Asked on Aug 31, 2014

What is this on my deck?

by N'neka
This is on my 10 year old, untreated deck. It's been there 2 or 3 years and it's only on one board. My hubby thinks it's mold but it looks like tree sap to me. We live on a wooded lot though we've cleared out several trees.
  16 answers
  • Donna Byram Donna Byram on Aug 31, 2014
    Looks like tree sap and tree sap fungus. Gardening Know how recommends using non-diluted Murphy’s Oil Soap. I would scrape off what I could then pour directly onto the affected surface. Allow it to sit for about fifteen minutes. Then scrub with a brush and rinse. The oil-based solution softens the sap residue, making it easier to remove. One note – this works best on finished or sealed decks.
  • Adrianne C Adrianne C on Aug 31, 2014
    It's sap. Try mineral spirits and a scraper to remove. I'd pressure wash the deck and stain it with a good deck stain.
  • Hope Williams Hope Williams on Sep 01, 2014
    It's sap! If it's hard, you can sand it. If it's wet, just use mineral spirits. Either way, it should just come right off after after 3 years.
  • Anny Palanzi Anny Palanzi on Sep 01, 2014
    It seems odd that sap would be on only one of the boards. Without actually seeing and touching it for myself, I suspect mold. Pressure washing is the best idea, then if it's still there, you might contact a lumber yard. Otherwise, after washing, seal it with a waterproofing stain.
  • Bonnie S Bonnie S on Sep 01, 2014
    That board was not treated /processed correcting. Have you tried turning it over?
  • Kelly S Kelly S on Sep 01, 2014
    It could be pitch coming up out of the board itself if it was installed green. Some of my front porch posts were doing that. I let it harden and then scraped it off with a razor blade scraper. Does your roof line extend from the house to about there? If it does, part of the mess could be bird or bat poo or roof edge drippings. Put on a respirator and scrape then bleach to get rid of it. Replace the board if all else fails. It appears to be screwed down so that should be relatively easy.
  • Patty Gonzales Patty Gonzales on Sep 01, 2014
    If it is mold, I would suggest wearing a mask when you try scrape it, so that you don't breath in any dust. Not a bad idea to do, no matter what it is, though!
  • Linda Tatom Peterson Linda Tatom Peterson on Sep 01, 2014
    Well, since you say your deck is un-treated, I'd say its fungus. I would go to a lumber store and ask what treatment to use.
  • Joanne williamson Joanne williamson on Sep 01, 2014
    I agree with Kelly.I have seen this before and it is only on one board.
  • Lauretta Cofer Lauretta Cofer on Sep 01, 2014
    We have a roof over our Deck and on hot days there is Pitch or Tree sap that drips down form some of the Boards up there onto the Deck and furniture. My husband Scrapes it off with a putty knife and we wash it with Dish soap! We used all treated Lumber to build it. Sometimes this happens. If it's a fungus Cleaning it with baking Soda should kill it!
  • That is sap, or pitch depending on what you want to call it. The only proper way to fix is to remove it by replacing the board. This is very common on lower grade treated lumber found at most of the home centers. Simply pull it off and put a new board back on. The wood itself is compromised and is not as strong as it should be. Any scraping and effort to repair most likely will be short lived and it will need to be gone over again in short order.
  • N'neka N'neka on Sep 01, 2014
    Thanks for the answers. I've never flipped the board and the roof line doesn't extended over it. I actually want to rebuild the entire deck so there's good info here for when we do that.
  • Gail lichtsinn Gail lichtsinn on Sep 01, 2014
    some of it is tannin from the wood and also black mold and or mildew
  • SandyG SandyG on Sep 02, 2014
    As stated, it is sap/pitch from the board. Pine boards are the worst to do this. Not all treated boards are treated correctly or dried correctly or long enough and the sap from inside the board. Even with using deck stain or paint, it will still seep through. Flip it over or replace it.
  • Connie S Connie S on Sep 02, 2014
    I worked in a mill, doug fir will do this too, pitch AKA Sap, will seep out of the boards over time. Any really bad like this you should just replace... some you will just see a few drips as green lumber dries, then no more..those are okay to leave, just clean up the mess.
  • Paula Hanson Paula Hanson on Sep 02, 2014
    It's pitch or sap seeping from the board. My deck did this as well.