How to protect 2x4 deck framing from ground moisture?

Question

I am building a deck on a concrete patio. The patio already does have a roof. It won’t get too wet during a rain and we don’t get much rain during the summer (Northern California).

I plan to do the framing with 2x4s, which will sit on the concrete.

• What should I use to best break the contact between the framing and the concrete to not trap moisture? Shims or tape flashing?
• Should I use brown colored ground contact 2x4‘s or should regular ones be fine? (Slab is raised and about 8 inches above the soil)

Answer 1

Use ground contact (GC) rated pressure treated wood. Don't skimp by using untreated wood or even above-ground; ground contact or bust! In addition to its rot resistance it is generally less susceptible to damage from bugs since it has undesirable chemicals.

Per https://www.homedepot.com/c/ab/types-of-pressure-treated-wood/9ba683603be9fa5395fab9052c50759

Ground-contact pressure-treated wood:

• Ground-contact pressure-treated lumber can be used either above ground or in contact with the ground.
• Has twice the level of chemical retention and protection compared to above-ground treated wood.
• Must be used when lumber is less than 6 inches from the ground or has poor ventilation.
• Must be used for applications where wood is difficult to maintain or replace.

If you use pressure-treated and there is seldom standing water on the concrete then that deck will easily outlast your lifetime.

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If you wanted to really overkill things then you could cut PVC shims from trim board and glue/tack them to the bottom of all your joists so that they float 1/2"-1" above the concrete.

Flashing tape could also work but my fear is that it has the potential to lock moisture into the wood.

Answer 2

You do not want any direct contact between your wood and the concrete. The wood will draw moisture out of the concrete which will rot your wood over time.

There are any number of commercial products that you can buy that will break the contact between the wood and the concrete. It's hard to say what will work best since it's not really clear how your patio is designed. But something like this may be helpful:

Wood Support