• The Beam Guy

Let's Talk About Brown Rot

Dry Rot Usually Refers To Brown Rot

If we break down wood-rotting fungi into the three most common groups we get: brown rot, soft rot and white rot. Each destroys wood in its own unique way. I'll let Wikipedia give you the synopsis if you're interested. For our purposes here, the focus is brown rot -- the usual suspect in residential dry rot.


In the Sacramento - Davis - Elk Grove area, I mostly see brown rot in the exterior beam ends I inspect. The term dry rot in the building trades is often interchangeable with the term brown rot because of the appearance of brown rot decayed wood -- dry and crumbly.


Not all "dry rot" is caused by a brown rot fungi, but the results of an infestation of a brown rot fungi are unmistakable -- cubical fractures.

dry rot caused by the brown rot fungi Serpula lacrymans in an exterior beam
The checkered pattern of brown rot, in this case, Serpula lacrymans

The shape is the result of the way these fungi eat the wood. The way it breaks down the hemicellulose and cellulose produces hydrogen peroxide as a by-product. It's the hydrogen peroxide responsible for the shrinking into cubical bits and the brown color.


The science geek version of the explanation from Advances in Botanical Research, VLM 61:

" BRF [Brown Rot Fungi] selectively strip cellulose and hemicelluloses from wood, which rapidly loses its strength properties. In advanced stages of decay, the rotten wood is reduced to a residue of amorphous, crumbly, brown cubical pieces, mainly composed of slightly modified lignins. As BRF do not produce phenol oxidases, these residual lignins can be dealkylated, demethoxylated and demethylated to some extent, but their aromatic rings are not degraded.

During brown-rot decay, degradation occurs at a great distance from hyphae by diffusion of enzymes. BRF colonize wood tissues via the rays, from where the hyphae ramify out into the axial wood structure, penetrating through cells using pits or by bore holes (Daniel, 2003). The S2 layer of the secondary wall is depolymerized preferentially to other layers, probably owing to its reduced density and lower lignin content. At advanced stages of decay, much of the S2 layer is severely degraded and metabolized. By contrast, the more highly lignified primary wall and middle lamella generally resist BRF attack."


Brown rot's appetites leaves dry fragile wood fragments that easily disintegrate when touched -- it literally turns to dust in your hand.

Brown rot in an exterior beam that's turned to dust
A beam with brown rot turning to dust

I like to think of brown rot, aka dry rot, as "The Zombie Apocalypse Meets Alien" because the only hope you have is to kill it and seal it off from the world for all eternity. Otherwise, it will come back nastier and meaner.


Eventually, that checkered appearance on the end of a beam will deepen as the rot fungi follow the cellulose and hemicellulose back in towards the home.

Severe dry rot  caused by a brown rot in an exterior beam
Advanced Dry Rot in an Exterior Beam End

Severe dry rot caused by a brown rot in an exterior beam
Deep cubical fractures in an exterior beam

So far, I've shown you the damages done by brown rot fungi to beams. But there's no reason to wait for the telltale cubical fractures to form. There are plenty of other parts of the dry rot fungi itself to look for -- the mycelium and the fruiting bodies, aka mushrooms.


The mycelium is a visible collection of the hyphae, which absorb nutrients from the environment and transport it back to the main body (thallus) of the dry rot fungus. In essence, it's how brown rot that's already buried deep into a wood beam still manages to get the water it needs to keep eating your home.

Mycelium of a dry fungus on an exterior beam

dry rot mycelium on an exterior beam

large mycelium of brown rot on an exterior beam

Mycelium provides a network for the dry rot to establish complex and far-flung growth. Once fully established, the next phase of life for brown rot can begin -- reproduction.


Dry rot fungi reproduce via fruiting bodies, which we colloquially call mushrooms. These fruiting bodies release spores into the environment so the dry rot process can begin again.


These fruiting bodies can be entirely flat and appear to be a pancake covering an area of the wood, semi-spherical and partly attached, or even that of any typical mushroom, cap and all.

fruiting bodies of dry rot on an exterior beam

brown rot mushroom on exterior beam end

Once a fruiting body is evident, there is no doubt that dry rot damage has already occured.


Dry rot is often present without the invading fungi being visible to the naked eye though. Layers of paint or any type of covering can serve to hide the ongoing infestation. Early detection is the key to treating dry rot before the damage becomes extensive. For more, see "A Down and Dirty Visual Guide to Dry Rot in Roof Beams." The best course of action is prevention through maintenance of exterior beams.


The most important fact about brown rot is it cannot be ignored. It will continue to destroy the wood until there is nothing left. Once there is not enough food to sustain itself, it will make fruiting bodies and release spores to land on the nearest new source and begin eating at a new location. In the right conditions, these spores can be carried aloft and become widespread. The spores have the ability to sit dormant for millenia (we've brought millenia old spores from Egyptian tombs back to life) waiting for the required water and wood to begin feasting once again. Alien and Zombies have nothing on brown rot fungi.