Moisture barrier on OSB before glue down wood floor?

markocosic · July 6

How can I waterproof an OSB sheet whilst making it possible to glue down a floor over it?

I have been asked to do something silly by SWMBO - glue down wood floor over OSB subfloor - in a bathroom, kitchen, and at the entrance to the house.

We have done this before with engineered wood over chipboard on the 1st floor (UK, but main entrance on ground floor was tile) and engineered wood over a screed (Baltics, but the main entrance is 4 flights of stairs to leave the worst of the water and snow on) and gotten away with it.

I have now been asked to do this with solid wood (28 mm x 120 mm pine) in a country house. This is going to move more - there will be gaps at times - and there will be more rain and snow. I'm not worried about the solid wood. I am a little worried about the OSB if water seeps through the joints.

What can I do to "waterproof" the OSB so that any water is sucked up by the solid pine sponge and dried to the house from there; rather than soaking into the OSB sponge and turning it into wet weetabix? Spread a think layer of MS polymer floor adhesive and let that kick before spreading the main layer for the floor perhaps?

The entrance and bathroom are the risk areas.

I am allowed a permanently "cut in" entry mat in a tanked area (remove the pine and make a framed "pit" for a mat to sit in with just the OSB as the base) as a concession to the worst of it.

Should I be using MS Polymer here; or would a urethane the ties the OSB and pine together mire rigidly and forces the assembly to move as one be more appropriate? What would you use as a primer there?

saveasteading · July 6

2 hours ago, markocosic said:

there will be more rain and snow.

This is outdoors?! Don't use osb.

Conor · July 6

I'm struggling to understand the context of this. Outdoors? Indoors but not water tight yet? OSB3 is moisture resistant and won't soak up water unless exposed directly to persistent liquid water.

markocosic · July 7

Indoors; but the outdoors comes in when people come in. (impossible to remove all snow from shoes; design didn't allow for a porch/utility)

OSB (not OSB3) already down and not straightforward to change. Should have been OSB3 but was lost in translation.

markocosic · July 7

Reading around more I'm overthinking this.

By the time the OSB is wet enough to worry the wood above it will be plenty swollen and discoloured.

Lay it, and it, hit it with polyurethane, try not to abuse it. A dry winter heating season will do more harm (opening up gaps etc) than the odd bit of water that does make it down the gaps and into the pine boards. To actually soak the OSB would need a material leak over a material amount of time.

Do set in an entry mat though. Perhaps even pop a heating mat under it to dry it.

markocosic · July 7

Something like these. Either frame and tank a 28 mm deep "bucket" to catch the slush (and tuck a UFH mat under it if needs be) or sink an actual tray through the OSB subfloor onto a framed support:

https://www.senukai.lt/c/santechnika-ir-sildymas/nuoteku-surinkimo-ir-drenazo-sistemos/drenazo-sistemos/drenaziniu-lataku-sistema/batu-valymo-groteles-voneles/bv2

markocosic · July 7

It would bring a new meaning to "sweeping the dirt under the carpet" too lol

JohnMo · July 7

Liquid DPM, epoxy based? Paint or roller on.

markocosic · July 7

That shouldn't lift and should take floor adhesive. Good shout.

(I had "tanking" in mind as you'd do for tiles that's soft as heck)

markocosic · October 9

For what it's worth this is what we have done did:

MS Polymer floor adhesive, notch trowelled over SANDED OSB (told by man in shop who seems to know his stuff that it is important to sand to remove wax coating on the boards that's there for temporary site waterproofing), which at a 1300g/m2 application rate effectively gives you a thin layer of glue over the ENTIRE surface (aka an air moisture barrier - be sure to work it into the holes created by your wedge blocks as you go too) PLUS your raised ridges of glue to lay the wood onto. Secret screwed through the tongues too to keep it in place/persuade it to be straight/allow me to walk on it instantly.

https://renove.lt/lt/klijai-ms-elastic-400-12kg

Boards then graded by width (of course they're all bloody different once acclimatised, and indeed different end to end because they're trees) and wedged into submission (September being a "neutral" month for expansion/contractor) with the ends (where there has to be joints) routered/false tongue added/glued to avoid any moisture getting into the "ends" of the grain should anything get wet wet. Set the router depth off the face of the board to set these "flush" and make the glue take up any variation in board thickness (which is also all bloody different because wood).

Prime/stain/2K PU laquer to go over the top in the coming weekends.

For the entry mats I have left a gap (mat well) that is going to be edged in solid brass (say 10x5 mm, as a visual edge and to protect the wood; with the wood/brass sanded to be inlaid flush) then tanked before dropping in an underfloor heating mat and a coir mat to sit flush with the floor. That's hopefully enough to (a) leave wet shoes on safely and (b) dry reasonably quickly in winter. 🙂

Other tips - don't scratch your balls if you have slow cure floor adhesive on your hand and buy kneepads that you'll use and love. I rate these FWIW:

https://lt.misupplies.co.uk/clothing-c12/mens-workwear-c635/trousers-and-shorts-c512/knee-pads-c519/helly-hansen-workwear-79571-kneepad-xtra-protective-p53480?utm_campaign=pr_r&utm_source=www.misupplies.co.uk&utm_medium=wi_proxy&utm_content=lt_LT&utm_term=c

Start with a stringline, attached aline of blocks against it, wedge the heck out of your first boards to have them be bang straight. False tongue in the groove end and then lay boards from both directions having first checked where to start to avoid horrible part board cuts.

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