What Caused This Wood Deck Failure?

[Runner]

Something funny I noticed is that the blocking between the 2x10 joists, which is made from the cutoffs from the joists, is all in almost perfect condition. Whereas, the adjacent joists are rotted like crazy.

 

[sixdogs]

That's moisture coming up from the ground.A plastic barrier with gravel on top would have worked. I've read many times that most decks rot from the bottom up.

I would have thought the concrete slab underneath would be somewhat of a moisture barrier?
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I didn't see the concrete and you're right. I would see concrete as a least a partial barrier. Now I'm stumped as well.

 

[2manyrocks]

I would have thought the concrete slab underneath would be somewhat of a moisture barrier?
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Concrete may feel dry to the touch on the surface, but has plenty of moisture inside due to concrete acting like a sponge soaking up moisture.

 

[sixdogs]

Concrete may feel dry to the touch on the surface, but has plenty of moisture inside due to concrete acting like a sponge soaking up moisture.

I have seen concrete sweat from the ground up. Would that have been enough to cause the problem? Maybe the restricted airflow combined with the concrete sweating during temperature change?

 

[5030]

You screwed yourself with the solid skirt. Same deal here but I used .40 retention lattice skirting which allows moisture egress as well as air flow and ours is just fine. Like when I use .40 retention fence posts, I always encapsulate the in ground part of the post in hot tar before setting, always.

 

[JimRB]

I have not used this but I did see it in Fine Homebuilding or somewhere. https://trexprotect.com/ A tape to put on the top of deck framing to help shed water and resist rot.

 

[2manyrocks]

My suspicion is the skirting trapped the moisture coming up from the concrete. It doesn't have to sweat for that to happen. The concrete will absorb rainwater and groundwater like a wick.

A properly poured and finished concrete pad will outlast wood decks by a wide margin.

One thing they did is keep the pad below the house framing. That created a step down that is perhaps inconvenient, but the good news is the house framing shouldn't rot due to having a concrete pad poured up against it.

 

[5030]

In my humble opinion, .40 retention lattice looks a heck of a lot better than solid boards anyway. It's cheaper, easier to install as well.

 

[jyoutz]

I have never liked wood decks due to the maintenance issues. When our concrete patio started cracking and spalling, I just covered it with paver bricks. The outside row is mortared to form a solid edge, and the interior bricks are just sitting on the concrete or sand on the concrete where the slab was unlevel. This has been in place over 10 years and sees a fair amount of snow and frost heaving, but since bricks are floating on the slab, it works.

 

[2manyrocks]

Pavers can really upgrade the look of a concrete pad. Yours looks very nice.