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Moisture Behind Stucco: How Water Gets Behind It, the Drainage Plane, the Missing Kick-Out Flashing, How to Test Without Guessing, and What a Real Repair Costs
Exterior Finishes
Water Behind Stucco: It Does Not Come Through the Wall, It Comes in at the Edges, and the Edge That Does the Most Damage Costs Less Than Lunch to Fix
Stucco does not leak through the middle. A sound stucco wall sheds rain the way a sidewalk does. Where it fails is at every place something else meets it: a window, a roof, a deck, the ground. Water gets in at those edges, and on the old face-sealed walls it has nowhere to go but the sheathing, which rots in the dark for years before a stain shows inside. I have opened 1990s EIFS walls where the stucco looked perfect and the OSB came out in a shovel. Here is how it gets in, how a wall is supposed to drain, how to test it before anyone starts tearing, and what the repair costs when the test says yes.
Where it gets in, in order of how often I find it
- The roof-to-wall step with no kick-out. Where a lower roof runs into a wall, the step flashing carries water down the roof edge, and at the bottom of that run there has to be a kick-out, a bent piece of flashing that throws the water out over the eavestrough. Without it, every rain on that roof section runs straight down behind the stucco at the eave, and one missing kick-out can rot a wall from the eave to the footing. The chimney and wall flashing page shows where it goes.
- Windows with no sill pan. A window set with the stucco run tight to the frame and a bead of caulk leaks the day the caulk cracks, and the water runs down the inside of the stucco. The right detail is a sill pan under the window, head flashing over it, both lapped into the drainage wrap, which is $85 an opening on a re-side. Window leaks and water damage is the inside view of the same problem.
- Deck ledgers. A deck bolted through stucco with no flashing over the ledger is a shelf that holds water against the wall. Almost every one I have opened was wet.
- Stucco in the dirt. Stucco run below grade, or with mulch piled against it, wicks ground water up into the sheathing. The code wants cladding clear of the ground; I keep stucco 8 in. up with a starter track and something harder below it.
- No drainage plane at all. The 1980s and 1990s barrier EIFS: board glued to bare OSB with no wrap and no way out. Any of the four entries above is fatal on this wall, and the test below is not optional on a house of that age.
How a wall is supposed to drain
Every cladding on a wood-frame wall is allowed to let some water past it. The Ontario Building Code calls the cladding the first plane of protection and the sheathing membrane behind it the second, and the second plane is what keeps the sheathing dry. Behind stucco that means a drainage wrap on the sheathing, every opening flashed into it so water runs over the flashing and not behind it, a path down, which on EIFS is grooves in the board or vertical ribbons of adhesive, and a way out at the bottom through a starter track with weep slots. Water that gets past the stucco runs down the wrap and out, and the sheathing never knows.
On an ICF house none of this exists because none of it is needed; the foam and the concrete are the plane and there is no sheathing. That is a large part of why I finish ICF in stucco without worrying about it, and the stucco on ICF page has the detail at the windows that still has to be right. The flashing details page has the drawings for the wood-frame version.
Testing instead of guessing
You cannot see wet sheathing through stucco. By the time it shows as a stain inside or a soft baseboard, the wall has been wet for years. The tests, in order of how much they tell you:
| Test | What it does | What it finds |
|---|---|---|
| Knock and look | Tap the wall at window corners, below roof steps, at the deck. Look for staining, bulges, cracked caulk, stucco below grade. | Hollow spots where the stucco has let go, and the places to probe. Free, and where every inspection starts. |
| Infrared camera | Scans the wall after a rain or on a cold morning; wet sheathing shows as a temperature difference. | Where to drill. It does not say how wet, and it misses walls that are dry on the day. |
| Pin probe through the stucco | Two insulated pins drilled through the stucco into the sheathing at each suspect spot, read on a moisture meter, holes sealed after. | The actual moisture content of the sheathing. Under 15% is dry; 19% and up is wet; 28% and up is rot in progress. This is the test that settles it. |
| Cut and look | A 6 in. square of stucco removed at the worst probe reading. | Whether the sheathing is OSB mush or just damp, and what the wrap and flashing actually are. The only way to price a repair. |
Probe at the lower corners of every window, below every roof-to-wall step, beside the deck ledger, and at the bottom of the wall. A dozen holes on a house. What a qualified inspection charges, and who is qualified, is on the moisture inspection page. Do not pay anyone who offers to find the damage for free and fix it for a deposit.
What a real repair costs
One elevation of a 1998 two-storey EIFS house, 600 sq ft of wall, with a roof step and no kick-out, three windows on that side, and probes reading 30% at all three lower window corners and along the eave. The cut-and-look found OSB that crumbled and two studs soft at the window sills.
Tearoff of the stucco and board, 600 sq ft at $6.00, $3,600, and a bin, $550. Sheathing replaced over the wet areas, 150 sq ft at $6.50, $975. Framing repair at the three windows at $650 each, $1,950. A drainage wrap on the whole elevation, 600 at $0.85, $510. Sill pans and head flashing on the three windows and two other openings, 5 at $85, $425. The kick-out at the roof step, a few dollars, in the flashing allowance. New acrylic stucco, drainage EIFS, mid grade, 600 at $15, $9,000. Bands back on the openings, 90 lf at $18, $1,620. Scaffolding for the 200 sq ft above 20 ft at $1.50, $300. Total $18,930 before HST, $21,390.90 with.
The new stucco is less than half of that bill. The rest is the tearoff, the rot and the drainage layer the wall never had, and the drywall, insulation and mould cleanup inside are not in it; Health Canada’s position on mould is find the water, stop it, remove what grew, and that is a second contractor. The water behind siding page walks through the same house from the living room.
The same elevation on a house where the probes read 14% is a kick-out, flashing at the windows and a recoat. The test is what separates a $2,500 afternoon from a $21,000 wall.
Rules for a stucco house, old or new
- Find every roof-to-wall step and look for the kick-out. If there is none, that is today’s job, before anything else on this page.
- Buying a 1985 to 2005 stucco house on wood frame? Probe it before the conditions come off. The seller’s fresh recoat is not evidence of anything.
- Keep the bottom clear. Mulch back, grade 8 in. below the stucco, sprinkler off the wall.
- Flash the deck. The ledger gets flashed over and the stucco cut back and sealed to it, or the deck stands on its own posts.
- New stucco on wood frame gets a drainage layer, in writing. Wrap, flashing at every opening lapped into it, weep at the bottom, named in the quote. If the quote does not say, the wall will not have it.
Questions people ask
How do you know if there is moisture behind stucco?
Hollow sound when tapped, stains inside, soft baseboard, stucco run into the dirt, a missing kick-out at a roof step. None of those is proof. A pin probe through the stucco into the sheathing, read on a meter, is: over 19% is wet, over 28% is rotting.
What is a kick-out flashing?
A bent piece of flashing at the bottom of a roof-to-wall step that throws the water off the roof edge out into the eavestrough instead of down behind the cladding. It costs a few dollars, it is missed constantly, and it is the single most common cause of rotten walls behind stucco.
Does all EIFS have moisture problems?
No. Barrier EIFS from the 1980s and 1990s, glued to bare sheathing with no drainage, has them whenever water finds an edge. Drainage EIFS, with a wrap behind the board and the openings flashed into it, drains the way any cladding should.
How much does it cost to fix water damage behind stucco?
On one 600 sq ft elevation with rotten sheathing and framing at three windows, about $18,900 before HST: $6.00 a square foot tearoff, $6.50 for sheathing, $400 to $900 a window for framing, then the drainage layer, flashing and new stucco at $15. On a wall that tests dry, a kick-out and flashing repair is a fraction of that.
Can moisture get behind stucco on an ICF house?
Only at a badly flashed window, and even then there is no sheathing for it to rot; the foam and concrete do not care. The stucco base coat may let go locally and need a patch. That is the reason ICF is the stucco wall I am not worried about.
Price the wall after the test
The exterior cladding cost calculator carries tearoff, sheathing, framing per opening, wrap, flashing kits and new stucco as separate lines, so a repair number is built from what the probes found and not from a guess. If a contractor has already handed you a price for “hidden moisture damage” without a probe reading on it, the $99 review will tell you what is missing from it, starting with the evidence.
Sources and further reading
- Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.
- Addressing moisture and mould in your home: Health Canada: what to do when the sheathing under the siding turns out wet.
- Ontario’s Building Code, Section 9.27 Cladding: the first and second planes of protection, sheathing membrane, flashing at openings and transitions, and the clearances from grade and roofs that every siding and stucco job has to meet.
