Orange streaks on a stucco wall: tracing the rust back to where it starts

An orange streak on a pale wall gets noticed fast, usually by a neighbour before the owner. The calls I get about rust stains on stucco come with two worries: is the house rusting from the inside, and will it wash off. Sometimes it is a five-minute clean. Sometimes it is the first visible sign that water has been reaching the steel behind the finish for a few winters. The stain itself tells you which, as long as you read it from the right end.

Is a rust stain on stucco a problem?

It depends on where the streak starts. Rust washing down from a railing, a bracket or roof metal is a cleaning job plus a fix to that piece of steel. Rust coming out of the wall itself, from a crack, a corner or a small dot, means metal inside the stucco is wet, and that is a water problem with a stain on top.

Stucco on most GTA houses is full of steel. Cement stucco goes on over metal lath fastened with staples or nails, with a metal corner bead on every outside corner, casing bead around many windows and a weep screed along the bottom. All of it is galvanized, and the zinc coat lasts a long time as long as it stays mostly dry. Once water gets behind the finish and sits there through a January thaw and a refreeze, the zinc goes and the steel underneath starts to bleed. EIFS has far less metal in it, so on a foam wall the orange nearly always comes from something fastened through it or mounted above it.

Follow the streak up to where it starts

Rust runs downhill with the rain, so the bottom of a stain tells you nothing. Stand back, find the very top of the orange, and match it to the spot on the wall below. I read a house this way from the roofline down to the grass.

1. Starts at the roofline or under a chimney

   Roof metal and chimney caps

   Step flashing, a kick-out, a steel chase cover or an old satellite mount up on the roof is rusting and the rain carries it down the face. The wall below is usually fine, apart from the colour.

   Outside
2. Starts at the top corners of a window

   Casing bead or lath at the window head

   Water getting in at the head flashing or past a dried-out sealant joint wets the metal right at the corner, where stucco cracks first anyway. A short orange tail from each top corner is a classic.

   Inside
3. Starts at a fixture, a railing post or a bracket

   Something bolted to or through the wall

   Light fixture screws, house numbers, a hose reel, a balcony rail, a gas meter bracket. If the steel is on the surface, it is outside rust. If the stain comes out of the hole around the fastener, water is following the screw in.

   Outside or inside
4. Starts at a single dot in the middle of the wall

   A staple, a nail head or an iron speck

   On cement stucco, a dot that grows over a few years is often a lath staple near the surface getting wet. A sprinkle of tiny dots across a newer finish is iron in the finish or off a steel tool.

   Inside
5. Runs straight down an outside corner

   Corner bead with too little cover

   The bead sits right under the surface at every corner, and corners take hits from ladders, bikes and snow shovels. Once the thin coat over it cracks, the bead rusts along its length.

   Inside
6. Sits along the bottom of the wall

   Weep screed, sprinklers or lawn products

   A rusting screed shows as a line just above grade, worst where soil or mulch is piled against it. Splotches that stop at sprinkler height are often iron in the water, common on well-fed systems out in King or Caledon, or a lawn product with iron in it.

   Outside or inside

The two rungs at the bottom overlap with how close the stucco sits to the ground, which I cover in the post on stucco clearance at grade. The roofline rung is the same family of leaks as in the guide to the roof-wall junction, where a missing kick-out is the usual culprit.

How do you tell rust from outside from rust from inside?

Clean a small section and watch it. Outside rust lifts with a wet scrub and stays gone once its source is fixed or removed. Inside rust fades, then comes back within a few weeks of rain, because the steel behind the finish keeps getting wet. A hollow sound or a crack at the top of the streak points the same way.

You can run this test yourself in an afternoon and a couple of weeks of patience. You need a hose, a soft brush, a phone camera and a pencil.

1. Wait for a dry spell of three or four days, so the wall shows you its dry colour.
2. Find the top of one streak and zoom in on it with your phone. You are looking for a hairline crack, a pin-sized hole, a fastener head or a corner where the finish has chipped away to show metal.
3. Knock gently with your knuckle beside the top of the streak, then a foot away on plain wall. A duller, hollow note near the stain means the coat has let go of what is behind it.
4. Wet a strip of the stain about a hand wide and scrub it with plain water and a soft nylon brush. Photograph it once it is dry again.
5. Make a small pencil mark at the top of the cleaned strip and take the same photo after the next two decent rains.

If the strip stays clean, the rust was on the face. If a fresh orange line has crept back down from the same spot, the metal inside is wet. Do not dig at the dot with a screwdriver on a cement wall; you will open a hole into the lath and give the water a better way in.

What each source costs to fix

The prices below come from the repair list on our stucco repair page. Most rust jobs are one or two lines of it, not a whole wall.

The last row is the one to worry about. If the rust sits on a soft or hollow patch, read the six signs water is behind your stucco before you spend anything on cleaning, because cleaning a wall that is wet behind is money thrown at the wrong end of the problem.

A typical corner-bead job adds up like the receipt above. Twelve feet of failed sealant at the window over the corner, the corner itself opened and rebuilt with new bead, and the crack that ran off it routed and meshed. On a lot of the 1980s and 1990s stucco houses around Erin Mills and Bronte, the corners are exactly where I find this.

Cleaning off rust that came from outside

Once the source is gone, the stain usually comes off. Wet the wall first so the cleaner does not soak in, then use a rust remover sold for masonry or stucco, many of them based on oxalic acid, and follow its label to the letter. Test it on a spot behind a downspout before you touch the front of the house. Work from the bottom up so drips land on wet wall, scrub with a soft nylon brush and rinse until the water runs clear.

On EIFS, check that the product is allowed by the maker of the finish; some strong removers bleach an acrylic finish lighter than the wall around it. Cover the plants below and keep it off windows and anodized frames. A pressure washer held close does more damage to a stucco finish than the rust ever did, and muriatic acid etches cement stucco and can attack the very metal you are trying to protect. My full routine for washing a stucco wall, including the gentle pressure that is safe, is in the guide to cleaning stucco.

What a rust repair quote should tell you

A quote that says "clean and paint rust stains" and nothing else is a quote written from a car window. It should name where each streak starts, whether the metal behind it gets replaced or only cleaned, how the leak feeding it will be stopped, and how the texture will be matched on any patch. If the source is a failed sealant joint, our caulking and sealant work is often the whole fix.

If you have orange running down your wall and you cannot tell which rung of the ladder it starts from, take a photo of the top of the streak and one of the whole wall, and send them through the contact page or call (905) 973-6470. I usually call back within a day, and the quote is fixed, written and free.