OSB in new construction

I was taught over and over that lumber, if not treated should not touch the slap/concrete floor. How about OSB? Opinions welcome.

I have heard OSB is only made to withstand 8, 1 hour wettings. After that is starts falling apart/swelling.

I do not like using OSB but the price drives us to use it. I have had 3/4" OSB get punky after (1) 8 hour exposure to water before.

Any lumber touching concrete should be Pressure treated to withstand any moisture migration to it, and in MHO, OSB, should not be used in a moisture environment whatsoever.
They need to give it a good wax job if they expect the product to go from 5/8" to 7/8" after a few wettings.

Marcel :slight_smile: :slight_smile:

You are 100% correct.

I hope that this helps.

If you are going to have any type of wood in direct contact with concrete it has be pressure treated wood. If it is a 4x8 sheet of wood in direct contact with concrete I would strongly recommend that it be Marine Grade plywood.

The International Residential Code.
319.1 Location required.
In areas subject to decay damage as established by Table R301.2(1), the following locations shall require the use of an approved species and grade of lumber, pressure treated in accordance with AWPAC1, C2, C3, C4, C9, C15, C18, C22, C23, C24, C28, C31, C33, P1, P2 and P3, or decay-resistant heartwood of redwood, black locust, or cedars.

  1. All wood framing members that rest on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.

  2. Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.

319.2 Quality mark.
Lumber and plywood required to be pressure preservatively treated in accordance with Section R319.1 shall bear the quality mark of an approved inspection agency that maintains continuing supervision, testing and inspection over the quality of the product and that has been approved by an accreditation body that complies with the requirements of the American Lumber Standard Committee treated wood program.

319.2.1 Required information.
The required quality mark on each piece of pressure preservatively treated lumber or plywood shall contain the following information:

  1. Identification of the treating plant.
  2. Type of preservative.
  3. The minimum preservative retention.
  4. End use for which the product was treated.
  5. Standard to which the product was treated.
  6. Identity of the approved inspection agency.
  7. The designation “Dry,” if applicable.
    Exception: Quality marks on lumber less than 1 inch (25.4 mm) nominal thickness, or lumber less than nominal 1 inch by 5 inches (25.4 mm by 127 mm) or 2 inches by 4 inches (51 mm by 102 mm) or lumber 36 inches (914 mm) or less in length shall be applied by stamping the faces of exterior pieces or by end labeling not less than 25 percent of the pieces of a bundled unit.

Lets not leave out the other products that are super glued together. That are not to come into contact with masonry/concrete.


**1. go to product info **
**2.trim & fascia **
**3.support materials **
**4.installation **
**5. trim&fascia **
Look at the third one on the list under general

Here is an easier link to the installation PDF


Not to get off topic, but I see a lot of OSB roof decking with wooden shingle or shake roofs. About 1/2 the time (mostly in new constructions) about half the roof decking (north and east sides, mostly) is black and soft.

I don’t like OSB.

Just my opinion.

Will hope you are fine.

Could not agree more, I hate the OSB sh$it.
Nothing but problems everywhere I go. Use it for small footing work on the job sometime and then throw it away, because it went from 5/8" to 3/4" overnight, due the moisture. Flammable like no one would expect, I tested it. Unreal.

Marcel :stuck_out_tongue: :stuck_out_tongue:

Concur with the above, hate OSB… be we will not stop seeing it. Price drives its use…

IMHO if any wood (even pressure treated) touches concrete where it’s subject to occasional water (like a door sill or step) it should be prepped and painted as well…

but I’m a little paranoid about moisture intrusion…:roll:

10/4, but in the mean time, I guess we have to live with it and deal with it in the oncurring events of the aftermath.

Marcel :slight_smile: :slight_smile:

Could someone please try to download the document TM420 @
and let me know if it worked. I have trouble with it :mad: and the owners webmaster sees no problem. I tried from two different computers with the same result. :frowning:

Thanks :slight_smile:

Try this direct link or paste it into a new browser window. It takes a while (10 min.) to download on a dialup connection so wait for it.


Marcel, when it first came out they did wax it, and it had smooth sides, so you’d be up there trying to sheath the roof with that stuff, trying to walk around on it, and it was like trying to walk on ice.
Trying to place a bundle of it up on a roof with a lift after the metal bands had been broken was like trying to balance a marble on your nose.
Anyway, OSB’s made of wood and will decay like wood unless it’s treated.

I know, I was up there slipping and sliding, and hated the sh#t, then and still do.
I don’t think they wax it enough for subfloors until the roof is water tight though.

Marcel :slight_smile: :slight_smile: