Fixing RV Roof Ponding Water: Causes and Cures

After a Florida rain, a little water on an RV roof for a few hours is normal. A puddle still there the next day, and the day after, is ponding — and ponding is a leak looking for a way in. In roofing, ponding means water that remains 48 hours or longer. On an RV I use the same test: if it has not drained or dried in two days, do not ignore it.

I see this every month: an owner chasing a ceiling drip with sealant while a shallow lake sits around the air conditioner. Sealant is not made to live underwater. Fix the pond first.

Why ponding does real damage

Standing water is heavy and patient. A gallon weighs about 8.3 pounds, so a wide, shallow puddle adds real load right where the roof is already low — which makes the low spot lower. That is the cycle: sag holds water, water adds weight, weight deepens the sag.

Dirt and algae collect at the pond line, sealant breaks down faster, and any pinhole or seam under that water gets hours to leak instead of minutes. If the decking is soft, ponding is both symptom and cause.

Cause 1: The air conditioner pocket

This is the number one cause I find. A rooftop AC is heavy, it vibrates down the road, and on a lot of rigs it sits on light framing — rafters commonly spaced 16 or 24 inches apart — with thin decking between them. Over years, the area around that standard 14-inch opening dips. Water runs toward the AC instead of away, pools against the gasket, and eventually works inside.

Look for a stain ring around the AC and a gasket crushed flat on one side. No stain inside does not mean no problem on top.

Cause 2: Soft decking between rafters

If the pond sits away from any unit, walk it carefully. Decking that flexes or feels spongy is not just low — it is failing. A past leak rots it, the roof loses its crown, and it holds water even in dry storage. Coating over it just traps the problem.

Cause 3: Level, gutters, and dams we build ourselves

Sometimes the roof is fine and the setup is not. A rig stored nose-low or leaning will pond on a roof that drains perfectly when level. Blocked gutter rails, leaf dams at the front cap, and debris behind a vent do the same.

So does our own work. I have peeled up ridges of old self-leveling sealant that acted like a curb, trapping water behind a vent. Layers of Dicor 501LSW-1 Self-Leveling Lap Sealant built up year after year can create the pond you are trying to cure. Dicor is the right sealant for horizontal roof seams — thin, on a clean, drained surface. Piled up in standing water, it will still fail.

How I diagnose it

• Mark the puddle outline right after rain, then check it at 24 and 48 hours. Photograph it — memory lies about size.
• Check the rig's level first. Re-level and re-check before you blame the structure.
• Clean the gutters and the whole drainage path to the edge. Retest with a hose, low to high.
• Feel around the pond for soft decking, and check the ceiling and upper cabinets inside for moisture.
• Note what sits in the pond: AC, vent, cap seam, or just low decking. That tells you which cure fits.

The cures, in order

Start simple. Level the rig for storage with a slight, consistent pitch, clear the gutters, and carefully remove any sealant dam without tearing the membrane. Reseal the exposed seam and hose-test it.

For a cap seam at the edge of a drained area, EternaBond RoofSeal tape can be a durable fix — on a clean, dry, sound surface, rolled down hard. It is not a slope fix. Tape across a pond still sits in water.

For the AC pocket, the fix is support, not sealant. Power off, remove the AC, and plan on a new gasket. A proven reinforcement is 3/4-inch plywood fitted inside the roof cavity through the opening, spanning the rafters front and back, so the load spreads onto structure. Reinstall, torque evenly, and confirm water runs away.

For soft decking, there is no surface cure. Cut back to solid wood, replace the section with decking matching the original thickness, land seams on framing, and rebuild the crown. Behind a wide AC or vent, a small tapered cricket — a shallow wedge that splits water around the unit — can restore drainage. Then reseal and hose-test it.

What not to do

• Do not fill a low spot with self-leveling sealant. It levels, ponds, then cracks — with more to remove later.
• Do not coat a roof to fix a slope problem. Coating over rot hides the failure until it is bigger.
• Do not caulk around the base of the AC. The seal is the gasket underneath; caulk on top traps water and tears the membrane at the next removal.
• Do not add more weight to a sagging bay. It makes the dip worse.

A roof that drains lasts. Find why it is low, fix the structure or drainage — then seal it once.

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