I’m contemplating building an elevated deck. It’s small and will be supported by two 6 X 6 treated posts. The distance from grade to the underside of the beam that the posts will support is about 7′. Is the best practice to put these posts into the ground below the frost line, or to pour piers and attach the post to the top of the piers? Putting them in the ground would seem to provide more stability, but of course it makes the posts more subject to rot.
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
The FHB podcast crew chats about whether or not coil stock on fascia and rake trim is necessary.
Featured Video
Related Stories
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
Assuming that the deck is fastened to the house, the posts need only act to support that side of the deck so...better to pour the piers to above grade.
Technically the posts/support has to be to the frostline like any other foundation. By code. Pour a pier so it ends higher than grade. You may need to add shear bracing if columns aren't continuous into the ground.
Concrete piers(sonotubes) to below frost line and just above finish grade, posts set on top with proper hardware(stand-offs), attach posts to beam with appropriate hardware(Simpson connectors). Diagonal bracing at post/beam intersection will prevent racking or install lateral (diagonal)bracing under the deck framing.
Geoff
So far, everyone likes the concrete pier, above grade solution. You said it's a small deck with only two posts...so I assume the post spacing is somewhwere around 7 - 9 feet center to center?
If so, skip all the ballywhoo with sonotubes and such. Dig your holes below frostline, and install your posts. Posts need to be rated for "in-ground treatment"...do not use treated posts rated for "ground contact only" as they do not have enough chemicals to guard against rot. Set your posts, bury them with a soil/cement mix and be done with it.
Soil/cement mix is just backfilling with soil and adding some bags of quickcrete (dry) to the soil. This will help firm up the soil. Do not encase posts in regular, wet concrete. Over time, concrete encasement will seperate some from post allowing rainwater to become trapped between post and concrete causing post to eventually rot. Regular soil mix will do no harm.
Having the posts buried into the ground at least 4 ft or so, will eliminate the need for you having to late install cross bracing between posts when deck is finished.
I've done many a deck like this...seen many more constructed like this, and never witnessed any posts rotting and/or needing replaced. Sonotube/pier construction is perferred method by many, but it is more time consuming, expensive, and most definately requires "ugly" cross bracing when finished. For a small deck, why complicate things? The deckboards will most likely need replaced long before you need worry about any post problems. The decks I have built are over 10 years old now. Decks I've witnessed being built with soil method by others are over 15 years old now...all are still free from any rot.
Good luck on which ever way you decide to go.
Davo
we bury our posts... have for the last 30 years
we pour a footing below frost (40 ")...and backfill with gravel
all posts have to be treated to .40 retention... unless they are for pole buildings.. then we use .60 retention Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Actually, on most small decks knee braces or posts set into the ground are unnecessary if you install the decking diagonally or install a diagonal brace across the bottom of the deck surface.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Actually, on most small decks knee braces or posts set into the ground are unnecessary if you install the decking diagonally or install a diagonal brace across the bottom of the deck surface.
Huh? What does diagonal bracing of the diaphragm have to do with post racking?
k
If the deck floor can't rack and the house can't twist, and the deck floor is tightly tied to the house, then you don't need rigid posts to make the deck rigid. Of course, in practice there's always some motion, which is why I said "small deck".
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Actually, on most small decks knee braces or posts set into the ground are unnecessary if you install the decking diagonally or install a diagonal brace across the bottom of the deck surface.
Actually, the decking does not take the place of bracing on the posts. It can't - it's in the wrong direction.
From an engineering standpoint, the purpose of the bracing on the posts or burying a significant length of the post is to keep the post from tipping over or racking vertically. The decking surface, regardless of how it is laid, provides no strength to prevent post tipping. The posts cannot be practically attached strongly enough to the deck frame to prevent this tipping without bracing or substantial post embedment.
According to the engineer and building codes.
When I frame a deck, I use a diagonal brace to keep the deck square after I install the first end joist. I work off that. I take it off after I have laid decking up to it. Whether the decking is laid diagonally or perpendicular to the joists, the deck isn't moving once the decking is on. Think about sheathing on a wall during the framing process. No need for diagonal bracing once the sheathing is on. Same goes for the decking on a deck. Post to frame or post to post bracing serves to keep the POSTS from moving, not the frame. I think you would need to hit one of the posts with a sledge to significantly budge one under load though. Unless they are buried or conected to the footing with some sort of bracketry. Then they aren't moving at all. For a free standing deck, I burry the posts in the ground. Pour your footing below frost line, install some sort of galvanized bracket/post base to prevent wood-concrete contact. BRACE your post before you backfill of course. I have just poured wet concrete around a post that was braced in a footing before with good results. But in the future I would use the method I outlined above, which is basically what the other's are recomending you do.
> Whether the decking is laid diagonally or perpendicular to the joists, the deck isn't moving once the decking is on.I'd argue with you if I had the energy. Laid perp the deck may be rigid to start with, but won't stay that way as things shrink up and weather a bit.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
> Whether the decking is laid diagonally or perpendicular to the joists, the deck isn't moving once the decking is on.
Laid perp the deck may be rigid to start with, but won't stay that way as things shrink up and weather a bit.
Won't make any difference, no matter how it shrinks or weathers, although if you are getting much shrinkage, you probably should be using better and graded material.
If you aren't convinced, perhaps you know an engineer who could run through the calculations for you. The decking orientation very clearly makes no practical difference in the strength of the deck, and certainly does not add any strength to stabilize the posts.
I don't see any problem with what you are doing.
Whether the decking is laid diagonally or perpendicular to the joists, the deck isn't moving once the decking is on.
The decking, assuming sufficient attachment, will keep the joists and deck from racking, as you note. Diagonal decking will provide a bit more strength, but as you imply, not enough to make any practical difference.
Same goes for the decking on a deck. Post to frame or post to post bracing serves to keep the POSTS from moving, not the frame. I think you would need to hit one of the posts with a sledge to significantly budge one under load though.
Two common failure modes are when the deck is overloaded, causing the posts to tip and collapse, and when the attachment to the house fails, causing the deck to tilt and fall. Decking, diagonal or otherwise, will not help these failure modes. I've not seen a failure or read any failure reports where the orientation of the decking would have made any difference.
I might add that the problem being addressed is not the potential of collapse -- most deck posts are plenty strong and stiff to prevent that -- it's the attachments that fail. Rather, the problem of concern is the unsettling (and sometimes structurally threatening) sideways motion of the deck surface. I've been on many a deck where someone coming up the stairs would make the whole deck shake like an earthquake.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Ok, I'm going to stand up for you that diagonal layout will make the diaphragm stiffer.
And, you have qualified your initial statement by adding the part about the deck being secured to the house, and the house not moving. As of your first post, no one had ascertained whether the deck was freestanding or attached, and if attached, whether it was adequately so.
If you're making assumptions about attachment, you should do that up front, when you make statements about the diaphragm.
k
The usual reason for adding knee braces and the like after the fact is because the deck is "wobbly".I'll agree that a free-standing deck needs more bracing, but if it's a high deck (over, say, 5 feet up) then knee braces or ordinary 4x4 posts in the ground aren't sufficient. And even for a free-standing deck diagonal bracing of the deck surface will add significant overall rigidity and strength.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
And even for a free-standing deck diagonal bracing of the deck surface will add significant overall rigidity and strength.
To the diaphragm, yes. To the racking resistance of the posts, not one iota.
k
But with a rigid deck floor the posts reinforce each other. Without a rigid floor they tend to move independently.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Pour piers to the frostline into a Sonotube seated in an integral footing form (looks like a big plastic cone). Before the concrete dries, insert a post bracket in the top of the pier. You can then use standard 6x6 lumber instead of the poisoned stuff, since the bottom of the post will never be sitting in water.
The tricky part about pouring piers is to get them exactly where they need to be, because you can't really move them once they're done.
Best way to do that is to build your deck frame first, brace it square, and support it on temporary posts offset from the pier locations to give you room to dig. Dig your holes by eye and drop your Sonotubes and footing forms into the holes. Hang a plumb-bob from the center point of where each post will seat on the beam. Cut four slots in the top of each Sonotube to hold cross-strings to locate the center of it, and put that center right under the point of the bob.
Now you can backfill around the Sonotubes, tamping lifts of fill progressively and keeping an eye on the plumb bob to make sure the tube doesn't shift.
Pour.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Edited 7/4/2009 11:54 am ET by Dinosaur
I'd do just as you said except I mark the post/sonotube locations and dig out before buiding the deck. Dig the holes just big enough for some play room. Then builld the deck and pour the sonotubes/ install posts. That way I dont have the deck in the way when Im digging the holes
. Live by the sword, die by the sword....choose your sword wisely.
I'd do just as you said except I mark the post/sonotube locations and dig out before buiding the deck. Dig the holes just big enough for some play room. Then builld the deck and pour the sonotubes/ install posts. That way I dont have the deck in the way when Im digging the holes
Yeah, that'll work too. If the deck is going to be 6' or more above grade it doesn't really matter much (unless you're going to dig with a backhoe), but as long as you put the crib up where it needs to be before you backfill around the tubes, you can get the piers spot on.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
QS,
Both methods are commonly used and both have pros/cons.
For most decks I build for clients, I bury the 6x6's. I worked with a crew once who just built decks and so I use their method: auger holes, drill 1" holes through bottom 18" of post, drop in precast pad cut to fit hole, drop post in hole, drop in 4 bags dry conc. mix, back fill with soil, sledge hammer top of post till it stops moving, start building deck.
For porches or screen rooms, or any "deck" with a roof attachment I use concrete piers. Whereas some guys pour to a mark inside the Sonotube, I like to set the pier forms to the exact level needed.
To start, you need to auger to the correct depth; this means auguring shy of 42" then hand digging to EXACTLY 42". To set my pier forms I use a combination of string lines, a laser and a 16" level. I screw a 1x4 to each side and screw two stakes to each of the 1x4's. I adjust height, level, and alignment with the screws until it is perfect. Since the tube form is 48" and the hole is 42", you want the top of the form to set at exactly 8" above grade. This means the bottom of the form is hovering 2" off the bottom of the hole.
If you are mixing by hand, which I usually do, mix the first batch a little stiff and pour it in about 1 foot thick. It will ooze out and create a widened footing. Mix the rest normally and pour to the top then mag level. Don't forget your rebar; 4@42" each for a 12" pier.
I don't like to imbed metal connectors in the wet concrete; since pouring is such an intense/high anxiety time there is too much going on that might cause error. Instead I wait a day or two for the piers to cure then I drill into them and imbed threaded rod using Rockite anchoring cement.
Here's a pic showing the tube form alignment method.
View Image
good luck
DC
Here's my detail. Footing left 8" min. below final grade. Post centered w/ 3/8" rebar pin.
Be tough to move one with a freaking farm tractor!
K.I.S.S.
View Image
View Image
The best reward for a job well done is the opportunity to do another.