Why stucco fails: caulking, flashing and the window problem

The system keeps water out, so it keeps water in

A traditional cement stucco wall lets water in and lets it dry again. An EIFS wall is a barrier: water stays on the face and runs off. That is a strength until water gets behind the barrier. Then it sits against the sheathing, the insulation stays wet, and nothing dries because the wall is sealed from the outside. Rot, mould and blisters follow, usually unnoticed for years.

So the whole question of EIFS durability comes down to one thing: keeping water out of the joints. That is where the three causes come from.

Cause one: caulking

Every place the stucco stops and something else starts is a joint: window and door frames, expansion joints, the line where stucco meets stone or brick, every pipe, vent, light and railing through the wall. Each one needs a flexible sealant that stretches as the materials move and keeps water out for years.

Here is what we find on the walls we repair: no sealant at all, because the installer skipped it; hardware-store caulking that is not compatible with acrylic finishes and let go in a year or two; a thin bead with no backer rod that tore the first winter; or good sealant that did its job for fifteen years and was never renewed. Weather dries sealant out. It is a maintenance item, like a roof, and most owners have never been told that.

A proper joint is cut clean, backed with foam rod to the right depth, primed where the manufacturer requires it, filled with a low-modulus sealant made for EIFS, and tooled so it bonds to both sides and sheds water. Done that way it lasts many years. Done any other way it does not.

Cause two: flashing

Sealant handles joints that move. Flashing handles places where water is directed: the top of every window, the line where a roof meets a wall, the bottom of the wall, decks and balconies, and the transitions between materials. A flashing is a piece of metal or membrane shaped so water that reaches it is sent out over the face of the wall instead of behind it.

The flashing we find missing most often is the kick-out: a small angled piece at the bottom of a roof edge where it meets a wall, which kicks the water running down the roof into the gutter. Without it, every rain pours a stream of water down the wall behind the stucco at that exact spot. We have opened walls under missing kick-outs and found sheathing you could push a finger through.

Head flashings over windows are the second. Without one, water running down the wall goes straight into the top of the window frame joint.

Cause three: the window problem

Windows are where the two causes meet. A window is a hole in the wall with a frame in it, and the frame moves differently from the stucco. Water runs down the wall, hits the head of the window, runs along the frame, and arrives at the bottom corners, which is why the stains on a failed wall are almost always at the bottom corners of windows.

A window done properly in an EIFS wall has a head flashing above it, a sealant joint around the frame with backer rod, a sill that slopes away from the window with a drip edge underneath, and jambs sealed to the frame. On the walls we repair, the sill is often flat, so water and snow sit against the frame; the joint is often a smear of caulking with no backer; and there is often no head flashing at all. Any one of those is enough. Together they are a leak at every window on the house.

Windows that were replaced after the stucco went on are a special case. The new window is often set in without resealing to the stucco, or with a bead of whatever the window installer had on the truck. If your windows were changed on an EIFS house, have the joints checked.

The other causes

• Stucco below grade. The system run into the soil wicks water up and traps it, and invites insects into the foam.
• No drainage plane on wood frame. Older barrier systems on framed houses had nowhere for incidental water to go. Modern drainage EIFS fixes this.
• Sprinklers and downspouts. A wall soaked daily will find a way to fail.
• Decks and railings. Ledger boards bolted through the stucco and railing posts through the finish are penetrations that are rarely flashed or sealed.

How to check your own house

Walk around it after a rain. Look at the bottom corners of every window for dark streaks. Look at the sealant around frames: is it there, is it cracked, has it pulled away? Look where any roof edge meets a wall: is there a kick-out flashing, and is the wall below it stained? Press any area that looks lifted. Check that the stucco stops above the soil and mulch. Each of those takes a minute and tells you most of what a site visit would.

What a good installation does differently

Flashings go in before the system, not after. Every window gets a head flashing, a sloped sill with a drip, and a proper sealant joint with backer rod. Every penetration is sealed. The system stops above grade with a starter track. On wood frame, a drainage plane goes behind the board. And the owner is told, in writing, that the sealants are a maintenance item and when to check them. That is how we install, and it is why most of our repair work is on other people's walls.