On a bright afternoon, go into the attic with the lights off. If you see thin lines or points of daylight coming through the roof deck, you have found a place where water can get in. The check costs nothing and tells you where to look before anyone climbs a ladder. This page explains how to run the check safely, what the common light patterns usually mean, and what to ask a roofer to write down before they touch the shingles.
Why the attic is the first place to look
Water that gets through a roof does not usually fall straight down. It follows the underside of the deck, runs along a rafter or a nail, and drips off at a low point, often several feet from the entry. That is why a stain on your ceiling can sit far from the hole. The attic side of the roof shows the entry point directly, before the water has had time to wander.
Daylight is the most reliable signal you can get without tools. Sunlight travels in a straight line, so any gap large enough to pass light can also pass rain when the wind pushes it sideways. A line of light along a seam means the gap runs in that direction. A single bright point usually means a nail hole, a pipe penetration, or a gap at a flashing edge.
How to run the daylight check
Pick a clear day, ideally late morning or early afternoon, when the sun is high enough to light the underside of the deck from outside. Turn off the attic light and let your eyes adjust for two minutes. Use a flashlight only to move around, and keep it pointed down so it does not wash out the small points you are looking for.
Work the attic in a grid. Start near the ridge and move down each rafter bay toward the eave. Look at the underside of the sheathing between rafters, around every pipe and vent, along the valleys, and at every spot where two slopes meet. Mark each light point with a piece of painter's tape on the rafter, or take a photo that includes a tape measure. Do not poke at the deck with a tool to see whether it gives, because that can knock a loose nail further out and make the problem worse.
Stay on the joists or rafters, never on the drywall or the ceiling below. In an unfinished attic, keep your weight on the framing and do not step between joists where insulation hides the gaps. A ceiling that gives way can take you with it. If the attic is low or hard to reach, bring a helper who can watch your footing.
What the common light patterns mean
- A thin line along a sheathing seam often points to a gap where two panels of deck meet, usually near a valley or a slope change. Light running down the slope is a different signal from light running across it.
- A bright point beside a nail shank usually means a nail has backed out or a shingle has lifted over it. Check the shingle courses directly above that point from outside before deciding the problem is in the deck.
- Light around a pipe, vent or chimney points to the flashing or boot at that penetration. Look at the collar from the attic side first, because the collar often ages before the shingles around it do.
- Light at the top of a rafter bay near the ridge can mean the ridge vent or ridge cap is letting wind-driven rain past the cap. The ridge is often the source, not the field shingles below it.
- No light at all, only a dark wet spot on the deck, may be condensation or an older leak that has already dried. Note its location, but do not assume it is the current entry point until you see it again after the next rain.
A single point of light is a lead, not a diagnosis. Two or three points lined up along a path usually show the route the water takes, and the highest point on that path is often the real entry.
What to photograph before you call anyone
Take each photo with a reference object in frame, such as a tape measure or a level phone held against the rafter. Shoot from the attic side toward the light, then shoot the same spot a few feet back so the rafters and nearby penetrations are visible. Write the rafter bay number on painter's tape and include it in the frame. Record the date and time, because the sun angle changes how the light looks from one visit to the next.
Keep these photos as originals on your phone or in your cloud account. Do not rely on screenshots, since screenshots lose the original file date. If you later share them with a roofer or your insurer, send the originals.
What to ask a roofer to write down
When a roofer comes out, ask them to check the same points you marked. A written scope should list each light point or suspected entry by location, with the photo that shows it. It should name the repair at each point, such as replacing a boot, resealing a valley, or re-nailing a lifted course, and it should say whether the deck needs replacement or only a patch.
Ask who will do the work and who will come back if the spot leaks again. Ask what the proposal does not cover, such as interior drywall, insulation, or framing that has gotten wet. If a proposal says the repair is "roof area only," it should name that area so you know what is left open.
Keys Roofing documents the damage it finds, puts a written scope with photos and measurements in the homeowner's hands, and attends the visit as the contractor. Keys does not represent or negotiate an insurance claim. Missouri law under RSMo 407.725 bars a roofing contractor from doing that, and Keys does not discuss a deductible. If you have an insurance question, take your photos and written scope to your own insurer and handle that conversation yourself.
When to call, and what to say
If you find light along a seam or around a penetration, call a roofer before the next rain. Tell them the exact location, the number of light points, the date you checked, and whether the stain below changed after the last storm. Those details let them plan the visit in one trip. Keys Roofing can be reached at (314) 220-2333.
Daylight is not the whole story. A shingle can keep water out at the surface while the deck below is already soft, so the attic check is a starting point rather than a final answer. It still costs nothing and leaves a record. Walk it on a clear day, mark what you see, and bring those marks to whoever climbs the roof.