Water intrusion is rain getting past a building’s exterior and into the walls or roof. Building Science Corporation (BSC) calls driving rain the largest single source of moisture for most walls and roofs. Common installation errors include missing or reversed flashing, house wrap lapped the wrong way, and cladding with no drainage path behind it. The damage often stays hidden inside the wall.
Last reviewed: September 2026. Sources are listed at the end of this page.
Where does water in a wall come from?
Rain is the biggest single source. BSC says driving rain is the largest single source of moisture for most walls and roofs.[1]
The cladding alone will not keep it out. The Building America Solution Center (BASC), a U.S. Department of Energy (DOE) resource run with Pacific Northwest National Laboratory (PNNL), says most exterior wall cladding systems leak. Wind pressure and capillary action push rain through cracks, joints and small gaps. BASC treats caulk as a temporary barrier that needs to be reapplied from time to time.[2]
How is a wall supposed to handle rain?
A wall should shed most rain at the surface, drain what gets behind the cladding, and then dry out. Building scientists describe this as layered control.
- BSC’s three-part approach. BSC groups rain control into deflection, then drainage, storage or exclusion, and then drying.[1][3]
- The “4 Ds.” Hazleden and Morris (1999) list deflection, drainage, drying and durable materials. They describe this as defense in depth. Deflect rain first, drain water that gets in, dry the assembly, and finally rely on decay-resistant wood.[4]
The code builds in the drainage step. BASC quotes 2018 International Residential Code (IRC) section R703.1.1. It requires a water-resistive barrier behind the exterior cladding and a way to drain water that gets past the cladding back to the outside.[2]
The barrier only works if it is lapped correctly. BASC says drainage-plane materials go on from the bottom up, with each course overlapping the one below, like shingles. They must never be underlapped. Typical overlaps are 4 to 6 inches at vertical seams and 2 inches at horizontal seams.[2]
Where do leaks usually start?
Leaks usually start where the wall is interrupted: windows, doors, roof-wall intersections and the base of the wall. This table lists the errors the sources name.
| Location | What goes wrong | What correct work looks like |
|---|---|---|
| Windows and doors | Corners not flashed, missing side or head flashing, tape that is poorly stuck or not rolled down, gaps between the house wrap and window flange, pan flashing not fully adhered, and no tie-in to the drainage plane.[5] BSC says mulled window joints and window corners often leak.[1] | Continuous waterproof flashing with an outward slope.[1] Installed in the order shown below.[5] |
| Roof-wall intersections | Missing kickout flashing, or step flashing lapped in reverse so water runs behind it.[6] | Kickout flashing at least 4 inches up the wall and 4 inches out, made of 26-gauge (0.019 inch) galvanized steel, with at least a 4-inch overlap over the diverter. BASC cites IRC R703.4 and R903.2.1.[6] |
| Base of the wall | Flashing set too close to the ground. | The bottom edge of the base flashing at least 8 inches above grade.[7] |
| House wrap and building paper | Upper layers tucked behind lower layers (underlapped). | Shingle-style laps from the bottom up.[2] |
| Caulked joints | Caulk treated as a permanent seal. | Caulk checked and reapplied over time.[2] |
Kickout flashing sends water from the end of a roof-wall intersection into the gutter. Without it, BASC says water can damage wall sheathing, framing and insulation and cause mold inside the wall.[6]
What is the correct window flashing order?
Each layer should lap over the one below it, so water always runs out and down. BASC gives this sequence.[5]
- Pan (sill) flashing.
- The window.
- Jamb (side) flashing, lapped over the pan flashing.
- Head flashing, lapped over the jamb flashing and extending 1 inch past it.
- House wrap, laid over the head flashing.
BASC warns that window flashing damage is often not found until the wall has been badly damaged.[5]
Why do stucco and stone veneer need a drainage gap?
Stucco and adhered stone can stay wet without a drained space behind them. Joseph Lstiburek of BSC gives three reasons stucco walls have moisture problems.[8]
- Modern building papers bond to the stucco and stop drainage.
- Oriented strand board (OSB) sheathing does not become more vapor-permeable when wet, as plywood does.
- Polymer-modified stuccos dry poorly.
His fix is a ventilated or drained gap, such as 3/8 inch of drainage mat between two layers of building paper.[8] BASC recommends two drainage layers behind stucco: building paper, then a grooved house wrap.[2]
Adhered manufactured stone veneer has a similar problem. The stone is back-coated and fairly impermeable, so the veneer stays wet where it meets the thin-set mortar. BSC says failures cluster where rain exposure is higher and over OSB or foam sheathing. The remedy is a drained, vented cavity of 3/16 inch or more behind the thin-set and stucco.[9] For stucco and EIFS (Exterior Insulation and Finish Systems), see EIFS vs. stucco.
What signs of water intrusion can you see?
Look for staining, soft or rotting wood, and peeling paint near windows, doors and roof lines. BASC’s do-it-yourself guides list these signs.[10][11]
- Outside: discoloration, bubbling or peeling paint, soft or warped siding and trim, water stains near windows and gutters, algae, mold, rot, and trim that dents easily when you press it.
- Inside: stains below windows, damp carpet along exterior walls, musty odors, and carpenter ants.
BSC adds three more signs: water stains below windows (called “mustaching”), moss or algae on mortar joints, and wood fibers that come off by hand.[3] On masonry or concrete, white mineral deposits called efflorescence show that moisture is moving through the material.[12]
Some damage is hard to see. BASC says fiber cement, vinyl siding and brick veneer can hide the evidence of a missing kickout flashing for years.[6] BSC puts wood above about 25 to 30% moisture content in the mold-growth range, and says 28% or more supports decay fungi.[3]
How do you trace a leak to its source?
Start at the wet spot and work backward and upward. That is Lstiburek’s advice in BSC’s guide to diagnosing moisture problems. He also suggests testing suspect areas with a gently applied garden hose.[13]
Act quickly once you find water. The U.S. Environmental Protection Agency (EPA) says the key to mold control is moisture control. EPA also says to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth.[14] See mold after water intrusion. Before any repair, record what you found. Our guide on documenting a construction defect explains how.
This page is general information, not engineering advice. Have a licensed professional engineer or qualified inspector evaluate your home.
Related guides
- What is a construction defect?
- EIFS problems, codes and care
- EIFS vs. stucco: how to tell them apart
- Roof and flashing defects
- Mold after water intrusion
- How to document a construction defect
Sources
- John Straube, Building Science Corporation, “BSD-013: Rain Control in Buildings,” Aug. 22, 2011.
- Building America Solution Center (DOE/PNNL), “Drainage Plane Behind Exterior Wall Cladding,” last updated Sept. 9, 2020.
- Kohta Ueno, Building Science Corporation, “PA-1904: How to Look at a House like a Building Scientist (Part 4: Water),” Aug. 5, 2021.
- D.G. Hazleden and P.I. Morris, “Designing for Durable Wood Construction: The 4 Ds,” Durability of Building Materials and Components 8, 1999.
- Building America Solution Center (DOE/PNNL), “Windows and Doors are Fully Flashed,” last updated Sept. 9, 2022.
- Building America Solution Center (DOE/PNNL), “Step and Kick-Out Flashing at Roof-Wall Intersections,” last updated Sept. 14, 2020.
- Building America Solution Center (DOE/PNNL), “Flashing at Bottom of Exterior Walls,” last updated June 13, 2017.
- Joseph Lstiburek, Building Science Corporation, “BSI-029: Stucco Woes, The Perfect Storm,” Jan. 22, 2010.
- Joseph Lstiburek, Building Science Corporation, “BSI-113: Stoned,” Dec. 16, 2019.
- Building America Solution Center (DOE/PNNL), “Look for Missing Roof and Wall Flashing,” last updated Sept. 11, 2025.
- Building America Solution Center (DOE/PNNL), “Look for Missing Window and Door Flashing,” last updated Sept. 2, 2025.
- Kohta Ueno and Joseph Lstiburek, Building Science Corporation and Building America, “BA-1015: Bulk Water Control Methods for Foundations,” Jan. 10, 2011.
- Joseph Lstiburek, Building Science Corporation, “BSD-108: Investigating and Diagnosing Moisture Problems,” Oct. 26, 2006.
- U.S. Environmental Protection Agency, “A Brief Guide to Mold, Moisture and Your Home,” last updated Feb. 18, 2026.