Cottage Conundrum: FPL Fields Cleaning Question

All of the research and expertise of the Forest Products Laboratory (FPL) is in vain if we can not use it to better the American people. Our lab receives many questions for our researchers, and recently, we received a question regarding T1-11 cedar siding.

Dear FPL,

I have a problem with the T1-11 Cedar Siding (plywood) that was installed on my cottage back in 1986. There is a black fungus on all sides of the cottage, even where the front is exposed to the direct sunlight most of the day. I would like to have a vinyl siding installed over the T1-11, but am not sure if I should have that done without washing the siding first. I have washed the siding in the past and the fungus has returned! I would appreciate any advice you can provide to me.

Cottage Conundrum

Luckily, Mark Knaebe, a Natural Resources Specialist with the Forest Products Marketing Unit, had the answer!

Dear Cottage Conundrum,

The most important question here is what do you really have?  If you used what is commonly called a “wood bleach” which is oxalic acid, and it brightened up, then you have iron stain. Iron stain is not a fungus.  


Although it leaves wood black, iron stain is not a fungus. The above example was caused by corroded fasteners.

If you used a chlorine or oxygen bleach and it brightened it up, then you have mildew, which is a form of fungus, and it may not be a huge problem except it that it can keep wood wetter for longer periods which can promote decay fungus (rot) in very wet areas.

I would not give up on the T1-11 just yet.

If you are set on putting vinyl over it, and it is iron stain, you can ignore the black. If you have mildew, you should find out why first, and if bleach cleans it up, you could just bleach and put up the new siding paying special attention to proper flashing. Traditional penetrating coatings contained oils which are food for mildew so as soon as sunlight destroys the mildewcide, mildew can quickly return, so that might be what you’re experiencing.

The bottom line is find out what it is first, and take the next steps from there. Thanks for your question!