There is a question a good roofer asks in the first thirty seconds of a phone call, and most homeowners have never thought about it: which rain makes it leak?
Not how bad the leak is, not how long it has been happening. Which rain. The answer eliminates whole categories of cause before anybody sets foot on a ladder, and if you have been through a repair that did not work, there is a reasonable chance nobody ever asked you.
The question that narrows it fastest
A roof is a water-shedding system, not a waterproof box. It is designed around the assumption that water arrives from above, runs down a slope, and leaves at the edge. Every component is lapped so that the piece above sheds onto the piece below, and that sequence works indefinitely as long as the water behaves the way the design expects.
Leaks happen when water stops behaving that way. There are only a few ways it can misbehave: it can arrive faster than the drainage can carry it, it can arrive sideways, it can be held in place instead of running off, or it can climb into a gap against gravity. Each of those failure modes produces a different trigger pattern — and the pattern is visible to you, the person living in the house, long before it is visible to anyone on the roof.
Four rain patterns and what each one means
It leaks in every rain, including light drizzle
This is the simplest case and usually the easiest to find. Something is genuinely open. A split pipe boot, a missing shingle, a hole from a fallen limb, a nail that has backed out and pushed through. Water does not need volume or wind to get through an opening, so even a gentle rain finds it. If you can get into the attic during rain, you can often watch it happen, which makes the diagnosis straightforward.
It leaks only in heavy rain, with or without wind
Nothing is open. The issue is capacity. A valley, gutter or drainage detail that handles a moderate rain perfectly well gets overwhelmed above a certain rainfall rate, water backs up above the edge of the shingle courses, and it enters sideways. Debris packed into a valley does this. Undersized valley metal does it. An overflowing gutter does it at the eave. The roof is intact and it leaks anyway.
It leaks only when the wind is up
This is wind-driven rain, and it is the most misdiagnosed pattern of the four. Water pushed horizontally at thirty or forty miles an hour goes places falling water never reaches: under shingle courses, behind siding at a wall junction, past a counter-flashing that has lifted a couple of millimetres, straight through a gable vent. There is no defect to find. Everything looks correct in dry weather because, for falling water, it is correct. The repair almost always lands in flashing and shingle repair territory.
It drips when it has not rained for days
Then it is probably not the roof. A sustained drip in dry weather points at an air-handler condensate pan that is overflowing or has a blocked drain line, a sweating duct, or a plumbing supply or waste line in the ceiling. All three produce staining that looks identical to a roof leak from below. Before paying anybody to go on the roof, check what is directly above the stain.
Write the pattern down the moment you notice it, with the date and roughly what the weather was doing. Three entries is enough to be genuinely diagnostic, and almost nobody arrives at a phone call able to say "heavy rain yes, light rain no, wind does not seem to matter." If you can say that, you have already done a meaningful part of the diagnosis.
Why volume is a different problem from water
The heavy-rain-only pattern confuses people because it seems like the roof should either keep water out or not. The reason it does not work that way is that every drainage path has a maximum rate, and roofs are full of places where the flow concentrates.
Consider a valley. Two roof planes meet, and everything landing on both of them funnels into one channel. If each plane is a few hundred square feet, a hard downpour delivers an enormous amount of water into a channel that might be a couple of feet wide. That channel has a capacity. Below it, everything runs down the valley and off the eave as designed. Above it, water piles up, rises above the level of the shingle edges on either side, and goes under them. The same thing happens at a gutter that cannot take the flow, at a downspout that is restricted, and at any low-slope area where water is meant to move laterally.
Add anything that reduces capacity and the threshold drops. Leaf litter in a valley, a gutter half full of debris, a downspout with a blockage, or a dip in a low-slope roof that holds standing water. A roof that coped fine last year can start leaking this year with no component having failed at all — the capacity has simply fallen below what the weather delivers. This is the single most common reason a sound-looking roof leaks, and it is why a drainage and valley repair is so often the answer rather than anything to do with the shingles.
What makes this worse in San Antonio
Two regional factors push local roofs toward exactly this failure mode.
The first is how it rains here. South Central Texas sits in the region widely known as Flash Flood Alley, and the defining characteristic is intensity rather than total volume: rain tends to arrive in short, very heavy bursts rather than steadily over hours. A drainage detail is sized, implicitly, for a rainfall rate. A roof that would never leak in a climate of long gentle rain can be over its threshold several times a year here.
The second is heat. Long, high-ultraviolet summers make sealant beads shrink and harden, make the neoprene collars on vent pipes go brittle, and work fasteners loose through daily expansion and contraction. None of that opens a hole. What it does is turn details that were comfortably watertight into details that are marginal — fine for falling water, not fine for water under pressure. The combination of marginal details and high-intensity rain is what produces a city full of roofs that only leak in the big storms.
Where the roof sits changes the odds too. Under the heavy oak canopy through Huebner / Leon Creeks, valleys load with organic debris year-round and the capacity problem is close to permanent. On newer construction out toward Kyle, the details themselves are more often the issue — a short piece of flashing or an unsealed boot base that survives ordinary rain indefinitely and fails the first time it gets a real downpour.
Keep a three-storm log
This is the most useful thing you can do between now and the roofer's visit, and it takes two minutes per storm. For the next three rain events, write down four things.
- The date and roughly how hard it rained. "Steady for an hour" versus "fifteen violent minutes" is a meaningful distinction.
- Whether there was wind, and from which direction if you noticed. Which elevation of the house the weather was hitting matters.
- Whether it leaked, and how long after the rain started. Immediately, or forty minutes in once something had filled up.
- How long it kept going afterwards. Minutes, hours, or into the next day.
Also mark the edge of the stain with a pencil line and date it. If the line is still the boundary after three storms, the leak is dormant or extremely slow. If it has moved, you have a measurement rather than an impression, and that is worth more to a roofer than any description.
Why a hose test can miss it entirely
When a roofer runs water on your roof to reproduce a leak, they are simulating rain. The problem is that a hose simulates one kind of rain quite badly.
A hose pointed down a slope delivers moderate volume from above. That reproduces the first pattern — an open defect — very well. It reproduces a capacity failure only if enough water is put into the right channel for long enough, which takes deliberate effort. And it reproduces wind-driven entry essentially never, because the water is not travelling sideways and there is no pressure behind it.
So a test that "passed" is not proof the roof is sound. It may only be proof that the test did not match the conditions that cause your leak. A proper leak detection visit accounts for this: sections isolated low to high so the location can be pinned down, sustained flow into valleys where capacity is suspected, and water directed horizontally at an elevation when the pattern says wind. If somebody tested your roof, found nothing, and the leak came back in the next storm, this is very often why.
Do not go up to look during or just after a storm. Wet shingles and wet flashing metal are extremely slick, and a roof that has been leaking may have soft decking that will not hold your weight. Every observation worth making can be made from the ground with binoculars, or from inside the attic.
What to tell the roofer
Lead with the pattern, not the symptom. "There is a stain on my ceiling" tells somebody almost nothing. "It only leaks when rain is heavy, it starts about half an hour in, and wind does not seem to make a difference" tells them to go and look at drainage capacity — valleys, gutters, low-slope transitions — rather than hunting for a hole.
Then add the three things that shorten the search: where the stain is and what is above it on the roof, the highest wet point you can see in the attic, and whether anything has been repaired up there before. If a previous repair was a smear of sealant over a junction, say so, because it changes the likely cause completely.
And ask one question back: how are you going to confirm it? A roofer who says they will run water in sections from low to high, or who asks you which direction the wind comes from when it leaks, is working from evidence. A quote produced from the driveway, with no attic visit and no test, is a price for a guess. If you want the fuller version of how this process should run, the main roof leak page for San Antonio covers it, and roof leak detection is the service that does this specific job.
Questions about intermittent roof leaks
If it only leaks sometimes, can I just ignore it?
An intermittent leak usually causes more cumulative damage than a constant one, because it runs for far longer before anybody takes it seriously. Each storm puts water into the same place, the decking and insulation absorb it, and the interval between storms is never long enough in a humid climate for everything to dry out properly. By the time an intermittent leak produces an obvious stain, the material around the entry point has often been wet on and off for a year or more.
Could it be condensation rather than a leak?
Check whether it tracks rain or tracks humidity. Condensation forms when warm, moist air meets a cold surface such as duct metal or the underside of the decking, so it appears in muggy weather whether or not it is raining, and it is spread fairly evenly across a broad area. A leak concentrates along one path and stops some hours after the rain does. If your drip happens on a humid morning with clear skies, the roof is probably not the culprit.
Why does the leak sometimes keep going after the rain stops?
Because the attic is still draining. Insulation, decking and framing hold a surprising volume of water, and it keeps migrating downhill and through the ceiling for hours after the roof surface is dry. A leak that continues for a day afterwards is normal. One that continues for several days with no rain at all is pointing at plumbing or an air-handler condensate problem instead.
Is a leak that only happens in wind more serious than one in ordinary rain?
Not more serious, but usually harder to find and more often misdiagnosed. Wind-driven entry means a detail is marginal rather than a hole being open, so the repair itself is frequently small. The difficulty is that nothing looks wrong in dry weather and a standard downward water test will not reproduce it, which is why these leaks commonly survive two or three repair attempts.
My gutters overflow in heavy rain. Is that related?
It can be the whole cause. When a gutter cannot take the flow, water backs up above the drip edge and gets under the bottom courses of shingles, and it also sheets down the fascia into the soffit. That only happens at high rainfall rates, which produces exactly the heavy-rain-only pattern. It is also one of the cheaper things on this list to fix, so it is worth checking before assuming the roof covering has failed.
Keep reading
If this described your leak, these are the next useful things to read or do.