Moisture Behind Siding: Causes, Warning Signs and How It Gets Fixed

By Bay Area Siding Company — Updated October 13, 2026

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Water gets behind siding mainly at openings and intersections: window and door edges, roof lines that run into walls, deck ledgers, and the bottom edge of the wall where siding sits too close to soil, paving or a roof. Every type of siding lets some water past, and a well-built wall is designed for that. Behind the siding, a weather-resistive barrier and flashing catch the water and send it back out. Moisture problems start when that barrier is missing, torn, lapped the wrong way or not tied into the flashing, so water is trapped against the sheathing instead of draining.

Where water gets in and what fixes it

Entry point	What you might see	Typical fix
Window and door openings	Stains or peeling paint below the lower corners, soft trim, interior drywall stains under the sill	Remove siding around the opening, install sill pan and side and head flashing lapped into the weather barrier
Roof-wall intersections	Rot and staining in a vertical streak below where a lower roof edge meets a wall	Install a kick-out flashing and repair the wall below it
Deck ledgers	Rot at the rim joist, staining on the ceiling or wall below the deck	Remove decking at the wall, flash the ledger, repair the rim joist and sheathing
Bottom of the wall	Swollen or crumbling bottom courses, paint failure along the base	Restore clearance to soil, paving and roofs; replace damaged boards and sheathing
Missing or failed weather barrier	Widespread soft spots, staining in many places, recurring leaks after small repairs	Remove siding on the affected walls and install a new barrier and flashings
Trim, penetrations and joints	Gaps at butt joints, open caulk at pipes, vents and fixtures	Flash above trim and at penetrations; caulk is a backup, not the primary seal

How a wall is supposed to handle water

Siding is the first layer of protection, and it sheds most rain. It is not watertight. Wind pushes water through laps and joints, and water runs behind trim and around windows. That is why the wall has a second line of defense, often called the drainage plane.

Weather-resistive barrier (WRB). House wrap or building paper over the sheathing. The California Residential Code requires at least one layer over the studs or sheathing of exterior walls, continuous to the top of the walls and behind deck ledgers.
Flashing. Metal or membrane pieces at windows, doors, roof-wall intersections, ledgers and horizontal trim that direct water outward over the WRB.
Shingle-lapping. Every upper layer overlaps the one below it, the way roof shingles do, so water running down the wall always stays on the outside face of the next layer down.
A way out. Water that reaches the WRB has to drain at the bottom of the wall and be able to dry. Siding that sits in soil or on a roof surface blocks that exit.

When any one of these is reversed, for example window flashing tucked behind the house wrap instead of over it, water gets directed into the wall instead of out of it.

House wrap vs building paper

Both are accepted weather barriers. They work in slightly different ways.

House wrap (Tyvek and similar)

DuPont describes Tyvek HomeWrap as a sheet made from very fine high-density polyethylene fibers fused together. The structure holds out liquid water and air while letting water vapor pass through, so a wall can dry outward. DuPont's installation guidance calls for working from the bottom of the wall up so each layer laps over the one below, with at least a 6-inch overlap, and for taping vertical seams with its own tape. DuPont rates HomeWrap for up to 120 days of UV exposure, so it should be covered by siding within about four months.

Building paper and felt

Asphalt-saturated kraft paper (often called Grade D paper) and asphalt felt are the older, traditional weather barriers. Many older Bay Area homes have one of these, and paper remains standard behind stucco, where building codes call for two layers (or an equivalent) over wood-based sheathing. Paper is installed in horizontal shingle-lapped courses. Old paper can be brittle, torn or missing in places, which is something to check during any re-side.

Which is better?

Either works when installed correctly. A house wrap goes on in fewer, larger pieces with fewer laps to get wrong. Paper is forgiving of small holes and is the usual choice behind stucco. What matters more is that the barrier is continuous, lapped the right way and integrated with every flashing. Integrated sheathing like ZIP System, which puts the barrier on the panel face, is covered in our sheathing guide.

Flashing details that prevent most leaks

Windows and doors

Building code calls for flashing at the head and sides of exterior window and door openings, and a pan flashing at the sill where the window maker's instructions do not provide another method. The common failure on older homes is no sill pan at all, so water that gets past the window frame drains straight into the wall below. If you are also replacing windows, see retrofit vs new construction windows, because only new-construction installs let the flashing be rebuilt properly.

Kick-out flashing

Where the lower edge of a sloped roof runs into a wall, rainwater sliding down the roof edge pours directly onto the siding. Residential code requires a flashing that diverts that water away from the wall. Guidance from the U.S. Department of Energy's Building America program describes the kick-out as the first piece of step flashing, sitting flat on the roof and flush to the wall. The building trade press has long described kick-outs as required but often overlooked. When one is missing, the water soaks the wall below the roof edge, and the damage can run down to the foundation before it shows.

Deck ledgers

A deck attached to the house is bolted through the siding area into the rim joist. The California Residential Code requires the weather barrier to continue behind deck ledgers, and its ledger flashing provision calls for flashing to extend at least 2 inches up the wall above the ledger. A ledger without flashing can rot the rim joist that holds the deck up, so this is a safety issue as much as a moisture issue.

Clearances at the bottom of the wall

The bottom edge of siding is where water collects and where cut edges absorb it. James Hardie's installation instructions call for a gap of 1 to 2 inches between its siding and roofing, decks, paths, steps and driveways, and clearance to finished grade according to local code. Separately, the model residential code requires wood siding, sheathing and framing that sits closer than 6 inches to the ground, or closer than 2 inches to exposed concrete slabs and steps, to be decay-resistant or preservative-treated. Raised planter beds, new patios and mulch piled against the wall often erase clearances that were correct when the house was built.

Bay Area conditions that make it worse

Coastal fog and wind-driven rain. In Pacifica, Half Moon Bay, Sausalito and other fog-belt areas, walls stay damp longer and dry slowly, so small leaks do more damage. West-facing walls take the most wind-driven rain.
Older housing stock. Many homes built from the 1920s through the 1980s have original paper, no sill pans and no kick-outs. T1-11 and hardboard siding from those decades absorb water at their bottom edges.
Irrigation. Sprinklers that hit the wall every day in summer can soak the base of a wall more than winter rain does.

Warning signs of moisture behind siding

Paint that blisters or peels in one area while the rest of the wall is fine
Dark stains or streaks running down from window corners, trim or roof lines
Siding that feels soft, spongy or loose when pressed
Swollen, delaminating or crumbling bottom edges on wood, T1-11 or hardboard siding
Gaps opening at butt joints or boards that have bowed outward
A musty smell in rooms on exterior walls, or stains and mold on interior drywall near windows
Insects such as carpenter ants or termites around the base of the wall, which are drawn to damp wood

Condensation on the inside of window glass is a different problem, usually caused by indoor humidity. Our window condensation guide covers that.

What a proper repair involves

Caulking over a stain rarely fixes moisture behind siding. A lasting repair goes to the source:

Find the entry point. Trace stains upward. The water usually enters above and to the side of where damage shows.
Open the wall past the damage. Remove siding until sound, dry sheathing is exposed on every side of the damaged area.
Repair the structure. Replace soft sheathing and any rotted framing, rim joist or sill plate. See our guide to dry rot in siding and trim.
Fix the cause. Add the missing kick-out, sill pan or ledger flashing, restore clearances, or redirect a downspout or sprinkler.
Rebuild the drainage plane. New weather barrier, lapped shingle-fashion under the existing barrier above and over it below, with flashing tapes integrated.
Re-side and finish. Install siding with correct clearances, seal cut ends where the manufacturer requires it, and paint.

When damage turns up on several walls, or the original barrier is failing everywhere, full replacement is usually the better investment than repeated patches. Our siding replacement page explains how a full re-side rebuilds the weather barrier and flashing on every wall. On coastal jobs like our Pacifica Hardie siding and window project, replacing windows and siding together allows the flashing at every opening to be done in one sequence.

Frequently asked questions

How does moisture get behind siding?

Mostly at windows, doors, roof-wall intersections, deck ledgers and the bottom edge of the wall. Wind-driven rain also gets through laps and joints in any siding. The weather barrier and flashing behind the siding are supposed to drain that water back out, and problems start when they are missing or installed in the wrong order.

Is Tyvek better than building paper?

Both are accepted weather barriers and both work when installed correctly. House wrap installs in larger sheets with fewer laps, while building paper is traditional and is standard behind stucco, where two layers are typically required. Continuity and correct lapping matter more than the brand.

Can you fix moisture behind siding without replacing it?

Often, yes, when the problem is confined to one area such as below a window or at a single roof line. The siding is removed around the damage, the sheathing and flashing are repaired, and the siding is reinstalled or replaced in that area. If damage shows up on many walls, a full re-side is usually more cost-effective.

What is kick-out flashing and do I need it?

It is a small angled flashing at the bottom of a roof-wall intersection that diverts water off the wall and into the gutter. Residential code requires one where a sloped roof eave meets a sidewall. If you see a streak of stains or rot below where a lower roof meets your wall, a missing kick-out is a likely cause.

Does moisture behind siding cause mold?

It can. Trapped moisture in sheathing and insulation can support mold growth and wood decay inside the wall. A musty smell or stains on interior walls near windows are signs to have the wall opened and checked.

Sources

The information provided here is intended for general reference and should not be considered professional advice. Before starting any project, consult a licensed contractor. Product features, specifications, and warranties may have been updated since this was published. Any mention of brands or products reflects personal opinion and does not constitute an endorsement or warranty.