Most Lewisville houses are steep-sloped and shingled, and then somewhere on the property there is a piece of roof that is not: a patio cover, a porch, a back addition that was added later, a garage tie-in that nobody thought about very hard. Those are the sections that hold water.
Two days after a storm the steep slopes are bone dry and the flat piece still has a puddle sitting in the middle of it, reflecting the sky. Homeowners tend to read that as normal because it has always done it. In roofing terms it has a name, a definition, and a list of consequences.
Ponding Has a Definition, and It Is Two Days
The Asphalt Roofing Manufacturers Association defines ponding water as water that remains on a roof for 48 hours or longer. That number is the whole distinction. Water sitting up there during a downpour is drainage doing its job. Water still sitting there on Wednesday after a Monday storm is a defect in the drainage, and it is going to keep happening until something changes.
The association lists the usual sources, and only one of them is rain. Ponding may result from rain, melting snow or ice, or runoff from rooftop equipment, and condensate is its own category: the guidance is to prevent condensate from draining onto the roof. On a North Texas house that last one usually means the condensate line off an air handler, which is a small pipe doing quiet damage in one spot for years.
What Standing Water Actually Does Up There
It starts with weight. ARMA's description is that the load on the roof increases, which may result in deck deflection. That is the problem feeding itself: water collects in a low spot, the weight pushes the deck down a little further, the low spot gets deeper, and more water collects next time. A puddle that has been there for five years is usually sitting in a dish the puddle made.
From there it spreads. Biological growth in standing water can damage the membrane, clogged drains cause further ponding, and the risk of water infiltration into the building and subsequent interior damage is amplified. ARMA's summary is blunt and worth quoting as the reason to do anything at all: failing to address ponding water may shorten the effective life of the roof membrane system.
- Deck deflection, which deepens the low spot and makes the next pond bigger.
- Biological growth that attacks the membrane where the water sits longest.
- Blocked drains and scuppers, which turn a slow problem into a fast one.
- A shortened service life for the whole low-slope system, not only the ponded area.
A Quarter-Inch Per Foot Is the Number to Know
Flat roofs are not flat, and that is the point most homeowners have never been told. ARMA's recommended slope for a low-slope roof is usually a minimum of a quarter inch per foot, specified so that the roof drains freely throughout the life of the building. Across a twelve-foot patio cover that is three inches of fall from one end to the other, which is far more than it looks like from the ground and considerably more than most add-on structures were built with.
Where the slope is right, the drainage has to be in the right places too. The guidance is that roof drains and scuppers should be included at properly designed low points in the roof system. And where the deck itself has no slope to give, there is a standard remedy: a tapered insulation system may be used to create positive roof drainage. That is the honest answer to a dead-flat addition that has ponded since the day it was built. Not a patch, not a coating, but a build-up that gives the water somewhere to go.
If the same puddle appears in the same place after every storm, the fix is drainage, not sealant. Coating over a low spot keeps the water out for a season and leaves the dish exactly where it was.
Shingles Are the Wrong Covering Below 2:12
This is where low-slope sections go wrong on otherwise ordinary houses, usually because somebody ran the main roof's shingles across a porch to save money. ARMA's guidance on lower-sloped roofs is unambiguous: never apply shingles on roofs with slopes lower than two inches per foot. Below that, the covering is simply outside what the product is designed for, and no amount of careful installation changes it.
Between 2:12 and 4:12, shingles are permitted but only if special procedures are followed. That means either a double layer of asphalt-saturated felt or synthetic underlayment, or a single layer of self-adhering underlayment, and where the low slope meets a steeper area the underlayment must extend at least 24 inches onto the steeper slope roof. There is a catch worth knowing about the self-adhering option: applying it over the entire deck creates a vapor barrier, so the installer has to make sure the building code's ventilation requirements are met.
ARMA also explains why these slopes behave differently in the first place. Water drains more slowly, creating a greater possibility of water backup, and shingles on lower slopes are more susceptible to water entry from wind-driven rain. North Texas supplies plenty of wind-driven rain, which is why a shallow porch roof that looked fine for a decade can start leaking in one bad spring.
Where This Turns Up on Lewisville Properties
Four places, in rough order of how often. Patio and porch covers, which are often built shallow for headroom and drain toward the house rather than away from it. Back additions, where the new roof had to tuck under existing eaves and lost most of its pitch doing it. Garage and carport tie-ins, where two planes meet and the valley between them is nearly level. And genuine low-slope commercial roofs on the small buildings along the business corridors, which are designed for it but still need their drains kept clear.
The common factor is that none of these are the part of the roof anybody looks at. They are below eye level, behind the house, or over a space nobody spends much time in. A section that ponds for years will usually announce itself through the ceiling underneath it rather than from above, by which point the deck is involved.
Low-Slope Roof Questions Lewisville Homeowners Ask
How do I tell what slope my porch roof actually has?
Lay a two-foot level on the slope, lift the low end until the bubble centers, and measure the gap underneath. Half an inch of gap over two feet is a quarter inch per foot. Four inches of gap over two feet is a 2:12 slope. It is a five-minute job from a ladder and it settles whether shingles belong there at all.
Can I just coat the flat section and be done?
A coating can extend the life of a sound low-slope roof, but it does not add slope and it does not drain anything. If the section ponds because the deck has deflected or because there is nowhere for water to go, coating it treats the symptom and hides the deflection from the next person who looks. Fix the drainage first, then decide whether a coating is worth it.
Is ponding water covered by insurance or a warranty?
Usually neither, because it is a design and drainage issue rather than a sudden event. ARMA is explicit that effects vary by material and directs owners to contact the individual roofing material manufacturer, which is worth doing before assuming anything about coverage. A long-standing pond is more often read as maintenance than as damage.
How urgent is it if nothing is leaking yet?
Not an emergency, but not something to leave indefinitely either, because the deflection cycle only runs one direction. The sensible move is to get the section measured and photographed while it is still dry inside, so the choice is between drainage work and a coating rather than between replacing a deck and replacing a ceiling. A free roof inspection will tell you which stage you are at.