Water Behind the Siding: Why the Inside Wall Is Wet When the Outside Looks Fine

One of the most confusing water damage calls we get in Greater Boston is the one where the drywall is soft, there is no plumbing in the wall, the roof is dry, and the siding looks perfect. The water is coming through the wall itself. Here is how it happens and what the fix actually involves.

Siding is a rain screen, not a seal

Vinyl and clapboard siding are designed to shed most of the water, not all of it. Wind-driven rain, which Boston gets plenty of from nor'easters, pushes water up and behind the laps. That is expected. What is supposed to stop it is the layer underneath: housewrap or felt, properly lapped, with flashing at every window, door, and roof-to-wall joint. When that layer is missing, torn, or installed with the laps backwards, water that gets behind the siding goes straight into the sheathing.

Where it almost always starts

• Window and door corners. No sill pan, no flashing tape, or tape applied over the wrap instead of under it. The bottom corners of windows are the single most common entry point we find.
• Where a lower roof meets a wall. Kick-out flashing at the bottom of that joint is missing on most houses in Quincy and Dorchester built or re-sided before 2010. Without it, the roof dumps water straight down the wall behind the siding.
• Deck ledgers. A deck bolted to the house without ledger flashing channels water into the rim joist. We see rot here on houses that are only fifteen years old.
• Old caulk. Caulk at trim joints lasts five to ten years. After that it is a crack that holds water against the wood.

What it looks like from inside

A soft spot low on an exterior wall, bubbling paint under a window, a baseboard that has pulled away, or a musty smell in a room with no plumbing. On a moisture meter the sheathing reads wet in a stripe that follows the leak path down from the entry point. By the time it shows on the inside, the sheathing has usually been wet for more than a season.

The repair

There is no fix from the inside. The siding comes off in the affected area, the wrap and flashing get redone correctly, rotted sheathing and framing get replaced, and the siding goes back. On the inside, the wet insulation and drywall come out, the cavity dries, and it is rebuilt. We do both sides with one crew, which matters because the two halves of the repair have to meet in the middle.

Is it covered by insurance?

Usually not, and it is worth knowing why before you file. Most Massachusetts homeowner policies exclude damage from repeated seepage, faulty workmanship, and wear. A siding leak is almost always one of those. A storm that physically damages the siding and lets water in during that storm is a different story and often is covered. If you are not sure which one you have, we will tell you before you call your carrier.