Growth Between Composite Deck Boards

Asked July 6, 2015, 11:17 AM EDT

Hello! Can anyone identify what is growing between the boards of my TimberTech composite deck? This problem began a few years ago when we added underdecking underneath the deck. It occurs mostly in the shady areas of the deck. I have removed part of the underdecking to look at the underside of the boards, but do not see anything growing there; it grows only between the deck boards. I have tried scrapping it out and using various cleaners, but it comes back. I am hoping that someone will know what it is, so I can find a product that will get rid of it permanently without damaging the deck boards.

The growth started this year about a month ago and is about 1/4" high. In the photos you might notice gray tips - this is where last year's growth was damaged by my cutting it with a putty knife.

Any information would be greatly appreciated..

Fairfax County Virginia

1 Response

I'm not sure what the growth is from the photo's. I assume that the growth is not attached to the deckboards but is coming from material trapped between the underdecking and the deck boards.

Regretfully, by adding underdecking you have created an area that will hold water and dirt, which can lead to growth of fungus and other plants. The decking material used, is rated for non-ground contact. While you are not in contact with the actaul ground, the underdecking has created conditions similar to ground contact (water holding and the ability to hold seeds, spores etc.). The best solution is the remove the underdecking to allow the area to dry after rain and thus preventing the growth.

Another suggestion would be to increase the distance between the underdecking and the decking and to assure that the underdecking has adequate slope for the draining of rainwater.