Water that comes through a roof rarely drops straight down from the hole you can see. On a shingle roof, the water that gets past the top layer runs down the underlayment, and it follows the joints between sheets until it finds a gap or a nail hole. Where it finally drips into the attic or onto the ceiling may be several feet from where it entered. That is why a repair that fixes only the spot under the stain often holds for one storm and then fails again.
How a lap is supposed to shed water
Roofing underlayment is laid in horizontal courses, and each course overlaps the one below it. The upper course covers the lower course with a lap, so water running down the roof lands on top of the sheet and keeps going down over the edge. Side laps work the same way: the sheet on one side overlaps the sheet next to it, so water crossing a seam keeps moving downhill instead of sliding under the edge. A properly lapped deck is a series of layers stacked like scales, and no single nail is supposed to be the only thing between a storm and the attic.
The trouble starts when a lap is short, when it runs the wrong direction, or when a crew has cut it to save a sheet. A lap that faces uphill makes a pocket. Water that reaches the joint sits behind the edge, and wind pushes more of it into the gap. Over a few seasons that gap widens, and the water that enters it does not show itself at the shingle surface.
Where laps fail on a St. Louis roof
Freeze and thaw cycles are the main reason. In a St. Louis winter the same roof can freeze overnight and thaw by afternoon several times in a month. Each thaw sends meltwater down the slope, and if the lap is short, meltwater pushes under the edge and refreezes there. Ice at the eave also holds water back, so the lower courses stay wet longer than the upper ones. Over time the wet lap softens, and the nail holes that hold the shingle above it begin to tear.
Spring storms add wind-driven rain. Rain pushed sideways under the upper courses reaches the side laps first, which is why leaks often appear near a rake, a valley, or the point where two slopes meet. On a lower-pitched section the water moves more slowly and spreads wider, so the stain on the ceiling can sit well away from the joint that let the water in.
Tracing the path from the ceiling
Start with the stain, then work back toward the roof along the direction water would travel. Note where the water shows on the ceiling, then go into the attic with a flashlight and look at the underside of the deck above that spot. Follow the wet wood uphill. Dark or stained sheathing usually marks the path, and the wettest point is often a seam or a nail line rather than the middle of a sheet. Mark that location on the outside of the roof before anyone climbs up, so the crew can find it without guessing.
Timing helps too. A stain that shows only during wind-driven rain points toward a side lap or a rake, while a stain that appears after a hard freeze and a thaw points toward the eave or the lower courses. Write down the date and the weather for each appearance. The pattern often narrows the search before anyone opens the roof.
Do not assume the lowest point of the stain is the entry point. Water on a joist or a truss runs along the wood before it drips. If the attic shows a wet trail across several framing members, the path may be long. A crew that opens the roof at the drip and finds nothing may be looking in the wrong place.
Leak and repair: what the repair should include
A repair that addresses a lap problem should replace or re-lay the affected underlayment, not just cover the seam with sealant. The shingles above the damaged area need to be lifted far enough to re-lay the sheet properly, and the nails should be placed where the install sheet calls for them. Keys installs Atlas and Malarkey shingles, and the install sheet in each bundle sets the nailing and lap details the repair has to follow. If the underlayment cannot be lifted without tearing, the damage probably runs further than the stain suggests.
Ask the crew to show you the underlayment before the shingles go back on. A clear photo of the lap, with a tape measure or ruler in frame, shows whether the overlap is correct. Photos taken before the shingles are set are more useful than photos taken after, because they record what was there.
What it costs
The cost of a lap repair depends on how much of the roof has to come up. A short run of underlayment near one slope is a small job. A lap failure that runs along a valley or a long rake can require lifting many shingles, and the price moves with the area opened. Ask for the repair as a line item that names the square footage or the number of courses involved. A single lump sum that does not say what area is being opened makes it hard to compare two quotes or to know whether the job is finished.
Watch for a quote that treats underlayment as an afterthought. Replacing the sheet is a material and labor line, not a free addition to the shingle work. If the estimate covers only the shingles above the leak, ask where the underlayment is addressed.
What to ask
- Where does the crew expect the water to enter, and how did they decide?
- Which courses of underlayment will be lifted, and how many shingles come off to reach them?
- Will the damaged sheet be replaced in full, or only patched?
- Will you receive photos of the laps before the shingles are set back?
- What does the written scope say about the underlayment and the nail pattern?
Keys documents what it finds and puts a written scope with photos and measurements in your hands. If you have a leak now, call the office at (314) 220-2333. Missouri law (RSMo 407.725) bars a roofing contractor from representing or negotiating your insurance claim, so any claim stays with you while Keys attends as the contractor on the roof.