What Caused This Wood Deck Failure?

[Runner]

The only thing that stuck in my head was "lowes" almost the last word in your post.
Have you contacted the supplier or yellawood?
This is just wrong and possibly keeping some moist air under the deck with concrete is probably better having the normal, wet dirt. I can't imagine that you skirting was anywhere near airtight either. Sorry about this, IMHO you didn't do anything wrong.

I have not contacted them, not sure that it would do any good. They probably have something in their installation guide saying "provide proper ventilation under decks" that they can fall back on if push came to shove.

I really thought I had this one figured out. No part of the structure touches dirt. The beam is sitting on concrete piers, 6" above grade (no posts in the ground) and the rest is 15" above the concrete.
I never in a million years thought that enclosing the area underneath could cause this kind of damage. The idea was to keep the trash and animals out from under there and make it look better. Oh well, live and learn.....

If I had to do it again, I probably would just use some kind of galvanized mesh screening mounted in a picture frame type arrangement for the skirting.

At this point, I just want to get this garbage out of here and get back to a patio. The hardest part will be fixing the siding I had to cut to install the ledger board. I have some extra siding from the original install, but not sure it I have enough. The siding has been on there for over 20 years, so not sure I'll be able to find a match if I have to buy new.

 

[EddieWalker]

A deck needs air flow. Adding vents to an enclosed deck will not provide enough air flow to keep the deck dry.

Most of your moisture under your deck is from condensation. This also what causes water to from inside metal buildings that make it feel like it's raining in them, and leaves the concrete floor wet in the morning. There might be some water from rain when that happens, but moisture coming up from the ground isn't happening unless you built it over a spring.

Just like in the attic of a house, when the air temperature drops to the point it cannot hold moisture vapor, it becomes solid, and forms water. An attic has vents in the lowest part of the roof that allows air to enter the attic, and vents at the highest point that allows air to exit the attic space. When the air gets hot in the attic, it rises, and creates air flow. An attic has to be hot for this to work. The air flow dries off the morning condensation that forms under the roof.

Your deck trapped the air under it. There was not a path for air to flow so it could dry off the underside of the deck. Warm temps, moisture, and the nutrients in the wood created the perfect place to grow mold. The treatment in the wood wasn't able to stop this, and the mold quickly ate up your wood. Treated wood cannot remain wet all the time unless it's specifically treated for underwater use. It's not that the treated wood you bought was bad, it's that you used it in a bad way.

 

[Michiganguy]

Interesting thread as I just had a deck built and the builder enclosed it with non vented mobile home skirting. Now I’m thinking I need to redo the skirting.
For any of you that actually have knowledge about deck ventilation would using vented skirting offer enough ventilation?
The skirting is similar to soffit.

 

[Diggin It]

If I had to do it again, I probably would just use some kind of galvanized mesh screening mounted in a picture frame type arrangement for the skirting.

That's what I did. I used wire fence but 1/4" hardware cloth would be fine. I added some regular window screen as a second layer to keep wasps out.

The hardest part will be fixing the siding I had to cut to install the ledger board. I have some extra siding from the original install, but not sure it I have enough. The siding has been on there for over 20 years, so not sure I'll be able to find a match if I have to buy new.

Consider brick veneer. Looks just like brick but is glued or nailed on and can be cut fairly easily.

 

[Runner]

I've learned my lesson on decks: Never Again.

And, while I'm saying never, no more raised ranch houses either....

Of course, my next house will probably be a slab-on-grade retirement community...

 

[Midniteoyl]

You should use some extra protection: Joist Tape: What Is It and Do I Need It? | Decks.com

 

[Runner]

You should use some extra protection: Joist Tape: What Is It and Do I Need It? | Decks.com

I think you're right. I notice the joists where the rot happened is also where all the screw penetrations are. The blocking between the joists (and where there are no screws) have almost no damage at all. I'll bet this product would help that situation.

 

[ponytug]

I would have thought the concrete slab underneath would be somewhat of a moisture barrier?
View attachment 875817

Yes, and no. Rainwater coming through the deck gets contained in the space, raising the humidity and increasing the speed of the corrosion and rot. As other have written, decks need free air flow with as coarse a screening as you are comfortable with.

Interesting thread as I just had a deck built and the builder enclosed it with non vented mobile home skirting. Now I’m thinking I need to redo the skirting.
For any of you that actually have knowledge about deck ventilation would using vented skirting offer enough ventilation?
The skirting is similar to soffit.

Soffits do not have the water load that decks do. Soffits only need to clear vapor from the house or roof. Decks need to be able to rapidly clear rain and snow to keep the wood dry. It is generally recommend to retreat / reseal decks, including the underside periodically, but very few folks do so.

Pressure treated wood is just one part of resisting decay. Getting a sealer, oil, or water based on the boards and supports when they are dry is important as well.

You should use some extra protection: Joist Tape: What Is It and Do I Need It? | Decks.com

To me this product should be required for all deck builds. I've seen some contractors brush out butyl sealant, but I'm not convinced that the coating is as uniform. (Nobody is perfect, right?)

Pressure treatment of wood only treats the exterior 1/8-1/4" or so. So any cracking, or cuts, or fasteners open up a route for water, insects and fungus to penetrate and begin the decay process. That's why resealing joists periodically is important, and why treating all cuts is important.

Pressure treated wood comes in various grades, and decks generally ought to have as resistant a grade as you can afford, preferably UCB4B. I think that folks often forget that any cut in pressure treated wood needs treating with copper naphthenate per code and manufacturer's instructions...

https://awpa.com/images/standards/ResidentialInfographic2021.pdf

I have a composite deck with pressure treated wood, built using joist tape. Prior to building it, I priced a concrete deck and a steel supported deck and both would have doubled or tripled the cost of the deck here. The prior deck was old growth redwood, and it rotted out in an area that did not have much air flow. (Basically, it is a corner that has a hill on both edges, so there isn't a great way to vent much air. When that deck area eventually fails, I will fill that area and pour a slab, but I didn't know then, what I know now.)

I think that slab on ground or pavers are great solutions, if they work for you.

All the best,

Peter

 

[Midniteoyl]

I think you're right. I notice the joists where the rot happened is also where all the screw penetrations are. The blocking between the joists (and where there are no screws) have almost no damage at all. I'll bet this product would help that situation.

The butyl versions are very good at resealing around the fasteners. I use butyl tape everywhere now, as flashing, under metal, etc..

 

[jyoutz]

Actually, the best alternative (and the most expensive) is engineered decking made from recycled plastic waste. The stuff never rots, looks just like wood, screws down easily and will drain your wallet quickly...lol

Besides, it's 'green' so it's all good, right?

And some of those products have wood fiber embedded in the plastic.