How to lay hardwood flooring over yellow tongue?

[wolive]

I have a new room in my house adjoining an existing room. The existing room has 19mm yellow tongue down with 14mm hardwood flooring laid on top. The new room also has yellow tongue and I intend to lay the same 14mm hardwood on it to match the exisiting room. The subfloor underneath the new room is a bit more enclosed than what the existing room is so the humidity can get higher under there. I’ve put ventilation in so not too worried about it, but I just wondered if I should put anything down on the yellow tongue first before I glue and staple the hardwood t&g boards down. Just thought if the humidity rises at certain times of year, it might transfer through the yellow tongue into the hardwood? I can’t put anything down like cement sheeting as I can’t build the height up otherwise the new floor won’t be the same level as the existing floor. Someone suggested applying a thin waterproofing or just damp proofing to the yellow tongue first. Something compatible with it that can still be glued and stapled to. Does anyone know anything about this and could suggest anything?

[JacobZ]

Hi @wolive,

Thank you for your question about laying hardwood flooring over YellowTongue.

YellowTongue is resin-infused, making it water resistant, so moisture transfer in the form of humidity won't really be an issue. If anything, temperature differences could cause moisture to condense on the underside of the YellowTongue which will then form beads and eventually drop off the underside of it.

You could always attach some builder's plastic to the top of the YellowTongue to act as a vapour barrier, although I don't think it is 100% necessary. This could give you some peace of mind for a relatively low cost.

Let me know what you think.

Jacob

[wolive]

I did initially think of something like that but then realised I wouldn't be able to glue to it. The hardwood flooring needs to be glued and nailed. I've just read about how moisture can make the boards cup and such, so I thought I should try and protect them with something and wondered if there is a paintable product that could be put down that could still be glued and nailed to. The dirt of the subfloor below experiences some rising damp (especially after a lot of rain), which has led to a little bit of mold forming on a couple of the bearers in the past. Hence the reason why I added the vent into one of the external walls to improve ventilation. I'm sure the yellow tongue will be fine, I just wonder if the hardwood flooring is more susceptible to moisture coming up through gaps in the yellow tongue and such. Maybe I'm worried about nothing, but thought I'd ask in case anyone knew. Should I instead apply some sort of primer to the yellow tongue to help with moisture and adhesive bond?

[JacobZ]

Hi @wolive,

Unless it is explicitly specified in the installation instructions for your hardwood flooring, there's no real need for anything between the YellowTongue and the flooring. If it was explicitly specified, then I would suggest contacting the manufacturer of the hardwood flooring for their guidance on what should be done.

I think it is fantastic that you are putting the thought in, but I just don't see a need for it and can't find anything stating that it is a requirement.

Adding the ventilation to the floor cavity is the best thing you could have done to prevent moisture issues. If anything, sealing any gaps in the YellowTongue with a bead of your flooring adhesive would be the best thing you can do at this stage.

Let me know what you think, and if you have further questions, please don't hesitate to ask.

Jacob

[wolive]

I sort the advice from a local hardwood flooring installer who also confirmed that nothing needs to be done. Just make sure the yellow tongue is flat (sand down any high spots or ridges) and then make sure the adhesive is troweled over the entire surface of the yellow tongue. I presume the adhesive acts as a bit of a barrier as well. Thought I'd leave this comment for anyone else that comes across this.

[JacobZ]

Thanks for coming back with that information @wolive. It's great to have a base of information that others can read in the future.

Don't forget to take some photos as you go. It would be great to see your flooring project shared as a project post once you are done.

Jacob