Building exterior stain cause diagnosis & removal FAQs:
Questions & answers about how to identify the cause of stains on the outside surfaces of buildings & how to remove those stains or marks.
This article series provides a catalog of the different types of stains or discolorations found on building exterior surfaces: walls, roof, trim, walks, patios, decks, etc.
Our page top photo shows Minneapolis home inspector Roger Hankey explaining why we find black soot stains on a roof below a chimney.
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These questions & answers about diagnosing and cleaning off various stains on building exteriors were posted originally at STAIN DIAGNOSIS on BUILDING EXTERIORS
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On 2017-09-08 by (mod) - brown water marks on roughcast exterior plaster/stucco
If the brown water marks are due to iron in the water then an iron-stain removing product should be helpful. Beyond that I can't guess as I don't know what's going on.
Could it be mold? If so any household cleaner will suffice. Be sure to rinse thoroughly so that you don't leave a bleach mark on the wall.
For other readers, from Wikipedia,
Roughcast or pebbledash is a coarse plaster surface used on outside walls that consists of lime and sometimes cement mixed with sand, small gravel, and often pebbles or shells. The materials are mixed into a slurry and are then thrown at the working surface with a trowel or scoop.
On 2017-09-08 by Diana
I have brown water marks on my roughcast due to overflow pipe water . Any ideas how l can clean this ? Thanks
On 2017-08-31 by (mod) - black mildew or mold on recent paint on a new home
It will not be mildew - mildew only grows on living plants.
It may be mold. Even if the wood was dry, once exposed to the outdoor elements of rain and humidity wood, and paint on wood can both support mold growth. That's more likely if the wood is in a shaded area.
at https://inspectapedia.com/mold/Mold-on-Building-Surfaces.php you will find photographs showing what mold looks like on building surfaces, including MOLD on PAINTED WOOD and MOLD on WOOD
Check out MOLD on PAINTED WOOD
Then use the page top or bottom CONTACT link to send me photos of the fascia boars on your home, both a distant view and up close sharp images and I can comment further. Let's do that before talking about the remedy and prevention of future mold growth on that trim.
On 2017-08-31 by Connie
We have a brand new home. 5 months after completion, we noticed black mold forming/staining the paint of a few facial trim boards. Our contractor used only kiln-dried trim & quality Sherwin-Wms paint.Is this likely mildew or mold?
On 2017-02-18 by (mod) - HardiePlank sprayed to kill bees left a stain
Start by finding out what was sprayed onto the home, and what was the actual spray mix including not just a pesticide but the vehicle that was used.
For example if an oil was used as the pesticide vehicle (the carrying liquid) you might have success with a TSP-substitute siding or deck cleaner; in the worst case the siding may need to be sealed and re-coated.
Check also with HardiPlank before choosing a new coating or sealant.
On 2017-02-17 by Anonymous
House is built of hardiplank . It was sprayed to kill bees in the eves of house. It left a dark stain. How can I remove stains.
On 2017-01-06 by (mod) - tips for realistic model builders in designing rust stains on a miniature castle
Thanks for the kudos, Minia. And nice tip for model builders.
On 2017-01-06 by Miniature Building Model Hobbies
Thank you so much for this extensive list - it is an invaluable resource for getting ideas on how to design realistic model buildings! A miniature castle with rust stains bleeding down the walls below metal fixtures looks very realistic and adds some colour.
Moss and pigments leeching from wood down the side of a stone inn really make it blend into the surrounding natural environment. Thanks again!
On 2016-11-17 by Anonymous
how to remove green stains from exterior walls
On 2016-08-16 by (mod) - OSB shiny area
OSB is often coated with a water repellant wax or other substance that can look shiny in some lighting conditions. Be warned that if exposed to wetting or to the elements OSB will deteriorate if left unprotected.
On 2016-09-25 by Judi shiny patch on some interior garage walls of unfinished osb.
Question - we have shiny patch on some interior garage walls of unfinished osb. Any idea what it might be?.
On 2016-08-16 by (mod) - getting rid of pollen stains on the exterior walls
I've posted our discussion at POLLEN STAINS on BUILDINGS
For the stains to be due to roof material bleed-out we'd expect to see the stain pattern occur wherever roof spillage was splashing up on walls; and we'e expect to find a roof product that's leaching or bleeding.
Your photos are a strong argument for blaming Hibiscus pollen
On 2016-08-16 by Gail - stains on walls in wet area of Florida
Thank you. My initial reaction was that the color was from a tile roof I had put on in February which is the same color.
I backed away from that because this color did not appear until after the bushes had bloomed and it's only on that side of the house. It is also the same shape and size of the hibiscus bushes across the depth of the house as you can see in the one picture.
Just sent 3 pics to the editor email.
I live in Florida so we get daily torrential rains and the pollen is no longer powder. I tried the tape and windex with no effect. I also tried full strength bleach and another time white vinegar.
Neither had any impact on the color. The wall is about 40 feet wide. The pollen is on my gutters and vinyl windows, too and does not come off.
I have pics, but not sure how to send them. Thanks, Gail
We just installed 2 lights on our outdoor garage and the siding has turned yellow above the lights as well as below. Any ideas? Might it be caused by electrical current? - Sharon Klingler 7/28/2012
Sharon you could be describing a FIRE hazard. Remove the light bulbs, check the label on the light fixture and make double sure no one installed an over-watted bulb. The result is a hot fixture that can discolor nearby vinyl or plastic materials. It wouldn't be the wiring or electrical current itself unless the wiring itself were overheating.
Every year we have a black substance appear on our cedar siding. The siding was stained originally on all sides before installing 17 years ago. We have used several cleaners, mainly JoMax made by Zinnser. and then re staining with Cabot full body water based stain. The next year it returns. Can you give us any solutions? - Bob I 8/7/12
Bob, I'd like to see some sharp photos of the siding, from a distance & up close. I'm out of the U.S. so can't easily check a sample.
The first step in effective and lasting cures for stains on wood siding is to understand just what the stain is and what is causing it. See if you can determine that it is being deposited on top of the surface rather than bleeding through - that's a first step.
Then compare the appearance of stains by where on the building and relate that to sun, shade, moisture, trees, grass, mowing, etc. Maybe it's artillery fungus.
7 years ago contractor changed our siding... Now when I am repainting walls inside, painter noticed that wall is wet. I think siding was not properly installed in many places. Installer says that "siding was not good". Where should I start? I can't pay again to redo the siding job. - Janna 11/30/12
(Mar 24, 2014) Eva Kaluzny said:
I read this article and found it very helpfu.. I have a situation where there are bronwn stains on the foundation wall inside of th house so want thinking that it might be a plumbing issue but then also notice brown stains on the foundation wall on the oustdie so trying to pin point if this is a plumbing issue or roofing issue. Any advise?
I can't say what's going on from just your email. If you like you can send us some photos using the CONTACT link at page bottom.
If there is a plumbing line that could leak into the foundation wall then that could be the problem; If there is water leaking into or through the wall from any other source of course that could be the issue. Take a look at our article on Effloresence for other brown stains that are deposits left by water leaks - see
(Apr 10, 2014) carol said:
categorically, what are the green stains on alum siding- ie- mold, algae etc
Ususally green stains are algae if they're in a shaded / wet area. But this is wild categorical arm-waving; there could be other sources of siding stains.
(June 3, 2014) Igor said:
Thanks for the article. There is lots of good information here. I have a question with regards to a stubborn black stain on lighter colored brick. Many years ago there was an old wood trellis attached to this spot and once removed the black stain was revealed. It ran along the edge of the piece of wood that was directly attached to the brick? What is the best chemical solution to clean this stain?
Check that someone didn't tar the brick surface.
Don't power-wash it. Gentle scrubbing with a TSP substitute may help.
(June 9, 2014) amy said:
What about a light orange-ish stain on white exterior brick?
(June 10, 2014) Linda said:
There are many small black raised dots on our clapboard in a few areas of the house. They are not low to the ground. Do you know what they could be?
Some possibilities include
Remnants of vines
(Aug 8, 2014) Ruth A said:
We just noticed white spots Om an upstairs window on the outside. What insect would cause this? The spots are round and in a 5" circle. Thanks for your help.
Ruth I really can't guess from just the information in your note. There are some insects whose egg deposits form white, round, flat structures on surfaces, though 5" sounds a bit big for bugs.
Re-posting without disallowed link (spam and virus protection for readers)
9/15/14 B Massey said:
I wanted to thank you for your site's guidance and advice. We bought this 23 years-old place three years ago.
When replacing the discolored bathroom linoleum recently, it was evaluated that the toilet seal needed redone and learned it had been leaking into the slab for several years!
Anyhow, appropriately helped us with that home repair. We of course left the backside of the toilet free of... so as to know if a future wax ring replacement is necessary.
Upon that lengthy time frame discovery, a 1 Cup Chlorine Bleach and 1 Gallon Water ratio was lightly applied to the slab in two coats with an unwanted sock since green colorings started showing up as well as blue.
That event eventually sent (Blue Green) algae to the exterior slab in sections [it was not there before]
I now know how to resolve that with same Sodium Hypochlorite ratio thanks to you! I would thank the toilet guy if I could (via e-mail or post). Great job looking out for your fellow Americans.
describes replacement of the wax ring seal on toilets using procedures that apply in most countries.
I have a section of an exterior wall that has become discoloured because of a leaking gutter above.
The discolouration is a mix of white and mouldy green. Please can you suggest any tips on how to clean it? E.g. do I need to use a particular cleaning solution? Is it OK to use a power washer? etc.
Here is a photo of the wall:
Damian your photo shows what looks like algae and white mineral effloresence on a yellow-gray brick wall.
Fix the leak gutter first or your cleaning effort will be wasted.
You can use a power washer on solid brick walls though if there are weep openings in the wall I would take care to avoid spraying water into those wall apertures.
Take care also that a power washer may leave the cleaned area of the wall lighter than surrounding areas or, if not handled with care, or if sprayed from too close a distance, may leave marks on the wall. Work gently and from a distance first to avoid these troubles or just use a cleaner like a TSP substitute and a garden hose.
Details on how to clean stone and brick surfaces are found at
Any household cleaner, detergent would be fine.
(Mar 20, 2015) Rick said:
I removed 3 cedar trees from the front of my house and discovered stains on the brick and vinyl eaves. Will a bleach solution remove these with no harm to the surfaces?
Bleach won't normally hurt brick but might leave a color change depending on what other debris or foreign materials are on the brick surfaces of the home; Bleach on vinyl siding is asking for trouble unless you work very carefully and experiment. Why not use a siding cleaning soution?
(Apr 12, 2015) Pam said:
Help!!! Two years ago, my grandson put roof tar on my older mobile home and we painted the entire exterior.
It looked wonderful until black streaks suddenly showed up on the sides. It appears to be coming from the roof. I have washed it and repainted it but, now it is even worse. It no longer will wash off so, I am thinking I need to redo the roof tar with something better and repaint the whole house again.
Any ideas how to stop this would be greatly appreciated. Thanks!!
Yes I've heard of this problem from readers and seen it a number of times myself, not just on mobile homes but on other buildings too. Black asphalt begins to wash out of the weathered bituminous coating on the roof and leaves stains on walls below.
Another reader wrote and sent photos of black stains down buiding walls below a roof parapet whose top had been sealed with tar and bituminous roof cement.
Check with your building supplier or mobile home or RV supplier to ask if they have a roof sealant paint that is compatible with an asphalt coating (I'm not too hopeful). That might reduce the problem. Otherwise we're left with needing to cover the whole roof with a new membrane, perhaps using rubber (EPDM) or a white plastic roof membrane.
(Jan 23, 2016) Anonymous said:
I have small bright red spots on our painted siding. Any ideas what it is?
From just your e-text I can't hazard a guess.
Use the page top or bottom CONTACT link to send me some sharp closeup photos as well as distant shots of the walls of the building and we can comment further.
(Feb 17, 2016) Peter said:
I just had my house resided with L.P. siding 2 months ago I live in Minnesota and it is winter time now. Ever since the new siding was installed there areally random yellow spots appearing all over. Can anyone one tell me what is going on and how to fix it
Peter from just your e-text I cannot diagnose the yellow spots. If "all over" means all sides of the building and thus varying conditions of sun and shade and weather exposure, one might think it's an artifact of the siding but without an onsite examination by an expert, sorry, I can't guess.
Use the page top or bottom CONTACT link to send me some sharp closeup photos as well as distant shots of the walls of the building and we can comment further.
(Mar 12, 2016) Shaun said:
We had a lot of green "mold" on the north side of our house sided with aluminum- came right off with a bleach solution.
The bottom two tiers had a mixture of the green mold and a red mold. The red mold did NOT come off with the bleach solution. I tried a little Bon Ami and with a lot of scrubbing it turned bright orange and after multiple scrubbing in the same place finally came off.
However, this would be a Very Long Job to do the entire north side of the house by hand. Ideas?
Algae on a shaded home exterior can also be mistaken for mold.
But in any case, if the stains are not easily removed I figure it's probably neither mold nor algae. Generally we can scrub off mold, leaving at most a modest stain in the paint pigment.
(June 1, 2016) dennis said:
how do i clean paint pigment run down from white siding on brick
If you are asking only about the simple deposits of runoff from a chalking painted surface, I recommend gentle cleaning of the brick with a natural bristle brush and a TSP substitute cleaning solution. You can also try household detergent.
Please see our warnings below about the dangers of aggressive cleaning or power washing of brick surfaces.
If you are asking about removal of actual paint that ran onto brick during painting of other building surfaces, see
(We may move that text to a separate article but if so there will be a forwarding link)
(June 15, 2016) Anonymous said:
Just finished a thermal bluestone patio looks like grey bloches coming thru the stone mai
Perhaps moisture-generated effloresence - search InspectApedia for EFFLORESENCE to read detasils
18 June 2016:
On some of our siding trim and soffit fascia boards I see round holes about the diameter of a pencil or a little larger. Below and around these holes I see what looks like yellow, brown and black tiny splash marks and particles. What's going on? - Chris
Chris, use the on page InspectApedia search InspectApedia for CARPENTER BEE DAMAGE to see what's probably going on with your soffits.
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