Asked on Oct 23, 2015

MDF board under roof on screen porch

Is there a right & wrong side to put up on a screen porch for the MDF panels that the roof is attached to? Two panels are turned on the smooth side showing inside on the porch roof, they're not flaking off. All other panels are turned on the rough side showing inside porch. Now flakes of wood are falling down on porch & furniture from rough side of panels. What can be done without having to remove the MDF plywood & roof,as this is a new porch? We paid a lot for this screen porch & now have found we had a bad contractor!! Can it be sealed with something to stop the flaking off of wood? Help me, all suggestions are appreciated. Thanks Donna
  18 answers
  • Z Z on Oct 23, 2015
    This is not MDF, but OSB or oriented strand board. If it has tongue and groove on the edges then there is a right and wrong, as they will not fit together properly. Those do have a marking tell you which side is which though. If there is no tongue and groove then there's no right or wrong way. I would suggest sealing and then painting the OSB to stop the chipping.
  • B. Enne B. Enne on Oct 23, 2015 whole garage has OSB--ceiling and walls, and it has never chipped like this. @Z although the boards should be printed on 1 side only, some are printed on both sides, as I have discovered on my laundry room wall in my basement, or not at all so unfortunately that is not always an indicator. The one seam looks a little black, could there be moisture or rotting between the roof and the panel? These links may help:
    • See 1 previous
    • B. Enne B. Enne on Oct 23, 2015
      @Z got you LOL
  • The printed lines are for guides so they are generally put right side up for the contractor or DIYer to use the lines as guides for roofing. or flooring. Flaking off seems a bit odd because I have used OSB in many projects and not had this. I would vacuum and seal it with a polyurethane. Or I would think about putting in a ceiling in your new room which could be planked with treated fence pickets, sheets of 4x8 siding, cedar planks, 1" thick lumber, or even paneling. That way you can insulate the ceiling if you ever decided to put storm windows or enclose the porch later. Just some thoughts
    • Paul M Paul M on Oct 09, 2016
      OSB flakes up to a point if it is protected from the environment. If it not protected it will continue to flake until there is nothing left. Depending on conditions this can take 1 to 5 years for total disintegration. OSB is not meant to be in an unprotected space exposed to the elements.
  • Stp1309848 Stp1309848 on Oct 23, 2015
    I used some old paint (white, light blue) once painted it made it smoother
  • Rus1058682 Rus1058682 on Oct 23, 2015
    I built houses for years, and the standard rule is "slick side down", which is the side with all the lettering, and is the sealed side. In your photos, it shows some are up, and some are down. If it were me, I would either paint it or clear seal (like Thompsons). I hope this helps...
  • PhilCo PhilCo on Oct 23, 2015
    Many people wont use OSB, but it's cheap and has it's place, in my opinion inside! or at least away from the elements, in a porch, it will get damp and start to break up if not sealed. As has been suggested I would paint it.
  • Sophie Breau Sophie Breau on Oct 24, 2015
    If you rather not paint it, seal it (polyurethane ...etc) low sheen... But Phil is right, it's particle board, it's bound to come appart with the change in weather. Good luck.
  • Dave Dave on Oct 24, 2015
    That is not mdf it's particle or Stirling board you should not use mdf on a roof
  • Rus1058682 Rus1058682 on Oct 24, 2015
    It's osb. standard roof material. paint or sealer are good choices, or you could put a ceiling in, using 1\4" B\C plywood, and seal it with a poly coat..
  • Jhunt10 Jhunt10 on Oct 24, 2015
    It's OSB ( oriented strand board ) seal two coats with Thompsons. Or seal with Kiltz ( white) it will prevent moisture from delaminating the surface.
  • Jlhicks Jlhicks on Oct 24, 2015
    cover it with strips of thin beadboard
  • Louis Lieberman Louis Lieberman on Oct 24, 2015
    fact - mdf is water sensitive- u can try to seal it with a lacquer-based or polyurethane wood sealer -i use something called sanding sealer
  • David Gnida David Gnida on Oct 25, 2015
    All good answers above. Just cover it with something. Kinda like the beadboard idea because it will look nicer than just putting a coat of sealer or paint over it. Or get creative and try using burlap sheets and 1x2 strips of wood painted whatever color you want to reinforce the ends. If you do it right the burlap looks pretty nice. Look up some pics. You might like it.
  • Pat Pat on Oct 25, 2015
    First of all, it is not mdf, it is osb. The best thing you can do is paint it with exterior paint.
  • Pat Pat on Oct 25, 2015
    First of all, it is not mdf, it is osb. The best thing you can do is paint it with exterior paint.
  • Pat Pat on Oct 25, 2015
    First of all, it is not mdf, it is osb. Best thing would be to paint it with exterior paint.
  • Paul M Paul M on Oct 08, 2016
    There is no way I would ever use that as a deck board. It needs to be painted with a very good outdoor grade paint, let it dry for a few days and put a real deck over it. That is subfloor material and should be treated accordingly. It's not going to stand the test of time outdoors unless you do something to it. Pressure treated plywood would be a better choice if you want to just have a single layer floor and don't want to deal with deck boards but OSB is not good unless protected from the elements, all of them.
  • MrsSherri MrsSherri on Oct 09, 2016
    Paul M is right. I think you could be looking at some water damage issues in the future. That being said you could always pretty up you're porch by adding bead board an painting it a relaxing color.