AG-rated pressure treated lumber in below-grade concrete; and 1/4″ rot-resistant material
Going to be building a 15×10 attached screened-in porch off the back of my house this year. Been working with an engineer I know regarding the structural details. For tie-in to resist rotational forces at the joint purposes, the corner posts are spec’d as 3 pressure-treated 2×4’s with a 1/4″ spacer between each one (to bring the outer ones out to the same thickness as the spec’d 1.75″ wide LVL’s) instead of a 4×6 deck post. The posts will be 36″ underground and sit on an 8″ deep 20″ wide footer that is 42″ underground, then have concrete poured around them the entire 20″ width of the hole to 6″ above grade with a tapered top to direct water flow away. The bottom of the posts will actually sit on 2″ tall composite spacers to get them above the height of the joint between the footer and the pier in case water penetrates that joint. One goal of the project was to minimize support posts to minimize visual breaks. This porch is being built off of the dining room where I put in the 6′ glass patio door last year, for those of you who saw that project. Great view of the woods. Burying the posts versus attaching them to the top of the piers with brackets adds substially greater resistance to lateral and rotational forces… which is important when asking 2 structural supports to do the job that at least 4 would usually be doing. So two questions I’d like your input on:
1. Any issues with using “above grade” -rated PT lumber in this application instead of “ground contact and fresh water immersion”-grade? The engineer doesn’t think so since the lumber will be encased in 20″ of concrete and spaced above the footer/pier joint on the composite material… but also encouraged me to do more research on the matter. I intend to e-mail the lumber supplier; but I’m expecting they’ll say “no good” simply for liability purposes since the product is stated as above-grade use. There will be no direct contact with the soil; but I know that water readily passes through concrete as it’s poreous. Anyone of you pros successfully doing this?
2. Looking for suggestions for the rot-resistant 1/4″ thick material to use as spacers between the PT 2×4’s. Thinnest PT plywood I can find is 1/2″ (too wide); and engineer says no deal substituting with 1/4″ untreated plywood. Needs to be at least 3.5″ wide. Total length needed will be ~48′ (~12′ apiece); but does not have to be a continuous 12′ section. Thought about the 1/4″ fan-fold XPS; but had concerns. Namely, I’d like something more rigid (like wood or composite material), there would be a lot of unused leftover wasted, and XPS really holds onto water once it gets wet (accelerating rate of breakdown of the wood its smashed between???)
Here’s the view I’m trying to interupt as little as possible (the various construction materials are cleaned up, now, by the way… this was taken right after I finished the project):
Replies
In general, embedding wood in concrete is worse than embedding it in dirt.
Also, 20" is an awfully large hole. Certainly the footings don't need to be that large, do they?
I was wondering because we built a pretty big enclosed deck and the footers were a max of 12". But we probably had more supports per square foot.
Whenever my engineer and I disagree, I always ask him what type of train he drives because a "structural engineer" would not have said that. Low blow in this case because I have no details. No offense meant. However, wood in concrete that can not be waterproofed is not permanent. Above ground contact treated lumber means above ground, 8". No rotting leaves stuck against it no snow standing around it. ABOVE GROUND, able to dry out.
Then there is the matter of the vault footings with the toothpick posts. If an engineer doesn't specify what material to space it with and how to fasten it toegther to get the strengh needed, how can one know the outcome of the forces.
View or no view, if you are spending the money and taking the time, search for permanence. I would consider a footer to +8" and a steel column bolted to it. It can then be trimmed out to flush with the trim you will be using for the LVL. You were going to cover the LVL on all six sides, right(waterpfoof)?
This is one of the nuttiest ideas I've read about here, in a long time. And you want to make it worse by using materials that are only suitable for above ground use. Good grief.
Get another engineer. Tell him to design a steel moment frame that sits on concrete footers with above ground connections. Enclose the steel with any suitable material of your choice if you don't like the appearance of steel.
There's nothing wrong with using wood, so long as it's a truly rot-resitant product. But (in addition to using foundation rated wood) use solid lumber rather than 3 sticks lashed together, and don't jacket it in concrete.
"truly rot-resitant
"truly rot-resitant product"
That has to be the funniest thing you've ever said!
I appreciate the insights and opinions (of varying levels of bluntness): that's why I come here. There are a couple of different views here; so I'm going to get further thoughts from both sides.
For those against the use of any type of wood underground, have you ever used and how to do you feel about these "Pro-Anchors"?
http://www.menards.com/main/building-materials/post-frame-materials/accessories/6-laminated-column-pro-anchor-3-ply/p-1807211.htm
I will ask the engineer about the steel moment frame. Images I found online look like they'd be a lot of overkill for such a small project; and possibly cost-prohibitive. I am certainly interested in learning as much as I can about all viable options, though.
For those in the "foundation/GC wood is fine underground" camp: I understand that encasing the posts in concrete is universally hated by both groups, what would you use as fill, instead? Compacted stone, pea gravel, sand, just the original dirt taken out of the hole, or something else? Still need to use a footer pad with the post secured to it with some kind of bracket, right?
Any experiences one way or another with the various plastic footer forms designed to be used in conjunction with sonotubes?
http://www.bigfootsystems.com/
http://www.soundfootings.com/Index.php
http://www.homedepot.com/p/Sound-Footings-Plastic-Concrete-Footing-Form-TB22/202532049#specifications
One approach is to embed a piece of rebar in the end of the post and then set the post ON TOP of the wet concrete footing (so there's no depression to collect water around the end of the post). Back fill with non-peaty soil or medium/fine crushed rock -- not pea gravel as it provides no lateral stability.
Note that structurally a post buried in the ground provides stiffness against tilting that a post sitting on a concrete pillar does not. If posts are placed on concrete pillars then additional diagonal bracing is needed to keep the posts from racking.
lateral forces
DanH wrote:
Note that structurally a post buried in the ground provides stiffness against tilting that a post sitting on a concrete pillar does not. If posts are placed on concrete pillars then additional diagonal bracing is needed to keep the posts from racking.
That is what dictated the original design: need to withstand those lateral and rotational forces without having to use bulky diagonal cross-bracing. Looks like the steel moment frame would accomplish this as well... I'll have to see what the estimated cost comes back as for that.
Do you see any benefit or detriment to wrapping the post in some drain wrap style house wrap before placing in the ground? Thought it might help by providing a slight air space between the post and compacted stone/soil... but then worried it might lower uplift resistance for that same reason.
This seems way overcomplicated to me but maybe I'm missing something. Why not use treated 6" X 6" posts dug down below your frost line, sitting on a 12" bed of compacted gravel and continuing up to the floor assembly? That's the way we build 99% of our decks and some of them are 10' off the ground.
Yeah, for once Florida and I agree. (Or, rather, Florida agrees with me.)
Really? Just a compacted gravel footing? That would never fly up here, but I guess I'm not sure why not.
I guess because we're building on very compacted sand. I've built dozens of piling houses with nothing under the piles at all. Certainly when we drive piles for houses nothing goes under them. Actually I've only started putting gravel under my posts in the last couple of years. I was spanked by a BI last year for not pouring "a bag," his words, of Sakrete around the posts of a beach front porch. Still not sure what a 60 pound bag of Sakrete will do but we did it anyway.