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EXTERIORS of buildings
ADHESIVES, EXTERIOR CONSTRUCTION
AGE of a BUILDING - how to determine
ALGAE, FUNGUS, LICHENS, MOSS
ANIMAL ENTRY POINTS in BUILDINGS
ANIMAL or URINE ODOR SOURCE DETECTION
ARCHITECTURE & BUILDING COMPONENT ID
ASBESTOS IDENTIFICATION IN BUILDINGS
ATTIC CONDENSATION CAUSE & CURE
BARK SIDE UP on DECKS & STEPS
BASEMENT WALKOUTS & COVERS
BEST CONSTRUCTION PRACTICES GUIDE
BRICK VENEER WALL LOOSE, BULGED
BRICK WALL DRAINAGE WEEP HOLES
BOOKSTORE - EXTERIORS
BUILDING SAFETY HAZARDS GUIDE
CAULK GUN TYPES, CHOICES
CAULKS & SEALANTS, EXTERIOR
CHIMNEY INSPECTION DIAGNOSIS REPAIR
COLUMNS & POSTS, DEFECTS
CONNECTORS, FASTENERS, TIES
DECK & PORCH CONSTRUCTION
DECK CONSTRUCTION BEST PRACTICES
DECK COLLAPSE Case Study
DECK FINISHES COATINGS PRESERVATIVES
DECK FLASHING LEAKS, ROT Case Study
DEFINITIONS of ENGINEERED WOOD OSB LVL etc
DRYWELLS, FRENCH DRAINS for FLAT SITES
EARTHQUAKE DAMAGED FOUNDATIONS
EIFS & STUCCO EXTERIORS
EXTERIOR WALL SIDING TRIM & FINISHES
EXTRACTIVE BLEEDING STAINS
FLASHING MEMBRANES PEEL & STICK
FLASHING for METAL ROOFS
FLASHING ROOF WALL DETAILS
FLASHING ROOF-WALL SNAFU
FLASHING SIDING DETAILS
FLASHING WALL DETAILS
FLASHING WINDOW DETAILS
FLASHING WOOD ROOF DETAILS
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FROST HEAVES, FOUNDATION, SLAB
FOOTING & FOUNDATION DRAINS
GALVANIC SCALE & METAL CORROSION
GLUES ADHESIVES, EXTERIOR CONSTRUCTION
GRADING, DRAINAGE & SITE WORK
GUTTERS & DOWNSPOUTS
HEAT TAPES & CABLES for ROOF ICE DAMS
HOUSE PARTS, DEFINITIONS
HOUSEWRAP / SHEATHING WRAP
HOUSEWRAP INSTALLATION DETAILS
HOUSEWRAP PRODUCT CHOICES
HOUSEWRAP at SILLS, SOLES, TOP PLATES
HUMIDITY LEVEL TARGET
ROOF ICE DAM LEAKS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INSECT INFESTATION / DAMAGE
KIT HOMES, Aladdin, Sears, Wards, Others
LEAD POISONING HAZARDS GUIDE
LEAD TEST KIT for HOME USE
LEED GREEN BUILDING CERTIFICATION
LOG HOME GUIDE
METAL LATH, PLASTER & STUCCO
MOISTURE CONTROL in BUILDINGS
MOLD DETECTION & INSPECTION GUIDE
MVOCs & MOLDY MUSTY ODORS
ODORS GASES SMELLS, DIAGNOSIS & CURE
PAINT & STAIN GUIDE, EXTERIOR
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION
PAINT FAILURE DICTIONARY
PAINT LAB SAMPLE PREPARATION
PAINT SURFACE PREPARATION
PORCHES & Sunrooms
PORCH CONSTRUCTION & SCREENING
RAILINGS, DECK & PORCH
RETAINING WALL DESIGNS, TYPES, DAMAGE
RETAINING WALL GUARD RAILINGS
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF DORMER TYPES - PHOTO GUIDE
ROOF VENTILATION SPECIFICATIONS
ROT RESISTANT LUMBER
ROT, TIMBER FRAME
ROT, TIMBER ASSESSMENT
SEARS KIT HOUSES
SHEATHING, GYPSUM BOARD
FIBERBOARD SHEATHING, Celotex Homasote & Other
SHEATHING, FOIL FACED - VENTS
SIDING TYPES, INSTALLATION, DEFECTS
SINKHOLES, WARNING SIGNS
SOUND CONTROL in buildings
STAIN & BIODETERIORATION AGENT CATALOG
STAINS on & in BUILDINGS, CAUSES & CURES
STAINS on CONCRETE
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
STAINS & Thermal Tracking
STAIN DIAGNOSIS on ROOFS
STAINS on STONE, DIAGNOSE & CURE
STAIRS, RAILINGS, LANDINGS, RAMPS
STONE SURFACE CLEANING METHODS
STONE VENEER WALLS
STRAW BALE CONSTRUCTION
STUCCO WAll FAILURES DUE TO WEATHER
STUCCO WALL METHODS & INSTALLATION
STUCCO OVER FOAM INSULATION
STUCCO PAINT FAILURES
SURFACE GRADING, SITE DRAINAGE
TEST KITS for DUST, MOLD, PARTICLE TESTS
THERMAL EXPANSION CRACKS in BRICK
THERMAL EXPANSION of MATERIALS
THERMAL IMAGING, THERMOGRAPHY
THERMAL IMAGING MOLD SCANS
THERMAL MASS in BUILDINGS
TREES & SHRUBS, TRIM OFF BUILDING
TRIM, EXTERIOR CHOICES, INSTALLATION
VAPOR BARRIERS & CONDENSATION in BUILDINGS
VENTILATION in BUILDINGS
VINYL CHLORIDE HEALTH INFO
VINYL SIDING or WINDOW PLASTIC ODORS
VOCs VOLATILE ORGANIC COMPOUNDS
WALL CONSTRUCTION BARRIER vs CAVITY
WATER BARRIERS, EXTERIOR BUILDING
WATER ENTRY in BUILDINGS
WIND ENERGY SYSTEMS
WIND TURBINES & LIGHTNING
WINTERIZE A BUILDING
Building exterior stain cause diagnosis:
This article describes and provide photographs and microscopic photographs of algae, moss, lichens, or fungal growth that occurs on buildings and in nature.
We include links to references useful in the identification of algae, moss, lichens, and mold. Our photo at page top shows moss on an asphalt shingle roof. Our page top photo shows algae growing on the shaded side of a vinyl-clad building
Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.
A Detailed Online Guide to Types of Stains or Marks on Building Exterior Surfaces: roofs, walls, decks, siding, trim
[Click to enlarge any image]
We distinguish among the following stuff that may stain or be found growing building roofs, walls, or other surfaces, with extra focus on asphalt shingle roofs as well as other roofing materials such as wood shingles, wood shakes, roll roofing, and even slate or tile roofs.
Our photo (left) contributed by professional home inspector Hugh Cairns in British Columbia is a classic example of the combination of thermal bypass temperature variations marking the locations of studs in a wood framed exterior wall.
Temperature variations across a building surface can often explain the pattern of algae growth and in some cases mold growth on building surfaces as well as the more harmless accumulation of dust and debris. [Click to enlarge any image at InspectApedia] Mr. Cairns writes:
Some types of building stains or discoloration are only cosmetic in nature, while others may indicate growths that are likely to reduce the life of the material such as roofing, siding, or trim.
Our photo at above right is a magnified view of tiny black dots on building siding that will probably be identified as a fungus, but algae is more common than mold on most exterior siding, particularly vinyl siding where it may appear as a green, brown, or black "stain".
Mold on a building exterior surface indicates damp or shaded conditions, is not normally itself a hazard to building occupants, but it might indicate conditions that have also produced an indoor mold hazard.
As reported in Best Practices Guide to Residential Construction:
Our clapboard staining photo below left shows staining in an inside corner below a roof valley where we suspect there was gutter spillage or leakage down the wall, exacerbating siding staining. Look at the edge of the stains and you'll see some extractive bleeding. Our stained clapboard siding photo at below right is a closeup of stains found on the home shown in our photograph just above.
Mildew, which is comprised of two groups of fungi within the larger class of molds, grows only on living plants: mildew is an obligate parasite that is found, for example, on grapes.
The mildew sub-group of molds includes itself two families: Oidium-Erysiphe (powdery mildew) and Peronosporacae (downy mildew). But in both cases, these grow only on living plants.
So if you see what you think is mildew on a building exterior, unless the exterior is made of living plants like grapes, it's mold, but it's not not mildew.
However because the terms mildew and mildewcide are used very widely among the building trades we have permitted that topic name in this article. To learn more about mildew and to see photos of what real mildew looks like
Algae or Mold Stains on Building Siding may be due to a combination of shade, moisture, and a building surface that uses a material or coating that is particularly conducive to algae or mold growth.
Our photo (above, left) shows black staining on painted wood clapboards on an older building in New York state. Without closer examination and maybe a lab test, we're not sure if the black stains are algae or a mold, but in either case the causes are the same: moisture, shade, and from the stain pattern, possibly the absence of insulation and a moisture barrier in the wall structure. Also
Various species of outdoor mold will grow on just about any surface with sufficient moisture and heat. In new construction, it can be minimized by storing wood off the ground and providing adequate ventilation. Although sealers and stains contain a mildewcide, any mildew should be removed before finishing or refinishing, or it will continue to grow through the new finish. As detailed in Best Practices Guide to Residential Construction:
To remove mold ("mildew") from building surfaces, use a sodium hypochlorite solution, which can be made with household chlorine bleach. Depending on the severity of the problem, the solution should range from 1 to 8 parts bleach to 1 part water.
Spray the solution onto the siding (avoid sprayers with aluminum parts), starting at the top and working down. If two applications do not remove the stains, you may need to scrub in the solution with a brush. Thoroughly rinse everything with water.
Bleach can harm plants, discolor the finishes on trim, and corrode aluminum, brass, and copper. It is best to cover plants with tarps and protect any stained or painted surfaces.
For complete information about recognizing and dealing with mold growth in or on building surfaces
Moss tends to hold more water and moisture on the roof surface. So in our opinion moss is more likely to damage a roof surface than lichens or algae. Also see ALGAE, FUNGUS, LICHENS, MOSS COMPARED.
White stucco blotches or stains may be caused by painting over high pH surfaces (over pH of 11), or over moisture, both conditions found when new stucco is insufficiently cured can lead to white blooms or efflorescence on the painted surface as well as a short life of the paint job as our photo shows (left).
See PAINT on STUCCO, FAILURES for an explanation of white blotches or white bloom that appears on painted stucco building surfaces.
Also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION for a complete inventory of causes and signs of types of paint failure on buildings.
Readers should see STAIN DIAGNOSIS on ROOFS
For identifying and diagnosing stains on indoor building surfaces
See STAINS on STONE, DIAGNOSE & CURE for additional examples of diagnosing, cleaning, and preventing stains on building materials and artifacts.
In the following guide we list types of stains by stain color & appearance, by building location or material, and by stain cause. We distinguish among the following stuff that may stain or be found growing building roofs, walls, or other surfaces, with extra focus on asphalt shingle roofs as well as other roofing materials such as wood shingles, wood shakes, roll roofing, and even slate or tile roofs.
Some of these types of roof stains or discoloration are only cosmetic in nature, while others may indicate growths that are likely to reduce the roof covering life. A more detailed, illustrated version of the list below is given
Continue reading at STAINS on & in BUILDINGS, CAUSES & CURES or select a topic from the More Reading links shown below.
Suggested citation for this web page
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Frequently Asked Questions (FAQs)
Question: siding has turned yellow around the lights installed on an exterior wall
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.
Question: black substance appears on cedar siding
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.
Question: wet interior walls make me suspect siding leaks - where do I start?
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
Question: brown stains on foundation
(Mar 24, 2014) Eva Kaluzny said:
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
Question: categorical green stains on aluminum siding
(Apr 10, 2014) carol said:
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.
Question: Igor's stubborn black stain on brick
(June 3, 2014) Igor said:
Check that someone didn't tar the brick surface.
(June 9, 2014) amy said:
Question: black spots, raised, on siding
(June 10, 2014) Linda said:
Some possibilities include
Remnants of vines
Question: white spots on the window
(Aug 8, 2014) Ruth A said:
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.
Questions & answers or comments about how to identify types & causes of stains on building exterior surfaces.
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