Brown Mold vs Water Stains, Tannin Bleed and Wood Rot

A brown mark is usually a record of water before it is anything else. The ranking pages for this search mostly skip that and go straight to killing a colony that may not exist. This page sorts the mark first, by surface, then gives the fix.

What you are looking at, surface by surface

Surface	Brown thing it may be	What gives it away	Source
Painted drywall or ceiling	Water stain	Flat discoloration from a leak, flood or condensation	NIOSH Dampness and Mold Assessment Tool
Painted drywall or ceiling	Mold growth	Spots colored differently from the paint, fuzzy, musty or earthy smell	NIOSH Dampness and Mold Assessment Tool
Painted cedar or redwood trim	Extractive (tannin) bleed	Yellow to reddish-brown, appears after wetting, through paint	Wood Handbook, Chapter 16
Bare pine near a fastener	Rust	Brown halo around a corroding steel nail or screw	Wood Handbook, Chapter 16
Bare framing or subfloor	Brown rot	Darker wood cracked across the grain into cubes, crumbles	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Bare lumber, sapwood	Fungal or chemical brown stain	Colour in the wood, not on it; does not brush off	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Fixtures or tile on well water	Manganese or iron	Black to brown (manganese) or reddish-orange (iron) where water dries	EPA secondary drinking water standards

Real brown fungi are common. Mycology Online describes Cladosporium colonies as mostly olivaceous-brown to blackish-brown, sometimes buff or brown (Mycology Online: Cladosporium); Aureobasidium as cream or pink, later brown or black; and Alternaria as black to olivaceous-black with pale brown spores. Those are lab-media descriptions: a lab names a genus from structures, not shade.

Score it before you scrub it

The NIOSH Dampness and Mold Assessment Tool (Publication 2019-114) was written for schools, but its method fits any brown mark: score damage or stains, visible mold, and wet or damp separately for each surface, then note odor. A brown stain with no fuzz and no dampness is a different job from fuzzy spots on a wet wall.

Score	Size of the affected area	What NIOSH says to do with it
0	None	Record it
1	Up to the size of a sheet of paper	Compare over time to see if problems get worse
2	Larger than a sheet of paper, up to a standard door	Compare over time to see if problems get worse
3	Larger than a standard interior door (32 by 80 inches)	Immediate attention to find the source and remediate

Then run the brown-specific checks, in order:

1. Touch and look closely. Fuzz or raised spots point to growth; a flat edge with a darker rim points to a water stain. Why this differs from generic advice: most guides assume the brown is alive.
2. Bleach drop on a coin-sized spot. Per the Wood Handbook, Chapter 16, a drop or two of 5% sodium hypochlorite usually bleaches mildew in a few seconds; what does not bleach is probably dirt, extractives bleed or iron stain. Why this differs: it is a test, not a treatment.
3. Meter the substrate. On wood, compare the reading with the 20% and 30% lines per the Wood Handbook. Why this differs: a dry stain is history; a wet one is an active job.
4. Probe wood with a screwdriver. Cubes and crumbling mean brown rot; spongy and bleached means white rot. Why this differs: cleaning does nothing for decay.

Brown mold on walls and ceilings, or a water stain

On drywall and plaster, the NIOSH Dampness and Mold Assessment Tool draws the line: stains are discoloration from leaks, flooding or condensation; damage includes peeling paint, efflorescence, rust and warping; mold is patches colored differently from the material, typically gray, brown or black, that can look fuzzy and smell musty or earthy. EPA's Mold Cleanup in Your Home adds that mold may cause staining, so a stain and a colony often sit together. Ceiling stain patterns are on water damage ceiling and general wall growth on mold on walls.

Feature	Water stain	Brown mold	Tannin bleed on painted wood
Surface	Flat, often a darker rim	Raised spots, fuzzy or powdery	Flat, through the paint film
Bleach drop (FPL test)	Not described by FPL	Usually lightens in seconds	Does not lighten
Smell	None of its own	Musty or earthy (NIOSH)	None of its own
What fixes it	Find and stop the source (NIOSH)	Fix moisture; clean or remove the material	Stain-blocking primer, then topcoat
What does not	Painting over a wet wall	Painting or caulking over it (EPA)	More coats of the same paint (FPL)

Brown mold on wood, or brown rot

The USDA Forest Products Laboratory Wood Handbook, Chapter 14 separates four things that look brown on wood. Stain fungi give the most common sapwood discoloration, bluish to grey to brown. A chemical brown stain in softwoods forms when natural chemicals oxidize during slow drying, with no fungus involved. Abnormal mottling with unnatural brown areas is often evidence of decay. And brown-rot fungi remove cellulose, leaving wood browner and cracked across the grain into cubes.

Type	Look	What it does to the wood	Source
Mold and sap stain	Fuzzy surface growth or colour in the sapwood	Affects strength only slightly	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Brown rot	Browner, cracks across the grain into cubes	Can shrink, collapse and crush to powder	USDA Forest Products Laboratory Wood Handbook, Chapter 14
White rot	Bleached, no cross-grain cracks, often spongy	Keeps its dimensions until severely degraded	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Soft rot	Shallow, soft when wet, firm just beneath	Most damaging to thin pieces	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Rust halo (pine)	Brown stain around a steel fastener	Slight degradation near the iron	Wood Handbook, Chapter 16

The handbook calls the name dry rot incorrect, because wood must have moisture to decay, and notes that some brown-rot fungi form strands that carry water from the soil into buildings. Depth and sanding are on mold on wood; plain water marks on furniture and floors are on water stains on wood.

Threshold	Value	What it means	Source
Fully air-dried wood	20% or less per the handbook	Reasonable margin against fungal damage	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Fiber saturation point	Average 30% per the handbook	Serious decay only above this	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Humid air alone	Below fiber saturation	Supports some mold, not significant decay	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Iron stain migration	Above about 16% per the handbook	Iron or its stain can move through wood	Wood Handbook, Chapter 16

Heartwood decay resistance	Common building species in that group (domestic)
Resistant	Cedars including western and eastern redcedar, old-growth redwood, white oaks
Moderately resistant	Douglas-fir, western larch, young-growth redwood, old-growth eastern white and longleaf pine
Slightly or nonresistant	True firs, hemlocks, spruces, pines other than those listed, maples, birches

Brown mold in shower

Brown in a shower has three usual explanations, and only one is a fungus. Per Mycology Online, Aureobasidium turns from cream or pink to brown or black and Cladosporium runs olive-brown, so aging growth in caulk can read brown. The EPA secondary drinking water standards lists black to brown color for manganese above 0.05 mg/L and reddish or orange staining for iron above 0.3 mg/L. And the Wood Handbook, Chapter 16 names bathrooms among rooms where extractives bleed through paint on wood trim.

Brown in the shower	Test	Fix	Why this differs
Speckles in silicone caulk	Bleach drop lightens it	Remove the moldy bead before re-caulking	EPA: do not caulk over mold
Brown-black streaks below the head or spout	Follows the water path; often well water	Water test for manganese and iron	Cleaning cannot outrun the supply
Brown bloom on painted window trim	No change with bleach	Stain-blocking primer, then topcoat	Extra topcoats will not stop it (FPL)

Grout, drains and general shower growth are covered on mold in shower.

Brown mold removal, case by case

How to get rid of brown mold depends on which brown it turned out to be. The generic method and EPA’s 10 square foot line are on DIY mold remediation.

• Growth on paint or tile: per EPA's Brief Guide to Mold, clean up the mold and fix the water problem, or it will most likely come back. Why this differs: the stain beside brown growth often shows where that water ran.
• Growth on drywall paper or ceiling tile: remove it, since EPA says absorbent materials may have to go. Why this differs: bleaching the face leaves growth inside.
• Surface mold on softwood: per the Wood Handbook it can often be brushed or surfaced off. Why this differs: on oak the stain may run too deep to sand out.
• Brown rot: dry the wood and repair weakened members. Why this differs: per the handbook, wood kept air dry will not decay and existing decay will not progress.
• Tannin bleed: clean, stain-blocking primer, cure, topcoat, per the Wood Handbook.
• Rust or iron stain: oxalic acid removes iron stain, per the handbook, but it returns unless the iron source goes.

When a lab is worth it

Usually it is not: the EPA Mold Course, Chapter 3 says sampling is unnecessary in most cases when mold is visible. It also says sampling can tell mold from soot or dirt, which is the brown case, and describes clear tape pressed on a spot as one method.

Per the EMSL list, three hours costs $328 against $68 at two weeks. Sampling strategy is on mold testing.

When brown means the structure is involved

Sign	What it suggests	Source
Brown wood cracked into cubes, or crumbling to powder	Brown rot; check strength	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Brown strands or fans spreading across a wall or floor	Decay fungus surface growth or water-conducting strands	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Wood reading above about 30% moisture content	Conditions for serious decay	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Any column scored 3, larger than an interior door	Immediate attention to source and remediation	NIOSH Dampness and Mold Assessment Tool
Strong mold odor with nothing visible	Search for hidden mold	NIOSH Dampness and Mold Assessment Tool
Visible brown growth on a wall face	Hidden growth may surround the visible area	EPA Mold Course, Chapter 3

For contractors

Brown calls split three ways: cosmetic stains that need a primer, growth that needs remediation, and decay that needs a carpenter. ANSI/IICRC S520 (fourth edition, 2024) is the procedural standard for mold remediation, and a 2024 Cal State LA mold assessment quotes its conditions: Condition 3 is actual growth, active or dormant, visible or hidden; Condition 2 is settled spores or fragments from a Condition 3 area; Condition 1 is normal fungal ecology. A tannin bleed or a dry water stain is none of them.

Item	What to record	Why an adjuster asks
Scores per surface	Stain, visible mold and wet columns kept separate, plus odor	Shows what is cosmetic and what is active
Moisture map	Pin readings on wood against the handbook lines, by location	Ties brown to a live or past water source
Probe results	Cubical cracking, crumbling or sponginess, with photos	Separates remediation from structural repair
Framing species, if known	Group under Wood Handbook Table 14-1	Explains why one member decayed and the next did not
Cause of loss	The leak, condensation or water supply behind the stain	Ties the stain to an event or a long-term condition

Write brown rot as structural repair with its own drying line, and tannin bleed as painting, so the remediation line covers only growth.

Questions people actually ask

Is brown mold dangerous?

Colour does not decide it. CDC says mold can look like spots of many colours and that you do not need to know the type, so brown growth gets the same response as any other: fix the moisture, then clean or remove by material and area. Many brown marks are stains or rot, not mold. See the scope note below.

What does brown mold on wood look like?

Surface mold is fuzzy or powdery and often brushes off softwood. Per the Wood Handbook, stain fungi colour the sapwood itself, while brown rot turns wood browner and cracks it across the grain into cubes that crumble. Probe and meter it: per the handbook, serious decay needs an average of 30% moisture or more.

Is the brown on my wall mold or a water stain?

The NIOSH assessment tool separates them: a stain is flat discoloration from a leak, flood or condensation, while mold is spots colored differently from the material, typically gray, brown or black, often fuzzy with a musty smell. A drop of household bleach usually lightens mildew within seconds, per the Forest Products Laboratory.

Why is there brown mold in my shower?

Aging growth in caulk can turn brown: Mycology Online describes Aureobasidium going from cream or pink to brown or black. On well water, brown-black streaks may be manganese, which EPA says causes black to brown color above 0.05 mg/L. If a bleach drop does not lighten the spot, it is probably not mildew; test the water.

How do I get rid of brown mold?

Match the fix to the cause. Growth on tile or paint: clean it and fix the water problem, per EPA. Growth on drywall paper: remove the material. Brown rot: dry the wood and repair weakened members. Tannin bleed: stain-blocking primer, per the Wood Handbook. Rust: oxalic acid and removal of the iron source.

Should I test brown mold?

Rarely. EPA’s mold course says sampling is unnecessary in most cases when mold is visible, but notes it can tell mold from soot or dirt, which suits an ambiguous brown stain. Per the EMSL 2026 price list, a tape lift exam runs $68 at two weeks to $328 at three hours.

More in our mold hub.