up again during a hard rain, and you will have most of what a paid inspection would tell you.
What to bring
A bright handheld light, not a headlamp alone. A headlamp throws light along your line of sight, which is exactly the geometry that hides surface texture; a light held off to the side rakes across the sheathing and makes stains and swelling pop. Bring a dry white rag, a moisture meter if you own one, a phone for photographs, and a screwdriver or awl. Wear a respirator if there is loose fiberglass or blown cellulose, and step only on the joists.
Pass one: the daylight check
Go up at midday and kill your light. Let your eyes adjust for a full minute, then look at the underside of the deck.
Pinpricks of daylight along a nail line are usually just fasteners that missed a rafter, which is cosmetically ugly but not urgent. Daylight in a coherent shape, a slot or a crescent or a patch, is a hole. Daylight around a penetration, at a vent boot or a stack, means the flashing at that penetration has moved. Note the location by rafter bay and by distance from the ridge, because from the top side everything looks the same and you will never find it again without a reference.
Pass two: the deck itself
Light on, and work slowly across the sheathing with the beam raking sideways. You are reading four things.
- Stains. Dark rings or streaks on the underside of the deck. A crisp-edged ring with a lighter center is an old leak that dried. A soft-edged dark area that is still darkening at the margin is active.
- Delamination. Plywood that has separated into its plies, or OSB that has swelled and gone fuzzy at the edges. Press with the screwdriver handle. Sound sheathing is firm; failed sheathing gives.
- Fastener rust. Nail points coming through the deck should be bright or lightly oxidized. Heavily rusted points, especially rusted in a cluster, mean that area has been repeatedly wet, and the usual cause is condensation, not a leak.
- Frost or droplets. In cold weather, frost on the nail points or on the underside of the deck is condensation, full stop. That is a ventilation and air-sealing problem, and no amount of roofing work will fix it.
That last distinction is the single most useful thing an attic inspection produces. A leak is local, follows a path, and stains in a line from a point of entry. Condensation is diffuse, appears across whole bays, is worst on the north slope and worst at the coldest sheathing, and gets better when you open the ventilation. People spend real money re-roofing houses that had a condensation problem, and then the stains come back.
Pass three: during rain
Go back up in a steady rain with the light. Water entering the assembly will travel along a rafter or along the top edge of the ceiling drywall before it drops, sometimes several feet from where it came in. Follow it uphill. The point of entry is always higher than the point of appearance, and it is almost always at a transition: a valley, a chimney, a dormer sidewall, a vent, or the place where an addition ties into the original roof.
Mark the spot. A carpenter pencil on the rafter, a photograph with something in frame for scale, and a note of the distance from the gable end. That mark is what converts a vague complaint into a specific repair.
Where the homeowner's part ends
Everything above is inside work on a floor you can stand on. The next step is not. Confirming what the attic told you means getting onto the covering, lifting shingles to look at the underlayment, and inspecting flashing laps, and that combination of height, pitch, and the possibility of stepping onto soft sheathing is where do-it-yourself stops being sensible. If the attic shows a real entry point or a soft patch of deck, get a roofing contractor up there with your marked location in hand. You have just saved them the diagnostic hour and saved yourself the exploratory one they would otherwise bill.
What to fix yourself
Two findings are squarely homeowner work and both are common. First, a bath fan or dryer duct discharging into the attic instead of through a roof or gable termination. That is a moisture source sitting directly under your sheathing, and rerouting it is an afternoon with a hole saw and a proper cap. Second, insulation shoved into the eaves and blocking the soffit intake. Pull it back, install baffles in each rafter bay, and let the intake air get to the ridge.
Fix those two, then re-inspect in the next cold snap. A surprising share of what looks like a failing roof is a ventilation problem wearing a roof problem's clothing.