Wood in contact with Grout/Concrete: Flashing Solutions 1

[medeek]

This question is more about moisture barriers and what is acceptable use and practice than a specific structural question but it does affect the structure so here goes:

I have a client who has a restaurant with a slab on grade foundation. He is about to resurface the slab with about 2 inches of grout/cement/concrete and thereby present a finished surface that appears to be a fancy Italian marble floor or something along those lines. The interior of the entire structure is gutted so there is no drywall left on the walls. To prevent the grout from filling the wall cavities he has installed a 1/2 plywood "baseboard" around the entire perimeter. The plywood is untreated so naturally my first concern when looking at this was you probably need to provide some sort of flashing or moisture barrier to prevent direct contact between the plywood and grout per Section R319 of the IRC or Section 2304.11 of the IBC.

The plywood retainer/baseboard serves no structural purpose however it does prevent the concrete from directly contacting the wall studs. In this scenario what would be permissible or the best option:

1.) Replace 1/2 plywood retainer strips with pressure treated plywood.
2.) Flash plywood with 6 mil black plastic
3.) Flash plywood with 30# felt
4.) Flash plywood with stick on membrane (ie. Grace Ice and Water Shield)
5.) Flash with felt and metal flashing
6.) Remove the 1/2 plywood retainer strips and install drywall on interior walls then run the grout up to the drywall (this one seems problematic to me)

There may be better options these are just the six that came to mind at the moment. I may be completely overthinking this though so this question is a sanity check if nothing more.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[John2025]

In my neck of the woods, any wood that's in contact with the concrete is supposed to be PT, so I'd go with option 1. If you flash it, it'll still be in contact with the existing SOG.

 

[medeek]

That thought just occurred to me as I posted this, thank-you John2025. Follow up question would be would it make sense to flash the pressure treated strip with anything or would PT be enough. Personally I am partial to a pressure treated baseboard with a layer of stick on membrane, but this is probably overkill considering this is an internal application.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

I think the whole "wood in contact with concrete or masonry" is overblown unless there is an inherent moisture problem. I have looked at hundreds of houses where un-treated crawlspacw beams sit directly on masonry piers without incident. Every old building in our area have the joists and rafters embedded in the solid masonry walls and function fine for close to 100 years. If there is an area where water runs down the wall, that does cause a problem, however.

 

[medeek]

Anything I have ever read about wood in contact with concrete has always stated that the wood wicks the moisture out of the concrete and will eventually rot. The client has already installed about 200 lineal feet of the "baseboard", I hate to have to tell him to rip it all up and replace with pressure treated but that is what I'm about to do. Of course the entire question never arose until I stopped for a site visit on an unrelated matter.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

I hear that too, but my experience says otherwise.

 

[boo1]

Nathaniel, what happens if it does rot?

 

[msquared48]

House go boom!

Mike McCann, PE, SE (WA)

 

[Brad805]

You may wish to consider blueskin as an alternate to ripping out the existing ply. I have seen a lot of wood come off a foundation looking just fine with a simple sill gasket; however, I live in a dry climate.

 

[medeek]

How about apply some of the paint on wood preservative (Copper Naphthenate).

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Ron]

Option 3 is all you need.

 

[BUGGAR]

I've seen a section of asphalt composition shingle placed asphalt side up between wood beams and concrete. As long as water doesn't collect, this seems to work well.

 

[Teguci]

The weakness to any flashing or paint system would be the bottom plywood edge that probably sits on the existing SOG. Even so, I'd be tempted to just put an adhesive flashing on the face of the plywood and expect some moisture penetration at the bottom (not building a boat here). I have witnessed a house where wood (had) sat directly on the top of a stone retaining wall. That wall settled due to rot and insect infestation through the bottom foot or so. Another example is a garage where the slab, probably due to the interior drainage slope, was cast above the bottom wall plate. Again, some rot was encountered during a recent renovation.