Sink or not to sink deck posts
#1
06-27-06, 07:24 PM
Sink or not to sink deck posts
I am building a retaining wall of 2x6s against 6x6 deck posts. I was going to pour 9" concrete footers in 32" holes, then fill with dirt. This leaves 23" of dirt to hold the deck post below ground, so that it does not shift side to side. In addition, I am setting 48" rebar sunk through the footer on three sides of each deck post to further prevent "walking". The mini retaining wall is only 36" tall and the 6x6s are 48" on center.
All help is welcome,
Tmilan
All help is welcome,
Tmilan
#2
06-28-06, 04:21 PM
Don't bury the wood posts in dirt. Fill the bottom 12" (not 9") of the hole with concrete then put a 28" to 30" length of Sonotube on the wet concrete and fill that with concrete. (It will be about 8" to 10" above the top of the hole.) Backfill around the outside of the tube with dirt as you fill it. Once the tube is full, set a Simpson PBS44 (assuming that you are using 4X4 posts) in the wet concrete. Let that dry for a couple of days, then set your wood posts in the PBS.
#3
06-29-06, 10:02 AM
Tmilan,
The 9" footing at the bottom of the post is not necessary as there exists no structural bearing weight. What is most important here is lateral movement. The replaced soil around the posts will not offer any degree of supporting the wall in the long run. My suggestion would be to wrap a portion of your posts (all four sides and bottom that will be below ground) with Grace (or equal) ice & water shield then bury to ground level(finish grade) in concrete- sonotube unnecessary. You will want to place about 6 inches of crushed stone in the holes prior to setting your posts/pouring. I suggest you purchase .40 or higher retention treated wood. One more thing: 6x6s - 4 feet center could look pretty bulky... You might consider 4x6s or even 4x4s. (save sum bucks too)
Finally, the best method of construcing the wall would be to set each post and brace together/to ground with stakes (stringline end to end) then pour the holes all at once. It's always beneficial to slope the wall slightly inward (about 5 degrees). Before back filling your finished wall, nail tight woven galv. hardware cloth against the inside of the wall (eliminates your back filled stone from coming through gaps in 2x6s) then back fill with crushed stone to 6" below grade. Apply a thick layer of straw (or Amoco Propex---french drain fabric) overtop the stones to prevent clogging then overlay topsoil and plant
The 9" footing at the bottom of the post is not necessary as there exists no structural bearing weight. What is most important here is lateral movement. The replaced soil around the posts will not offer any degree of supporting the wall in the long run. My suggestion would be to wrap a portion of your posts (all four sides and bottom that will be below ground) with Grace (or equal) ice & water shield then bury to ground level(finish grade) in concrete- sonotube unnecessary. You will want to place about 6 inches of crushed stone in the holes prior to setting your posts/pouring. I suggest you purchase .40 or higher retention treated wood. One more thing: 6x6s - 4 feet center could look pretty bulky... You might consider 4x6s or even 4x4s. (save sum bucks too)
Finally, the best method of construcing the wall would be to set each post and brace together/to ground with stakes (stringline end to end) then pour the holes all at once. It's always beneficial to slope the wall slightly inward (about 5 degrees). Before back filling your finished wall, nail tight woven galv. hardware cloth against the inside of the wall (eliminates your back filled stone from coming through gaps in 2x6s) then back fill with crushed stone to 6" below grade. Apply a thick layer of straw (or Amoco Propex---french drain fabric) overtop the stones to prevent clogging then overlay topsoil and plant
#4
06-30-06, 09:30 AM
retaining wall/deck combo
Sorry, I left out that this retaining wall is actually the one side of a deck that is being built into a hill as a 4ft walk out around a pool. The deck will span the distance from pool edge to ground level on one side and wrap around. Right now there is a wall of dirt, with a 40" space between the dirt wall (uphill) and the 38ft long side of an 18x38 oval Doughboy pool. The posts need to support the downward weight of the deck as well as the weight of the dirt pushing sideways from the hill. I am running the deck beams out from the pool 10' into the hill (they will lay in trenches, where they will connect to their footers.
Clear as mud - Tmilan
Clear as mud - Tmilan
#6
06-30-06, 07:11 PM
Deck/retaining wall combination
Gentlemen,
I had another idea, and poured some concrete today. I sunk 24"x1/2" rebar in my concrete footers, and left it sticking out of the concrete @ 12" . The plan is to use the rebar to hold my posts in place. Before setting the posts, I will add 3" of stone on top of the concret, set and true the post, then back fill each hole with stone. Each hole will have a small drain trench leading to the main drain trench for the retaining wall. How long would you estimate 6x6 pressure treated posts would last in such a setup?
I had another idea, and poured some concrete today. I sunk 24"x1/2" rebar in my concrete footers, and left it sticking out of the concrete @ 12" . The plan is to use the rebar to hold my posts in place. Before setting the posts, I will add 3" of stone on top of the concret, set and true the post, then back fill each hole with stone. Each hole will have a small drain trench leading to the main drain trench for the retaining wall. How long would you estimate 6x6 pressure treated posts would last in such a setup?
#7
06-30-06, 08:05 PM
Tmilan,
A Simpson PBS44 or PBS66 is just one of many post bases and column bases that Simpson has. Find a Simpson stocking dealer (pretty much any lumber yard, HD, Lowes, etc.) and browse thru the Simpson catalog. I kinda thought that this wall was somehow going to be involved with supporting that deck. That's why I told you to put the footing at the bottom of the hole and then sonotube up to a level about 8" or 10" above grade.
I'm not exactly sure what you did with the rebar or why. It really wan't necessary. How's the rebar going to hold the post in place? And adding gravel on top of the footing means that there is no continuos support from the post to the footing. You blew it!
A Simpson PBS44 or PBS66 is just one of many post bases and column bases that Simpson has. Find a Simpson stocking dealer (pretty much any lumber yard, HD, Lowes, etc.) and browse thru the Simpson catalog. I kinda thought that this wall was somehow going to be involved with supporting that deck. That's why I told you to put the footing at the bottom of the hole and then sonotube up to a level about 8" or 10" above grade.
I'm not exactly sure what you did with the rebar or why. It really wan't necessary. How's the rebar going to hold the post in place? And adding gravel on top of the footing means that there is no continuos support from the post to the footing. You blew it!
#8
07-02-06, 02:54 PM
almost blew it
I have not yet set the posts, just poured the footers. If I use th tube former on top, will it still be a solid lateral connection? What about the old way of puoring concrete around the post, once I set it on the footer? Also, does dipping the PT lumber in Creosote add any longevity?
#9
07-02-06, 04:39 PM
Member
Sink or not to sink deck posts
You are building a 36" high retaining walls out of wood and are just now thinking of the durability of the wood?
Trying to get a 2x to span 48" and retain 3 feet of wet soil may work on paper if you look at allowable lumber stresses. You will not see it lasting very long with the new growth lumber and preservatives. Profesionals take alternate routes. How many mini retaining walls are built this way then streets and roads are widened or when additional level space is be created?
Right now you are looking at the details of connecting things together. I hope you have looked at the big picture.
A retaining wall is a very difficult thing to build and last. Subjecting the wood to alternate wetting and drying cycle is a classic condition for a short life (and greatly reduced strength and increased deflection). Having moisture behind the wall provides a source of moisture to carry the preservative out to the surface where it is no longer effective.
Your situation is complex from a soil mechanics situation. You could end up with the wall and posts keeping together as they move horizontally is the soil gets saturated (leaky pool, poor drainage, big storm?). Wet soil is much, much heavier than water. You could have 1000# trying to tip each post over and slide it on a steady basis. If your house foundations are anywhere near the wall and post supports you could have additional loads.
Code require walls over 4' to be engineered. Common sense says this wall should be looked at. So far, you have not even said what kind of soil you have and that is a key item, not to mention subsurface drainage.
If everything does not last without moving, this could be a tough repair.
Dick
Trying to get a 2x to span 48" and retain 3 feet of wet soil may work on paper if you look at allowable lumber stresses. You will not see it lasting very long with the new growth lumber and preservatives. Profesionals take alternate routes. How many mini retaining walls are built this way then streets and roads are widened or when additional level space is be created?
Right now you are looking at the details of connecting things together. I hope you have looked at the big picture.
A retaining wall is a very difficult thing to build and last. Subjecting the wood to alternate wetting and drying cycle is a classic condition for a short life (and greatly reduced strength and increased deflection). Having moisture behind the wall provides a source of moisture to carry the preservative out to the surface where it is no longer effective.
Your situation is complex from a soil mechanics situation. You could end up with the wall and posts keeping together as they move horizontally is the soil gets saturated (leaky pool, poor drainage, big storm?). Wet soil is much, much heavier than water. You could have 1000# trying to tip each post over and slide it on a steady basis. If your house foundations are anywhere near the wall and post supports you could have additional loads.
Code require walls over 4' to be engineered. Common sense says this wall should be looked at. So far, you have not even said what kind of soil you have and that is a key item, not to mention subsurface drainage.
If everything does not last without moving, this could be a tough repair.
Dick
#10
07-02-06, 05:17 PM
Tmilan,
If you had poured the footing and IMMEDIATELY added the sonotube and filled it, you would have had a monolithic pour. lateral stability in that case is taken care of. If, when you poured the footings, you added several pieces of rebar and have them projecting up a foot or so in such a way that a sonotube can cover them, it wont be as strong, but it will work. (Sonotubes come in a variety of sizes. You need at least an 8" tube for the Simpson post base or column base, but you may have to go to a 10" or 12" tube to get the rebar to all fit inside of it.)
Read carefully what Concretemasonary posted as well. Dick is telling you a lot of the same thing that I'm telling you -- sticking wood in the dirt is going to make for a very short-lived piece of wood.
If you had poured the footing and IMMEDIATELY added the sonotube and filled it, you would have had a monolithic pour. lateral stability in that case is taken care of. If, when you poured the footings, you added several pieces of rebar and have them projecting up a foot or so in such a way that a sonotube can cover them, it wont be as strong, but it will work. (Sonotubes come in a variety of sizes. You need at least an 8" tube for the Simpson post base or column base, but you may have to go to a 10" or 12" tube to get the rebar to all fit inside of it.)
Read carefully what Concretemasonary posted as well. Dick is telling you a lot of the same thing that I'm telling you -- sticking wood in the dirt is going to make for a very short-lived piece of wood.
#11
07-03-06, 05:23 AM
wood in dirt
Gentlemen, I appreciate both of you help in this endeavor. So it sounds like the .4 pressure treated posts, that I read would be fine in contact with the ground, are really not OK. That's difficult because the whole project was planned and drwn up out of wood. Drainage was planned as a 6" perforated pipe covered in stone up against the wall, fabric on top, then back filled. Dirt is a mixture of clay and topsoil. Are you guys guessing that i am setting myself up for a failed wall in 5 years?
Thanks - Tmilan
Thanks - Tmilan
#12
07-03-06, 01:04 PM
Member
Sink or not to sink deck posts
Treated wood in the ground is temporary in terms of the expected life of a home. The life will vary depending on the exposure and conditions.
Some of the old creosote projects give people the idea the newer wood and treatments will perform as well.
Your soil - topsoil and clay is the worst type of backfill that could be used. It holds moisture, has little strength on its own when damp and experts the greatest load on a wall.
Since you are attaching a retaining wall to a deck design, I am surprised your code people did not ask a few questions. There are standard ways of building things, but combining them can be tricky. This free code advice is one of the benefits is what you get with a permit.
I cannot fully understand your configuration in detail, but the choice of materials, combining structures together and the effect of the available moisture have always worried me in designs.
Dick
Some of the old creosote projects give people the idea the newer wood and treatments will perform as well.
Your soil - topsoil and clay is the worst type of backfill that could be used. It holds moisture, has little strength on its own when damp and experts the greatest load on a wall.
Since you are attaching a retaining wall to a deck design, I am surprised your code people did not ask a few questions. There are standard ways of building things, but combining them can be tricky. This free code advice is one of the benefits is what you get with a permit.
I cannot fully understand your configuration in detail, but the choice of materials, combining structures together and the effect of the available moisture have always worried me in designs.
Dick
